diff --git a/R/attributes.R b/R/attributes.R
index a75f4d2be3..13824155c1 100644
--- a/R/attributes.R
+++ b/R/attributes.R
@@ -433,7 +433,7 @@ graph.attributes <- function(graph) {
 #' @param name Name of the attribute to query. If missing, then
 #'   all vertex attributes are returned in a list.
 #' @param index An optional vertex sequence to query the attribute only
-#'   for these vertices.
+#'   for these vertices. The default `NULL` selects all vertices.
 #' @return The value of the vertex attribute, or the list of
 #'   all vertex attributes, if `name` is missing.
 #'
@@ -448,16 +448,19 @@ graph.attributes <- function(graph) {
 #' vertex_attr(g, "label")
 #' vertex_attr(g)
 #' plot(g)
-vertex_attr <- function(graph, name, index = V(graph)) {
+vertex_attr <- function(graph, name, index = NULL) {
   ensure_igraph(graph)
   if (missing(name)) {
-    if (missing(index)) {
+    if (is.null(index)) {
       return(vertex.attributes(graph))
     }
     return(vertex.attributes(graph, index = index))
   }
 
   check_string(name)
+  if (is.null(index)) {
+    index <- V(graph)
+  }
   myattr <-
     .Call(
       Rx_igraph_mybracket2,
@@ -479,7 +482,7 @@ vertex_attr <- function(graph, name, index = V(graph)) {
 #'   then `value` must be a named list, and its entries are
 #'   set as vertex attributes.
 #' @param index An optional vertex sequence to set the attributes
-#'   of a subset of vertices.
+#'   of a subset of vertices. The default `NULL` selects all vertices.
 #' @param value The new value of the attribute(s) for all
 #'   (or `index`) vertices.
 #' @return The graph, with the vertex attribute(s) added or set.
@@ -497,7 +500,10 @@ vertex_attr <- function(graph, name, index = V(graph)) {
 #' vertex_attr(g, "label") <- V(g)$name
 #' g
 #' plot(g)
-`vertex_attr<-` <- function(graph, name, index = V(graph), value) {
+`vertex_attr<-` <- function(graph, name, index = NULL, value) {
+  if (is.null(index)) {
+    index <- V(graph)
+  }
   if (missing(name)) {
     `vertex.attributes<-`(graph, index = index, value = value)
   } else {
@@ -512,7 +518,7 @@ vertex_attr <- function(graph, name, index = V(graph)) {
 #' @param graph The graph.
 #' @param name  The name of the attribute to set.
 #' @param index An optional vertex sequence to set the attributes
-#'   of a subset of vertices.
+#'   of a subset of vertices. The default `NULL` selects all vertices.
 #' @param value The new value of the attribute for all (or `index`)
 #'   vertices.
 #'   If `NULL`, the input is returned unchanged.
@@ -526,10 +532,14 @@ vertex_attr <- function(graph, name, index = V(graph)) {
 #'   set_vertex_attr("label", value = LETTERS[1:10])
 #' g
 #' plot(g)
-set_vertex_attr <- function(graph, name, index = V(graph), value) {
+set_vertex_attr <- function(graph, name, index = NULL, value) {
   call <- rlang::current_env()
   check_string(name)
 
+  if (is.null(index)) {
+    index <- V(graph)
+  }
+
   if (is_complete_iterator(index)) {
     return(i_set_vertex_attr(
       graph = graph,
@@ -554,7 +564,7 @@ set_vertex_attr <- function(graph, name, index = V(graph), value) {
 #' @param graph The graph.
 #' @param ... <[`dynamic-dots`][rlang::dyn-dots]> Named arguments, where the names are the attributes
 #' @param index An optional vertex sequence to set the attributes
-#'   of a subset of vertices.
+#'   of a subset of vertices. The default `NULL` selects all vertices.
 #' @return The graph, with the vertex attributes added or set.
 #'
 #' @family attributes
@@ -568,7 +578,7 @@ set_vertex_attr <- function(graph, name, index = V(graph), value) {
 #' set_vertex_attrs(g, !!!x)
 #' # to set an attribute named "index" use `:=`
 #' set_vertex_attrs(g, color = "blue", index := 10, name = LETTERS[1:10])
-set_vertex_attrs <- function(graph, ..., index = V(graph)) {
+set_vertex_attrs <- function(graph, ..., index = NULL) {
   call <- rlang::current_env()
   dots <- rlang::list2(...)
 
@@ -576,6 +586,10 @@ set_vertex_attrs <- function(graph, ..., index = V(graph)) {
     cli::cli_abort("All arguments in `...` must be named.")
   }
 
+  if (is.null(index)) {
+    index <- V(graph)
+  }
+
   for (attr_name in names(dots)) {
     attr_value <- dots[[attr_name]]
     graph <- i_set_vertex_attr(
@@ -660,10 +674,10 @@ i_set_vertex_attr <- function(
 }
 
 #' @export
-vertex.attributes <- function(graph, index = V(graph)) {
+vertex.attributes <- function(graph, index = NULL) {
   ensure_igraph(graph)
 
-  if (!missing(index)) {
+  if (!is.null(index)) {
     index <- as_igraph_vs(graph, index)
   }
 
@@ -674,7 +688,7 @@ vertex.attributes <- function(graph, index = V(graph)) {
     igraph_attr_idx_vertex
   )
 
-  if (!missing(index)) {
+  if (!is.null(index)) {
     if (!index_is_natural_vertex_sequence(index, graph)) {
       for (i in seq_along(res)) {
         res[[i]] <- res[[i]][index]
@@ -743,7 +757,7 @@ set_value_at <- function(value, idx, length_out) {
 #' @param name The name of the attribute to query. If missing, then
 #'   all edge attributes are returned in a list.
 #' @param index An optional edge sequence to query edge attributes
-#'   for a subset of edges.
+#'   for a subset of edges. The default `NULL` selects all edges.
 #' @return The value of the edge attribute, or the list of all
 #'   edge attributes if `name` is missing.
 #'
@@ -757,17 +771,20 @@ set_value_at <- function(value, idx, length_out) {
 #'   set_edge_attr("color", value = "red")
 #' g
 #' plot(g, edge.width = E(g)$weight)
-edge_attr <- function(graph, name, index = E(graph)) {
+edge_attr <- function(graph, name, index = NULL) {
   ensure_igraph(graph)
 
   if (missing(name)) {
-    if (missing(index)) {
+    if (is.null(index)) {
       edge.attributes(graph)
     } else {
       edge.attributes(graph, index = index)
     }
   } else {
     check_string(name)
+    if (is.null(index)) {
+      index <- E(graph)
+    }
     myattr <- .Call(
       Rx_igraph_mybracket2,
       graph,
@@ -790,7 +807,7 @@ edge_attr <- function(graph, name, index = E(graph)) {
 #'   then `value` must be a named list, and its entries are
 #'   set as edge attributes.
 #' @param index An optional edge sequence to set the attributes
-#'   of a subset of edges.
+#'   of a subset of edges. The default `NULL` selects all edges.
 #' @param value The new value of the attribute(s) for all
 #'   (or `index`) edges.
 #' @return The graph, with the edge attribute(s) added or set.
@@ -808,7 +825,10 @@ edge_attr <- function(graph, name, index = E(graph)) {
 #' edge_attr(g, "label") <- E(g)$name
 #' g
 #' plot(g)
-`edge_attr<-` <- function(graph, name, index = E(graph), value) {
+`edge_attr<-` <- function(graph, name, index = NULL, value) {
+  if (is.null(index)) {
+    index <- E(graph)
+  }
   if (missing(name)) {
     `edge.attributes<-`(graph, index = index, value = value)
   } else {
@@ -822,7 +842,7 @@ edge_attr <- function(graph, name, index = E(graph)) {
 #' @param graph The graph
 #' @param name  The name of the attribute to set.
 #' @param index An optional edge sequence to set the attributes of
-#'   a subset of edges.
+#'   a subset of edges. The default `NULL` selects all edges.
 #' @param value The new value of the attribute for all (or `index`)
 #'   edges.
 #'   If `NULL`, the input is returned unchanged.
@@ -836,9 +856,12 @@ edge_attr <- function(graph, name, index = E(graph)) {
 #'   set_edge_attr("label", value = LETTERS[1:10])
 #' g
 #' plot(g)
-set_edge_attr <- function(graph, name, index = E(graph), value) {
+set_edge_attr <- function(graph, name, index = NULL, value) {
   call <- rlang::current_env()
   check_string(name)
+  if (is.null(index)) {
+    index <- E(graph)
+  }
   if (is_complete_iterator(index)) {
     i_set_edge_attr(
       graph = graph,
@@ -928,10 +951,10 @@ i_set_edge_attr <- function(
 }
 
 #' @export
-edge.attributes <- function(graph, index = E(graph)) {
+edge.attributes <- function(graph, index = NULL) {
   ensure_igraph(graph)
 
-  if (!missing(index)) {
+  if (!is.null(index)) {
     index <- as_igraph_es(graph, index)
   }
 
@@ -943,7 +966,7 @@ edge.attributes <- function(graph, index = E(graph)) {
   )
 
   if (
-    !missing(index) &&
+    !is.null(index) &&
       !index_is_natural_edge_sequence(index, graph)
   ) {
     for (i in seq_along(res)) {
diff --git a/R/centrality.R b/R/centrality.R
index b874c4e401..ec70be7fb7 100644
--- a/R/centrality.R
+++ b/R/centrality.R
@@ -382,6 +382,7 @@ betweenness.estimate <- estimate_betweenness
 #' @aliases edge.betweenness.estimate
 #' @param graph The graph to analyze.
 #' @param v The vertices for which the vertex betweenness will be calculated.
+#'   The default `NULL` selects all vertices.
 #' @inheritParams rlang::args_dots_empty
 #' @param directed Logical, whether directed paths should be considered while
 #'   determining the shortest paths.
@@ -430,7 +431,7 @@ betweenness.estimate <- estimate_betweenness
 #'
 betweenness <- function(
   graph,
-  v = V(graph),
+  v = NULL,
   ...,
   directed = TRUE,
   weights = NULL,
@@ -469,6 +470,10 @@ betweenness <- function(
   }
   # END GENERATED ARG_HANDLE
 
+  if (is.null(v)) {
+    v <- V(graph)
+  }
+
   res <- betweenness_cutoff_impl(
     graph = graph,
     vids = v,
@@ -489,11 +494,12 @@ betweenness <- function(
 
 #' @rdname betweenness
 #' @param e The edges for which the edge betweenness will be calculated.
+#'   The default `NULL` selects all edges.
 #' @inheritParams rlang::args_dots_empty
 #' @export
 edge_betweenness <- function(
   graph,
-  e = E(graph),
+  e = NULL,
   ...,
   directed = TRUE,
   weights = NULL,
@@ -521,6 +527,10 @@ edge_betweenness <- function(
   }
   # END GENERATED ARG_HANDLE
 
+  if (is.null(e)) {
+    e <- E(graph)
+  }
+
   e <- as_igraph_es(graph, e)
   res <- edge_betweenness_cutoff_impl(
     graph = graph,
@@ -593,6 +603,7 @@ edge.betweenness.estimate <- estimate_edge_betweenness
 #' @aliases closeness.estimate
 #' @param graph The graph to analyze.
 #' @param vids The vertices for which closeness will be calculated.
+#'   The default `NULL` selects all vertices.
 #' @inheritParams rlang::args_dots_empty
 #' @param mode Character string, defined the types of the paths used for
 #'   measuring the distance in directed graphs. \dQuote{in} measures the paths
@@ -628,7 +639,7 @@ edge.betweenness.estimate <- estimate_edge_betweenness
 #'
 closeness <- function(
   graph,
-  vids = V(graph),
+  vids = NULL,
   ...,
   mode = c("out", "in", "all", "total"),
   weights = NULL,
@@ -667,6 +678,10 @@ closeness <- function(
   }
   # END GENERATED ARG_HANDLE
 
+  if (is.null(vids)) {
+    vids <- V(graph)
+  }
+
   closeness_cutoff_impl(
     graph = graph,
     vids = vids,
@@ -1451,6 +1466,7 @@ eigen_centrality <- function(
 #'
 #' @param graph The input graph.
 #' @param vids The vertices for which the strength will be calculated.
+#'   The default `NULL` selects all vertices.
 #' @inheritParams rlang::args_dots_empty
 #' @param mode Character string, \dQuote{out} for out-degree, \dQuote{in} for
 #'   in-degree or \dQuote{all} for the sum of the two. For undirected graphs this
@@ -1483,7 +1499,7 @@ eigen_centrality <- function(
 #' @export
 strength <- function(
   graph,
-  vids = V(graph),
+  vids = NULL,
   ...,
   mode = c("all", "out", "in", "total"),
   loops = TRUE,
@@ -1515,6 +1531,10 @@ strength <- function(
   }
   # END GENERATED ARG_HANDLE
 
+  if (is.null(vids)) {
+    vids <- V(graph)
+  }
+
   strength_impl(
     graph = graph,
     vids = vids,
@@ -1548,6 +1568,7 @@ strength <- function(
 #'   computation. If `NULL`, then the \sQuote{weight} attibute is used. Note
 #'   that this measure is not defined for unweighted graphs.
 #' @param vids The vertex IDs for which to calculate the measure.
+#'   The default `NULL` selects all vertices.
 #' @return A numeric vector, its length is the number of vertices.
 #' @author Gabor Csardi \email{csardi.gabor@@gmail.com}
 #' @references Nathan Eagle, Michael Macy and Rob Claxton: Network Diversity
@@ -1570,7 +1591,7 @@ diversity <- function(
   graph,
   ...,
   weights = NULL,
-  vids = V(graph)
+  vids = NULL
 ) {
   # BEGIN GENERATED ARG_HANDLE: diversity, do not edit, see tools/generate-migrations.R
   if (...length() > 0L) {
@@ -1581,7 +1602,7 @@ diversity <- function(
       recover_old = c("weights", "vids"),
       match_names = c("weights", "vids"),
       match_to = c("weights", "vids"),
-      defaults = list(weights = NULL, vids = V(graph)),
+      defaults = list(weights = NULL, vids = NULL),
       head_args = c("graph"),
       fn_name = "diversity"
     )
@@ -1594,6 +1615,10 @@ diversity <- function(
   }
   # END GENERATED ARG_HANDLE
 
+  if (is.null(vids)) {
+    vids <- V(graph)
+  }
+
   diversity_impl(
     graph = graph,
     weights = weights,
@@ -1625,7 +1650,7 @@ diversity <- function(
 #'   interprets edge weights as connection strengths. The weights of parallel
 #'   edges are effectively added up.
 #' @param options A named list, to override some ARPACK options. See
-#'   [arpack()] for details.
+#'   [arpack()] for details. The default `NULL` uses [arpack_defaults()].
 #' @inheritParams rlang::args_dots_empty
 #' @return A named list with members:
 #'   \describe{
@@ -1665,10 +1690,14 @@ hits_scores <- function(
   ...,
   scale = TRUE,
   weights = NULL,
-  options = arpack_defaults()
+  options = NULL
 ) {
   rlang::check_dots_empty()
 
+  if (is.null(options)) {
+    options <- arpack_defaults()
+  }
+
   hub_and_authority_scores_impl(
     graph = graph,
     scale = scale,
@@ -1788,6 +1817,7 @@ hub_score <- function(
 #'   default implementation from igraph version 0.5 until version 0.7. It computes
 #'   PageRank scores by solving an eingevalue problem.
 #' @param vids The vertices of interest.
+#'   The default `NULL` selects all vertices.
 #' @param directed Logical, if true directed paths will be considered for
 #'   directed graphs. It is ignored for undirected graphs.
 #' @param damping The damping factor (\sQuote{d} in the original paper).
@@ -1848,7 +1878,7 @@ page_rank <- function(
   graph,
   ...,
   algo = c("prpack", "arpack"),
-  vids = V(graph),
+  vids = NULL,
   directed = TRUE,
   damping = 0.85,
   personalized = NULL,
@@ -1906,7 +1936,7 @@ page_rank <- function(
       ),
       defaults = list(
         algo = c("prpack", "arpack"),
-        vids = V(graph),
+        vids = NULL,
         directed = TRUE,
         damping = 0.85,
         personalized = NULL,
@@ -1925,6 +1955,10 @@ page_rank <- function(
   }
   # END GENERATED ARG_HANDLE
 
+  if (is.null(vids)) {
+    vids <- V(graph)
+  }
+
   personalized_pagerank_impl(
     graph = graph,
     algo = algo,
@@ -1949,6 +1983,7 @@ page_rank <- function(
 #'
 #' @param graph The graph to analyze.
 #' @param vids The vertices for which harmonic centrality will be calculated.
+#'   The default `NULL` selects all vertices.
 #' @inheritParams rlang::args_dots_empty
 #' @param mode Character string, defining the types of the paths used for
 #'   measuring the distance in directed graphs. \dQuote{out} follows paths along
@@ -1985,7 +2020,7 @@ page_rank <- function(
 #'
 harmonic_centrality <- function(
   graph,
-  vids = V(graph),
+  vids = NULL,
   ...,
   mode = c("out", "in", "all", "total"),
   weights = NULL,
@@ -2024,6 +2059,10 @@ harmonic_centrality <- function(
   }
   # END GENERATED ARG_HANDLE
 
+  if (is.null(vids)) {
+    vids <- V(graph)
+  }
+
   harmonic_centrality_cutoff_impl(
     graph = graph,
     vids = vids,
@@ -2154,7 +2193,7 @@ bonpow.sparse <- function(
 #'
 #' @param graph the input graph.
 #' @param nodes vertex sequence indicating which vertices are to be included in
-#'   the calculation.  By default, all vertices are included.
+#'   the calculation. The default `NULL` selects all vertices.
 #' @inheritParams rlang::args_dots_empty
 #' @param loops Logical indicating whether or not the diagonal should be
 #'   treated as valid data.  Set this true if and only if the data can contain
@@ -2216,7 +2255,7 @@ bonpow.sparse <- function(
 #'
 power_centrality <- function(
   graph,
-  nodes = V(graph),
+  nodes = NULL,
   ...,
   loops = FALSE,
   exponent = 1,
@@ -2282,6 +2321,10 @@ power_centrality <- function(
   }
   # END GENERATED ARG_HANDLE
 
+  if (is.null(nodes)) {
+    nodes <- V(graph)
+  }
+
   nodes <- as_igraph_vs(graph, nodes)
   if (sparse) {
     res <- bonpow.sparse(
@@ -2395,8 +2438,8 @@ alpha.centrality.sparse <- function(
 #' @param graph The input graph, can be directed or undirected. In undirected
 #'   graphs, edges are treated as if they were reciprocal directed ones.
 #' @param nodes Vertex sequence, the vertices for which the alpha centrality
-#'   values are returned. (For technical reasons they will be calculated for all
-#'   vertices, anyway.)
+#'   values are returned. The default `NULL` selects all vertices.
+#'   (For technical reasons they will be calculated for all vertices, anyway.)
 #' @inheritParams rlang::args_dots_empty
 #' @param alpha Parameter specifying the relative importance of endogenous
 #'   versus exogenous factors in the determination of centrality. See details
@@ -2437,7 +2480,7 @@ alpha.centrality.sparse <- function(
 #'
 alpha_centrality <- function(
   graph,
-  nodes = V(graph),
+  nodes = NULL,
   ...,
   alpha = 1,
   loops = FALSE,
@@ -2482,6 +2525,10 @@ alpha_centrality <- function(
   }
   # END GENERATED ARG_HANDLE
 
+  if (is.null(nodes)) {
+    nodes <- V(graph)
+  }
+
   nodes <- as_igraph_vs(graph, nodes)
   if (sparse) {
     res <- alpha.centrality.sparse(
diff --git a/R/cocitation.R b/R/cocitation.R
index 5423dd7efb..f5c67ab277 100644
--- a/R/cocitation.R
+++ b/R/cocitation.R
@@ -39,7 +39,7 @@
 #' @param graph The graph object to analyze
 #' @param v Vertex sequence or numeric vector, the vertex IDs for which the
 #'   cocitation or bibliographic coupling values we want to calculate. The
-#'   default is all vertices.
+#'   default `NULL` selects all vertices.
 #' @return A numeric matrix with `length(v)` lines and
 #'   `vcount(graph)` columns. Element `(i,j)` contains the cocitation
 #'   or bibliographic coupling for vertices `v[i]` and `j`.
@@ -53,7 +53,11 @@
 #' cocitation(g)
 #' bibcoupling(g)
 #'
-cocitation <- function(graph, v = V(graph)) {
+cocitation <- function(graph, v = NULL) {
+  if (is.null(v)) {
+    v <- V(graph)
+  }
+
   res <- cocitation_impl(
     graph = graph,
     vids = v
@@ -68,7 +72,11 @@ cocitation <- function(graph, v = V(graph)) {
 
 #' @rdname cocitation
 #' @export
-bibcoupling <- function(graph, v = V(graph)) {
+bibcoupling <- function(graph, v = NULL) {
+  if (is.null(v)) {
+    v <- V(graph)
+  }
+
   res <- bibcoupling_impl(
     graph = graph,
     vids = v
diff --git a/R/cohesive.blocks.R b/R/cohesive.blocks.R
index b27c2f859b..3cd2c138fb 100644
--- a/R/cohesive.blocks.R
+++ b/R/cohesive.blocks.R
@@ -259,8 +259,8 @@ blockGraphs <- function(blocks, graph) {
 #'   them. By default all cohesive blocks are marked, except the one
 #'   corresponding to the all vertices.
 #' @param layout The layout of a plot, it is simply passed on to
-#'   `plot.igraph()`, see the possible formats there. By default the
-#'   Reingold-Tilford layout generator is used.
+#'   `plot.igraph()`, see the possible formats there. The default `NULL` uses
+#'   the Reingold-Tilford layout generator.
 #' @param \dots Additional arguments. `plot_hierarchy()` and [plot()] pass
 #'   them to `plot.igraph()`.  [print()] and [summary()] ignore them.
 #' @return `cohesive_blocks()` returns a `cohesiveBlocks` object.
@@ -538,9 +538,13 @@ plot.cohesiveBlocks <- function(
 #' @importFrom graphics plot
 plot_hierarchy <- function(
   blocks,
-  layout = layout_as_tree(hierarchy(blocks), root = 1),
+  layout = NULL,
   ...
 ) {
+  if (is.null(layout)) {
+    layout <- layout_as_tree(hierarchy(blocks), root = 1)
+  }
+
   plot(hierarchy(blocks), layout = layout, ...)
 }
 
diff --git a/R/community.R b/R/community.R
index c966d54cff..d0849464d2 100644
--- a/R/community.R
+++ b/R/community.R
@@ -3130,7 +3130,7 @@ plot.communities <- function(
 
 #' @rdname plot_dendrogram.communities
 #' @export
-plot_dendrogram <- function(x, mode = igraph_opt("dend.plot.type"), ...) {
+plot_dendrogram <- function(x, mode = NULL, ...) {
   UseMethod("plot_dendrogram")
 }
 
@@ -3197,6 +3197,7 @@ plot_dendrogram <- function(x, mode = igraph_opt("dend.plot.type"), ...) {
 #' @param x An object containing the community structure of a graph. See
 #'   [communities()] for details.
 #' @param mode Which dendrogram plotting function to use. See details below.
+#'   The default `NULL` uses the `dend.plot.type` igraph option.
 #' @param \dots Additional arguments to supply to the dendrogram plotting
 #'   function.
 #' @param use.modularity Logical, whether to use the modularity values
@@ -3217,11 +3218,14 @@ plot_dendrogram <- function(x, mode = igraph_opt("dend.plot.type"), ...) {
 #'
 plot_dendrogram.communities <- function(
   x,
-  mode = igraph_opt("dend.plot.type"),
+  mode = NULL,
   ...,
   use.modularity = FALSE,
   palette = categorical_pal(8)
 ) {
+  if (is.null(mode)) {
+    mode <- igraph_opt("dend.plot.type")
+  }
   mode <- igraph_match_arg(mode, c("auto", "phylo", "hclust", "dendrogram"))
 
   old_palette <- palette(palette)
@@ -3600,7 +3604,8 @@ communities <- groups.communities
 #'   correspond to the vertices, and for each element the ID in the new graph is
 #'   given.
 #' @param vertex.attr.comb Specifies how to combine the vertex attributes in
-#'   the new graph. Please see [attribute.combination()] for details.
+#'   the new graph. Please see [attribute.combination()] for details. The
+#'   default `NULL` uses the `vertex.attr.comb` igraph option.
 #' @return A new graph object.
 #' @author Gabor Csardi \email{csardi.gabor@@gmail.com}
 #' @keywords graphs
@@ -3624,8 +3629,12 @@ communities <- groups.communities
 contract <- function(
   graph,
   mapping,
-  vertex.attr.comb = igraph_opt("vertex.attr.comb")
+  vertex.attr.comb = NULL
 ) {
+  if (is.null(vertex.attr.comb)) {
+    vertex.attr.comb <- igraph_opt("vertex.attr.comb")
+  }
+
   contract_vertices_impl(
     graph = graph,
     mapping = mapping,
diff --git a/R/conversion.R b/R/conversion.R
index 9f44f19361..28d218c7f2 100644
--- a/R/conversion.R
+++ b/R/conversion.R
@@ -422,7 +422,7 @@ get.adjacency.sparse <- function(
 #'   is present in the graph.
 #' @param sparse Logical, whether to create a sparse matrix. The
 #'   \sQuote{`Matrix`} package must be installed for creating sparse
-#'   matrices.
+#'   matrices. The default `NULL` uses the `sparsematrices` igraph option.
 #' @return A `vcount(graph)` by `vcount(graph)` (usually) numeric
 #'   matrix.
 #'
@@ -444,7 +444,7 @@ as_adjacency_matrix <- function(
   ...,
   weights = NULL,
   names = TRUE,
-  sparse = igraph_opt("sparsematrices"),
+  sparse = NULL,
   edges = deprecated(),
   attr = deprecated()
 ) {
@@ -468,7 +468,7 @@ as_adjacency_matrix <- function(
       defaults = list(
         weights = NULL,
         names = TRUE,
-        sparse = igraph_opt("sparsematrices"),
+        sparse = NULL,
         edges = deprecated(),
         attr = deprecated()
       ),
@@ -484,6 +484,10 @@ as_adjacency_matrix <- function(
   }
   # END GENERATED ARG_HANDLE
 
+  if (is.null(sparse)) {
+    sparse <- igraph_opt("sparsematrices")
+  }
+
   if (lifecycle::is_present(edges) && isTRUE(edges)) {
     lifecycle::deprecate_stop("2.0.0", "as_adjacency_matrix(edges = )")
   }
@@ -740,15 +744,19 @@ as_directed <- function(
 #'   `mode="collapse"` or `mode="mutual"`.  In these cases many edges
 #'   might be mapped to a single one in the new graph, and their attributes are
 #'   combined. Please see [attribute.combination()] for details on
-#'   this.
+#'   this. The default `NULL` uses the `edge.attr.comb` igraph option.
 #' @export
 as_undirected <- function(
   graph,
   mode = c("collapse", "each", "mutual"),
-  edge.attr.comb = igraph_opt("edge.attr.comb")
+  edge.attr.comb = NULL
 ) {
   # Argument checks
   ensure_igraph(graph)
+  if (is.null(edge.attr.comb)) {
+    edge.attr.comb <- igraph_opt("edge.attr.comb")
+  }
+
   mode <- igraph_match_arg(mode)
 
   # Function call
diff --git a/R/efficiency.R b/R/efficiency.R
index 51f6f34974..288d1fc86f 100644
--- a/R/efficiency.R
+++ b/R/efficiency.R
@@ -42,7 +42,8 @@
 #'   additionally, no edge weight may be NaN. If it is `NULL` (the default)
 #'   and the graph has a `weight` edge attribute, then it is used automatically.
 #' @param vids The vertex IDs of the vertices for which the calculation will be done.
-#'   Applies to the local efficiency calculation only.
+#'   Applies to the local efficiency calculation only. The default `NULL`
+#'   selects all vertices.
 #' @param directed Logical, whether to consider directed paths. Ignored
 #'   for undirected graphs.
 #' @param mode Specifies how to define the local neighborhood of a vertex in
@@ -107,7 +108,7 @@ global_efficiency <- function(
 #' @export
 local_efficiency <- function(
   graph,
-  vids = V(graph),
+  vids = NULL,
   ...,
   weights = NULL,
   directed = TRUE,
@@ -139,6 +140,10 @@ local_efficiency <- function(
   }
   # END GENERATED ARG_HANDLE
 
+  if (is.null(vids)) {
+    vids <- V(graph)
+  }
+
   local_efficiency_impl(
     graph = graph,
     vids = vids,
diff --git a/R/embedding.R b/R/embedding.R
index 626d9db1d0..77f324decc 100644
--- a/R/embedding.R
+++ b/R/embedding.R
@@ -62,10 +62,12 @@
 #' @param scaled Logical, if `FALSE`, then \eqn{U} and \eqn{V} are
 #'   returned instead of \eqn{X} and \eqn{Y}.
 #' @param cvec A numeric vector, its length is the number vertices in the
-#'   graph. This vector is added to the diagonal of the adjacency matrix.
+#'   graph. This vector is added to the diagonal of the adjacency matrix. The
+#'   default `NULL` uses
+#'   `strength(graph, weights = weights) / (vcount(graph) - 1)`.
 #' @param options A named list containing the parameters for the SVD
-#'   computation algorithm in ARPACK. By default, the list of values is assigned
-#'   the values given by [arpack_defaults()].
+#'   computation algorithm in ARPACK. The default `NULL` uses the values given
+#'   by [arpack_defaults()].
 #' @return A list containing with entries:
 #'   \describe{
 #'     \item{X}{
@@ -109,8 +111,8 @@ embed_adjacency_matrix <- function(
   weights = NULL,
   which = c("lm", "la", "sa"),
   scaled = TRUE,
-  cvec = strength(graph, weights = weights) / (vcount(graph) - 1),
-  options = arpack_defaults()
+  cvec = NULL,
+  options = NULL
 ) {
   # BEGIN GENERATED ARG_HANDLE: embed_adjacency_matrix, do not edit, see tools/generate-migrations.R
   if (...length() > 0L) {
@@ -131,8 +133,8 @@ embed_adjacency_matrix <- function(
         weights = NULL,
         which = c("lm", "la", "sa"),
         scaled = TRUE,
-        cvec = strength(graph, weights = weights) / (vcount(graph) - 1),
-        options = arpack_defaults()
+        cvec = NULL,
+        options = NULL
       ),
       head_args = c("graph", "no"),
       fn_name = "embed_adjacency_matrix"
@@ -146,6 +148,13 @@ embed_adjacency_matrix <- function(
   }
   # END GENERATED ARG_HANDLE
 
+  if (is.null(cvec)) {
+    cvec <- strength(graph, weights = weights) / (vcount(graph) - 1)
+  }
+  if (is.null(options)) {
+    options <- arpack_defaults()
+  }
+
   adjacency_spectral_embedding_impl(
     graph = graph,
     no = no,
@@ -272,8 +281,8 @@ dim_select <- function(sv) {
 #' @param scaled Logical, if `FALSE`, then \eqn{U} and \eqn{V} are
 #'   returned instead of \eqn{X} and \eqn{Y}.
 #' @param options A named list containing the parameters for the SVD
-#'   computation algorithm in ARPACK. By default, the list of values is assigned
-#'   the values given by [arpack_defaults()].
+#'   computation algorithm in ARPACK. The default `NULL` uses the values given
+#'   by [arpack_defaults()].
 #' @return A list containing with entries:
 #'   \describe{
 #'     \item{X}{
@@ -320,7 +329,7 @@ embed_laplacian_matrix <- function(
   which = c("lm", "la", "sa"),
   type = c("default", "D-A", "DAD", "I-DAD", "OAP"),
   scaled = TRUE,
-  options = arpack_defaults()
+  options = NULL
 ) {
   # BEGIN GENERATED ARG_HANDLE: embed_laplacian_matrix, do not edit, see tools/generate-migrations.R
   if (...length() > 0L) {
@@ -342,7 +351,7 @@ embed_laplacian_matrix <- function(
         which = c("lm", "la", "sa"),
         type = c("default", "D-A", "DAD", "I-DAD", "OAP"),
         scaled = TRUE,
-        options = arpack_defaults()
+        options = NULL
       ),
       head_args = c("graph", "no"),
       fn_name = "embed_laplacian_matrix"
@@ -356,6 +365,10 @@ embed_laplacian_matrix <- function(
   }
   # END GENERATED ARG_HANDLE
 
+  if (is.null(options)) {
+    options <- arpack_defaults()
+  }
+
   laplacian_spectral_embedding_impl(
     graph = graph,
     no = no,
diff --git a/R/games.R b/R/games.R
index 6a0cf53800..45a7ab8252 100644
--- a/R/games.R
+++ b/R/games.R
@@ -2248,9 +2248,10 @@ pa_age <- function(
 #' @inheritParams rlang::args_dots_empty
 #' @param edge.per.step The number of edges to add to the graph per time step.
 #' @param type.dist The distribution of the vertex types. This is assumed to be
-#'   stationary in time.
+#'   stationary in time. The default `NULL` gives a uniform distribution.
 #' @param pref.matrix A matrix giving the preferences of the given vertex
 #'   types. These should be probabilities, i.e. numbers between zero and one.
+#'   The default `NULL` sets all preferences to one.
 #' @param directed Logical, whether to generate directed graphs.
 #' @param k The number of trials per time step, see details below.
 #' @return A new graph object.
@@ -2268,8 +2269,8 @@ sample_traits_callaway <- function(
   types,
   ...,
   edge.per.step = 1,
-  type.dist = rep(1, types),
-  pref.matrix = matrix(1, types, types),
+  type.dist = NULL,
+  pref.matrix = NULL,
   directed = FALSE
 ) {
   # BEGIN GENERATED ARG_HANDLE: sample_traits_callaway, do not edit, see tools/generate-migrations.R
@@ -2293,8 +2294,8 @@ sample_traits_callaway <- function(
       match_to = c("edge.per.step", "type.dist", "pref.matrix", "directed"),
       defaults = list(
         edge.per.step = 1,
-        type.dist = rep(1, types),
-        pref.matrix = matrix(1, types, types),
+        type.dist = NULL,
+        pref.matrix = NULL,
         directed = FALSE
       ),
       head_args = c("nodes", "types"),
@@ -2309,6 +2310,13 @@ sample_traits_callaway <- function(
   }
   # END GENERATED ARG_HANDLE
 
+  if (is.null(type.dist)) {
+    type.dist <- rep(1, types)
+  }
+  if (is.null(pref.matrix)) {
+    pref.matrix <- matrix(1, types, types)
+  }
+
   res <- callaway_traits_game_impl(
     nodes = nodes,
     types = types,
@@ -2339,8 +2347,8 @@ traits_callaway <- function(
   types,
   ...,
   edge.per.step = 1,
-  type.dist = rep(1, types),
-  pref.matrix = matrix(1, types, types),
+  type.dist = NULL,
+  pref.matrix = NULL,
   directed = FALSE
 ) {
   # BEGIN GENERATED ARG_HANDLE: traits_callaway, do not edit, see tools/generate-migrations.R
@@ -2364,8 +2372,8 @@ traits_callaway <- function(
       match_to = c("edge.per.step", "type.dist", "pref.matrix", "directed"),
       defaults = list(
         edge.per.step = 1,
-        type.dist = rep(1, types),
-        pref.matrix = matrix(1, types, types),
+        type.dist = NULL,
+        pref.matrix = NULL,
         directed = FALSE
       ),
       head_args = c("nodes", "types"),
@@ -2380,6 +2388,13 @@ traits_callaway <- function(
   }
   # END GENERATED ARG_HANDLE
 
+  if (is.null(type.dist)) {
+    type.dist <- rep(1, types)
+  }
+  if (is.null(pref.matrix)) {
+    pref.matrix <- matrix(1, types, types)
+  }
+
   constructor_spec(
     sample_traits_callaway,
     nodes,
@@ -2399,8 +2414,8 @@ sample_traits <- function(
   types,
   k = 1,
   ...,
-  type.dist = rep(1, types),
-  pref.matrix = matrix(1, types, types),
+  type.dist = NULL,
+  pref.matrix = NULL,
   directed = FALSE
 ) {
   # BEGIN GENERATED ARG_HANDLE: sample_traits, do not edit, see tools/generate-migrations.R
@@ -2421,11 +2436,7 @@ sample_traits <- function(
       recover_old = c("type.dist", "pref.matrix", "directed"),
       match_names = c("type.dist", "pref.matrix", "directed"),
       match_to = c("type.dist", "pref.matrix", "directed"),
-      defaults = list(
-        type.dist = rep(1, types),
-        pref.matrix = matrix(1, types, types),
-        directed = FALSE
-      ),
+      defaults = list(type.dist = NULL, pref.matrix = NULL, directed = FALSE),
       head_args = c("nodes", "types", "k"),
       fn_name = "sample_traits"
     )
@@ -2438,6 +2449,13 @@ sample_traits <- function(
   }
   # END GENERATED ARG_HANDLE
 
+  if (is.null(type.dist)) {
+    type.dist <- rep(1, types)
+  }
+  if (is.null(pref.matrix)) {
+    pref.matrix <- matrix(1, types, types)
+  }
+
   res <- establishment_game_impl(
     nodes = nodes,
     types = types,
@@ -2464,8 +2482,8 @@ traits <- function(
   types,
   k = 1,
   ...,
-  type.dist = rep(1, types),
-  pref.matrix = matrix(1, types, types),
+  type.dist = NULL,
+  pref.matrix = NULL,
   directed = FALSE
 ) {
   # BEGIN GENERATED ARG_HANDLE: traits, do not edit, see tools/generate-migrations.R
@@ -2486,11 +2504,7 @@ traits <- function(
       recover_old = c("type.dist", "pref.matrix", "directed"),
       match_names = c("type.dist", "pref.matrix", "directed"),
       match_to = c("type.dist", "pref.matrix", "directed"),
-      defaults = list(
-        type.dist = rep(1, types),
-        pref.matrix = matrix(1, types, types),
-        directed = FALSE
-      ),
+      defaults = list(type.dist = NULL, pref.matrix = NULL, directed = FALSE),
       head_args = c("nodes", "types", "k"),
       fn_name = "traits"
     )
@@ -2503,6 +2517,13 @@ traits <- function(
   }
   # END GENERATED ARG_HANDLE
 
+  if (is.null(type.dist)) {
+    type.dist <- rep(1, types)
+  }
+  if (is.null(pref.matrix)) {
+    pref.matrix <- matrix(1, types, types)
+  }
+
   constructor_spec(
     sample_traits,
     nodes,
@@ -2665,15 +2686,17 @@ grg <- function(
 #' @inheritParams rlang::args_dots_empty
 #' @param type.dist The distribution of the vertex types, a numeric vector of
 #'   length \sQuote{types} containing non-negative numbers. The vector will be
-#'   normed to obtain probabilities.
+#'   normed to obtain probabilities. The default `NULL` gives a uniform
+#'   distribution.
 #' @param fixed.sizes Fix the number of vertices with a given vertex type
 #'   label. The `type.dist` argument gives the group sizes (i.e. number of
 #'   vertices with the different labels) in this case.
 #' @param type.dist.matrix The joint distribution of the in- and out-vertex
-#'   types.
+#'   types. The default `NULL` gives a uniform distribution.
 #' @param pref.matrix A square matrix giving the preferences of the vertex
 #'   types. The matrix has \sQuote{types} rows and columns. When generating
-#'   an undirected graph, it must be symmetric.
+#'   an undirected graph, it must be symmetric. The default `NULL` sets all
+#'   preferences to one.
 #' @param directed Logical, whether to create a directed graph.
 #' @param loops Logical, whether self-loops are allowed in the graph.
 #' @return An igraph graph.
@@ -2701,9 +2724,9 @@ sample_pref <- function(
   nodes,
   types,
   ...,
-  type.dist = rep(1, types),
+  type.dist = NULL,
   fixed.sizes = FALSE,
-  pref.matrix = matrix(1, types, types),
+  pref.matrix = NULL,
   directed = FALSE,
   loops = FALSE
 ) {
@@ -2752,9 +2775,9 @@ sample_pref <- function(
         "loops"
       ),
       defaults = list(
-        type.dist = rep(1, types),
+        type.dist = NULL,
         fixed.sizes = FALSE,
-        pref.matrix = matrix(1, types, types),
+        pref.matrix = NULL,
         directed = FALSE,
         loops = FALSE
       ),
@@ -2770,6 +2793,13 @@ sample_pref <- function(
   }
   # END GENERATED ARG_HANDLE
 
+  if (is.null(type.dist)) {
+    type.dist <- rep(1, types)
+  }
+  if (is.null(pref.matrix)) {
+    pref.matrix <- matrix(1, types, types)
+  }
+
   if (nrow(pref.matrix) != types || ncol(pref.matrix) != types) {
     cli::cli_abort(c(
       "{.arg pref.matrix} must have {.arg types} rows and columns.",
@@ -2805,9 +2835,9 @@ pref <- function(
   nodes,
   types,
   ...,
-  type.dist = rep(1, types),
+  type.dist = NULL,
   fixed.sizes = FALSE,
-  pref.matrix = matrix(1, types, types),
+  pref.matrix = NULL,
   directed = FALSE,
   loops = FALSE
 ) {
@@ -2856,9 +2886,9 @@ pref <- function(
         "loops"
       ),
       defaults = list(
-        type.dist = rep(1, types),
+        type.dist = NULL,
         fixed.sizes = FALSE,
-        pref.matrix = matrix(1, types, types),
+        pref.matrix = NULL,
         directed = FALSE,
         loops = FALSE
       ),
@@ -2874,6 +2904,13 @@ pref <- function(
   }
   # END GENERATED ARG_HANDLE
 
+  if (is.null(type.dist)) {
+    type.dist <- rep(1, types)
+  }
+  if (is.null(pref.matrix)) {
+    pref.matrix <- matrix(1, types, types)
+  }
+
   constructor_spec(
     sample_pref,
     nodes,
@@ -2893,8 +2930,8 @@ sample_asym_pref <- function(
   nodes,
   types,
   ...,
-  type.dist.matrix = matrix(1, types, types),
-  pref.matrix = matrix(1, types, types),
+  type.dist.matrix = NULL,
+  pref.matrix = NULL,
   loops = FALSE
 ) {
   # BEGIN GENERATED ARG_HANDLE: sample_asym_pref, do not edit, see tools/generate-migrations.R
@@ -2916,8 +2953,8 @@ sample_asym_pref <- function(
       match_names = c("type.dist.matrix", "pref.matrix", "loops"),
       match_to = c("type.dist.matrix", "pref.matrix", "loops"),
       defaults = list(
-        type.dist.matrix = matrix(1, types, types),
-        pref.matrix = matrix(1, types, types),
+        type.dist.matrix = NULL,
+        pref.matrix = NULL,
         loops = FALSE
       ),
       head_args = c("nodes", "types"),
@@ -2932,6 +2969,13 @@ sample_asym_pref <- function(
   }
   # END GENERATED ARG_HANDLE
 
+  if (is.null(type.dist.matrix)) {
+    type.dist.matrix <- matrix(1, types, types)
+  }
+  if (is.null(pref.matrix)) {
+    pref.matrix <- matrix(1, types, types)
+  }
+
   if (nrow(pref.matrix) != types || ncol(pref.matrix) != types) {
     cli::cli_abort(c(
       "{.arg pref.matrix} must have {.arg types} rows and columns.",
@@ -2973,8 +3017,8 @@ asym_pref <- function(
   nodes,
   types,
   ...,
-  type.dist.matrix = matrix(1, types, types),
-  pref.matrix = matrix(1, types, types),
+  type.dist.matrix = NULL,
+  pref.matrix = NULL,
   loops = FALSE
 ) {
   # BEGIN GENERATED ARG_HANDLE: asym_pref, do not edit, see tools/generate-migrations.R
@@ -2996,8 +3040,8 @@ asym_pref <- function(
       match_names = c("type.dist.matrix", "pref.matrix", "loops"),
       match_to = c("type.dist.matrix", "pref.matrix", "loops"),
       defaults = list(
-        type.dist.matrix = matrix(1, types, types),
-        pref.matrix = matrix(1, types, types),
+        type.dist.matrix = NULL,
+        pref.matrix = NULL,
         loops = FALSE
       ),
       head_args = c("nodes", "types"),
@@ -3012,6 +3056,13 @@ asym_pref <- function(
   }
   # END GENERATED ARG_HANDLE
 
+  if (is.null(type.dist.matrix)) {
+    type.dist.matrix <- matrix(1, types, types)
+  }
+  if (is.null(pref.matrix)) {
+    pref.matrix <- matrix(1, types, types)
+  }
+
   constructor_spec(
     sample_asym_pref,
     nodes,
@@ -3227,13 +3278,16 @@ smallworld <- function(
 #' @param n Number of vertices.
 #' @param edges Number of edges per step.
 #' @inheritParams rlang::args_dots_empty
-#' @param agebins Number of aging bins.
+#' @param agebins Number of aging bins. The default `NULL` uses `n / 7100`.
 #' @param pref Vector (`sample_last_cit()` and `sample_cit_types()` or
 #'   matrix (`sample_cit_cit_types()`) giving the (unnormalized) citation
-#'   probabilities for the different vertex types.
+#'   probabilities for the different vertex types. The default `NULL` uses
+#'   `(1:(agebins + 1))^-3` for `sample_last_cit()` and all-one probabilities
+#'   for the other two.
 #' @param directed Logical, whether to generate directed networks.
 #' @param types Vector of length \sQuote{`n`}, the types of the vertices.
-#'   Types are numbered from zero.
+#'   Types are numbered from zero. The default `NULL` gives all vertices
+#'   type zero.
 #' @param attr Logical, whether to add the vertex types to the generated
 #'   graph as a vertex attribute called \sQuote{`type`}.
 #' @return A new graph.
@@ -3245,8 +3299,8 @@ sample_last_cit <- function(
   n,
   edges = 1,
   ...,
-  agebins = n / 7100,
-  pref = (1:(agebins + 1))^-3,
+  agebins = NULL,
+  pref = NULL,
   directed = TRUE
 ) {
   # BEGIN GENERATED ARG_HANDLE: sample_last_cit, do not edit, see tools/generate-migrations.R
@@ -3258,11 +3312,7 @@ sample_last_cit <- function(
       recover_old = c("agebins", "pref", "directed"),
       match_names = c("agebins", "pref", "directed"),
       match_to = c("agebins", "pref", "directed"),
-      defaults = list(
-        agebins = n / 7100,
-        pref = (1:(agebins + 1))^-3,
-        directed = TRUE
-      ),
+      defaults = list(agebins = NULL, pref = NULL, directed = TRUE),
       head_args = c("n", "edges"),
       fn_name = "sample_last_cit"
     )
@@ -3275,6 +3325,13 @@ sample_last_cit <- function(
   }
   # END GENERATED ARG_HANDLE
 
+  if (is.null(agebins)) {
+    agebins <- n / 7100
+  }
+  if (is.null(pref)) {
+    pref <- (1:(agebins + 1))^-3
+  }
+
   res <- lastcit_game_impl(
     nodes = n,
     edges_per_node = edges,
@@ -3297,8 +3354,8 @@ last_cit <- function(
   n,
   edges = 1,
   ...,
-  agebins = n / 7100,
-  pref = (1:(agebins + 1))^-3,
+  agebins = NULL,
+  pref = NULL,
   directed = TRUE
 ) {
   # BEGIN GENERATED ARG_HANDLE: last_cit, do not edit, see tools/generate-migrations.R
@@ -3310,11 +3367,7 @@ last_cit <- function(
       recover_old = c("agebins", "pref", "directed"),
       match_names = c("agebins", "pref", "directed"),
       match_to = c("agebins", "pref", "directed"),
-      defaults = list(
-        agebins = n / 7100,
-        pref = (1:(agebins + 1))^-3,
-        directed = TRUE
-      ),
+      defaults = list(agebins = NULL, pref = NULL, directed = TRUE),
       head_args = c("n", "edges"),
       fn_name = "last_cit"
     )
@@ -3327,6 +3380,13 @@ last_cit <- function(
   }
   # END GENERATED ARG_HANDLE
 
+  if (is.null(agebins)) {
+    agebins <- n / 7100
+  }
+  if (is.null(pref)) {
+    pref <- (1:(agebins + 1))^-3
+  }
+
   constructor_spec(
     sample_last_cit,
     n = n,
@@ -3343,9 +3403,9 @@ last_cit <- function(
 sample_cit_types <- function(
   n,
   edges = 1,
-  types = rep(0, n),
+  types = NULL,
   ...,
-  pref = rep(1, length(types)),
+  pref = NULL,
   directed = TRUE,
   attr = TRUE
 ) {
@@ -3358,11 +3418,7 @@ sample_cit_types <- function(
       recover_old = c("pref", "directed", "attr"),
       match_names = c("pref", "directed", "attr"),
       match_to = c("pref", "directed", "attr"),
-      defaults = list(
-        pref = rep(1, length(types)),
-        directed = TRUE,
-        attr = TRUE
-      ),
+      defaults = list(pref = NULL, directed = TRUE, attr = TRUE),
       head_args = c("n", "edges", "types"),
       fn_name = "sample_cit_types"
     )
@@ -3375,6 +3431,13 @@ sample_cit_types <- function(
   }
   # END GENERATED ARG_HANDLE
 
+  if (is.null(types)) {
+    types <- rep(0, n)
+  }
+  if (is.null(pref)) {
+    pref <- rep(1, length(types))
+  }
+
   res <- cited_type_game_impl(
     nodes = n,
     types = types,
@@ -3398,9 +3461,9 @@ sample_cit_types <- function(
 cit_types <- function(
   n,
   edges = 1,
-  types = rep(0, n),
+  types = NULL,
   ...,
-  pref = rep(1, length(types)),
+  pref = NULL,
   directed = TRUE,
   attr = TRUE
 ) {
@@ -3413,11 +3476,7 @@ cit_types <- function(
       recover_old = c("pref", "directed", "attr"),
       match_names = c("pref", "directed", "attr"),
       match_to = c("pref", "directed", "attr"),
-      defaults = list(
-        pref = rep(1, length(types)),
-        directed = TRUE,
-        attr = TRUE
-      ),
+      defaults = list(pref = NULL, directed = TRUE, attr = TRUE),
       head_args = c("n", "edges", "types"),
       fn_name = "cit_types"
     )
@@ -3430,6 +3489,13 @@ cit_types <- function(
   }
   # END GENERATED ARG_HANDLE
 
+  if (is.null(types)) {
+    types <- rep(0, n)
+  }
+  if (is.null(pref)) {
+    pref <- rep(1, length(types))
+  }
+
   constructor_spec(
     sample_cit_types,
     n = n,
@@ -3447,9 +3513,9 @@ cit_types <- function(
 sample_cit_cit_types <- function(
   n,
   edges = 1,
-  types = rep(0, n),
+  types = NULL,
   ...,
-  pref = matrix(1, nrow = length(types), ncol = length(types)),
+  pref = NULL,
   directed = TRUE,
   attr = TRUE
 ) {
@@ -3462,11 +3528,7 @@ sample_cit_cit_types <- function(
       recover_old = c("pref", "directed", "attr"),
       match_names = c("pref", "directed", "attr"),
       match_to = c("pref", "directed", "attr"),
-      defaults = list(
-        pref = matrix(1, nrow = length(types), ncol = length(types)),
-        directed = TRUE,
-        attr = TRUE
-      ),
+      defaults = list(pref = NULL, directed = TRUE, attr = TRUE),
       head_args = c("n", "edges", "types"),
       fn_name = "sample_cit_cit_types"
     )
@@ -3479,6 +3541,13 @@ sample_cit_cit_types <- function(
   }
   # END GENERATED ARG_HANDLE
 
+  if (is.null(types)) {
+    types <- rep(0, n)
+  }
+  if (is.null(pref)) {
+    pref <- matrix(1, nrow = length(types), ncol = length(types))
+  }
+
   pref[] <- as.numeric(pref)
   res <- citing_cited_type_game_impl(
     nodes = n,
@@ -3503,9 +3572,9 @@ sample_cit_cit_types <- function(
 cit_cit_types <- function(
   n,
   edges = 1,
-  types = rep(0, n),
+  types = NULL,
   ...,
-  pref = matrix(1, nrow = length(types), ncol = length(types)),
+  pref = NULL,
   directed = TRUE,
   attr = TRUE
 ) {
@@ -3518,11 +3587,7 @@ cit_cit_types <- function(
       recover_old = c("pref", "directed", "attr"),
       match_names = c("pref", "directed", "attr"),
       match_to = c("pref", "directed", "attr"),
-      defaults = list(
-        pref = matrix(1, nrow = length(types), ncol = length(types)),
-        directed = TRUE,
-        attr = TRUE
-      ),
+      defaults = list(pref = NULL, directed = TRUE, attr = TRUE),
       head_args = c("n", "edges", "types"),
       fn_name = "cit_cit_types"
     )
@@ -3535,6 +3600,13 @@ cit_cit_types <- function(
   }
   # END GENERATED ARG_HANDLE
 
+  if (is.null(types)) {
+    types <- rep(0, n)
+  }
+  if (is.null(pref)) {
+    pref <- matrix(1, nrow = length(types), ncol = length(types))
+  }
+
   constructor_spec(
     sample_cit_cit_types,
     n = n,
@@ -4750,8 +4822,8 @@ sample_forestfire <- function(
 #'   graph (the adjacency matrix being used as a vector).
 #' @inheritParams rlang::args_dots_empty
 #' @param p A numeric scalar, the probability of an edge between two
-#'   vertices, it must in the open (0,1) interval. The default is the empirical
-#'   edge density of the graph. If you are resampling an Erdős-Rényi graph and
+#'   vertices, it must in the open (0,1) interval. The default `NULL` uses the
+#'   empirical edge density of the graph. If you are resampling an Erdős-Rényi graph and
 #'   you know the original edge probability of the Erdős-Rényi model, you should
 #'   supply that explicitly.
 #' @param permutation A numeric vector, a permutation vector that is
@@ -4777,7 +4849,7 @@ sample_correlated_gnp <- function(
   old.graph,
   corr,
   ...,
-  p = edge_density(old.graph),
+  p = NULL,
   permutation = NULL
 ) {
   # BEGIN GENERATED ARG_HANDLE: sample_correlated_gnp, do not edit, see tools/generate-migrations.R
@@ -4789,7 +4861,7 @@ sample_correlated_gnp <- function(
       recover_old = c("p", "permutation"),
       match_names = c("p", "permutation"),
       match_to = c("p", "permutation"),
-      defaults = list(p = edge_density(old.graph), permutation = NULL),
+      defaults = list(p = NULL, permutation = NULL),
       head_args = c("old.graph", "corr"),
       fn_name = "sample_correlated_gnp"
     )
@@ -4802,6 +4874,10 @@ sample_correlated_gnp <- function(
   }
   # END GENERATED ARG_HANDLE
 
+  if (is.null(p)) {
+    p <- edge_density(old.graph)
+  }
+
   correlated_game_impl(
     old_graph = old.graph,
     corr = corr,
diff --git a/R/glet.R b/R/glet.R
index 81e97cda8a..718d93f80c 100644
--- a/R/glet.R
+++ b/R/glet.R
@@ -71,7 +71,8 @@ graphlets.candidate.basis <- function(graph, weights = NULL) {
 #' @param niter Integer scalar, the number of iterations to perform.
 #' @param cliques A list of vertex IDs, the graphlet basis to use for the
 #'   projection.
-#' @param Mu Starting weights for the projection.
+#' @param Mu Starting weights for the projection. The default `NULL` uses a
+#'   weight of one for each clique.
 #' @return `graphlets()` returns a list with two members:
 #'   \describe{
 #'     \item{cliques}{
@@ -178,7 +179,7 @@ graphlet_proj <- function(
   weights = NULL,
   cliques,
   niter = 1000,
-  Mu = rep(1, length(cliques))
+  Mu = NULL
 ) {
   # BEGIN GENERATED ARG_HANDLE: graphlet_proj, do not edit, see tools/generate-migrations.R
   if (...length() > 0L) {
@@ -189,11 +190,7 @@ graphlet_proj <- function(
       recover_old = c("weights", "cliques", "niter", "Mu"),
       match_names = c("weights", "cliques", "niter", "Mu"),
       match_to = c("weights", "cliques", "niter", "Mu"),
-      defaults = list(
-        weights = NULL,
-        niter = 1000,
-        Mu = rep(1, length(cliques))
-      ),
+      defaults = list(weights = NULL, niter = 1000, Mu = NULL),
       head_args = c("graph"),
       fn_name = "graphlet_proj"
     )
@@ -208,6 +205,10 @@ graphlet_proj <- function(
 
   # Argument checks
   ensure_igraph(graph)
+  if (is.null(Mu)) {
+    Mu <- rep(1, length(cliques))
+  }
+
   if (is.null(weights) && "weight" %in% edge_attr_names(graph)) {
     weights <- E(graph)$weight
   }
diff --git a/R/hrg.R b/R/hrg.R
index e03d7bfca1..d01be95016 100644
--- a/R/hrg.R
+++ b/R/hrg.R
@@ -266,11 +266,11 @@ fit_hrg <- function(
   ensure_igraph(graph)
   if (is.null(hrg)) {
     hrg <- list(
-      left = c(),
-      right = c(),
-      prob = c(),
-      edges = c(),
-      vertices = c()
+      left = numeric(),
+      right = numeric(),
+      prob = numeric(),
+      edges = numeric(),
+      vertices = numeric()
     )
   }
   hrg <- lapply(
@@ -830,6 +830,7 @@ rlang::on_load(s3_register("ape::as.phylo", "igraphHRG"))
 #' @param x An `igraphHRG`, a hierarchical random graph, as returned by
 #'   the [fit_hrg()] function.
 #' @param mode Which dendrogram plotting function to use. See details below.
+#'   The default `NULL` uses the `dend.plot.type` igraph option.
 #' @param \dots Additional arguments to supply to the dendrogram plotting
 #'   function.
 #' @return Returns whatever the return value was from the plotting function,
@@ -846,9 +847,12 @@ rlang::on_load(s3_register("ape::as.phylo", "igraphHRG"))
 #'
 plot_dendrogram.igraphHRG <- function(
   x,
-  mode = igraph_opt("dend.plot.type"),
+  mode = NULL,
   ...
 ) {
+  if (is.null(mode)) {
+    mode <- igraph_opt("dend.plot.type")
+  }
   if (mode == "auto") {
     have_ape <- requireNamespace("ape", quietly = TRUE)
     mode <- if (have_ape) "phylo" else "hclust"
diff --git a/R/layout.R b/R/layout.R
index 7c6d472e70..aa15547ea6 100644
--- a/R/layout.R
+++ b/R/layout.R
@@ -619,8 +619,12 @@ component_wise <- function(merge_method = "dla") {
 #' Scale coordinates of a layout.
 #'
 #' @param xmin,xmax Minimum and maximum for x coordinates.
-#' @param ymin,ymax Minimum and maximum for y coordinates.
-#' @param zmin,zmax Minimum and maximum for z coordinates.
+#' @param ymin,ymax Minimum and maximum for y coordinates. When omitted,
+#'   they follow `xmin` and `xmax`; `NULL` disables normalization along
+#'   this axis.
+#' @param zmin,zmax Minimum and maximum for z coordinates. When omitted,
+#'   they follow `xmin` and `xmax`; `NULL` disables normalization along
+#'   this axis.
 #'
 #' @family layout modifiers
 #' @family graph layouts
@@ -631,11 +635,27 @@ component_wise <- function(merge_method = "dla") {
 normalize <- function(
   xmin = -1,
   xmax = 1,
-  ymin = xmin,
-  ymax = xmax,
-  zmin = xmin,
-  zmax = xmax
+  ymin = NULL,
+  ymax = NULL,
+  zmin = NULL,
+  zmax = NULL
 ) {
+  # NULL is a legal value here (norm_coords() skips normalization along an
+  # axis with a NULL limit), so the fallback to the x limits applies only
+  # when an argument is not supplied at all.
+  if (missing(ymin)) {
+    ymin <- xmin
+  }
+  if (missing(ymax)) {
+    ymax <- xmax
+  }
+  if (missing(zmin)) {
+    zmin <- xmin
+  }
+  if (missing(zmax)) {
+    zmax <- xmax
+  }
+
   args <- grab_args()
 
   layout_modifier(
@@ -760,8 +780,8 @@ as_bipartite <- function(...) layout_spec(layout_as_bipartite, ...)
 #'
 #' @param graph The graph to layout.
 #' @inheritParams rlang::args_dots_empty
-#' @param center The ID of the vertex to put in the center. By default it is
-#'   the first vertex.
+#' @param center The ID of the vertex to put in the center. The default
+#'   `NULL` uses the first vertex.
 #' @param order Numeric vector, the order of the vertices along the perimeter.
 #'   The default ordering is given by the vertex IDs.
 #' @return A matrix with two columns and as many rows as the number of vertices
@@ -783,7 +803,7 @@ as_bipartite <- function(...) layout_spec(layout_as_bipartite, ...)
 layout_as_star <- function(
   graph,
   ...,
-  center = V(graph)[1],
+  center = NULL,
   order = NULL
 ) {
   # BEGIN GENERATED ARG_HANDLE: layout_as_star, do not edit, see tools/generate-migrations.R
@@ -795,7 +815,7 @@ layout_as_star <- function(
       recover_old = c("center", "order"),
       match_names = c("center", "order"),
       match_to = c("center", "order"),
-      defaults = list(center = V(graph)[1], order = NULL),
+      defaults = list(center = NULL, order = NULL),
       head_args = c("graph"),
       fn_name = "layout_as_star"
     )
@@ -815,6 +835,9 @@ layout_as_star <- function(
     # vertices
     return(layout_in_circle(graph))
   }
+  if (is.null(center)) {
+    center <- V(graph)[1]
+  }
   # Use the _impl function
   layout_star_impl(
     graph = graph,
@@ -998,7 +1021,8 @@ layout.reingold.tilford <- function(..., params = list()) {
 #' @param graph The input graph.
 #' @param order The vertices to place on the circle, in the order of their
 #'   desired placement. Vertices that are not included here will be placed at
-#'   (0,0).
+#'   (0,0). The default `NULL` selects all vertices, in the order of their
+#'   IDs.
 #' @return A numeric matrix with two columns, and one row for each vertex.
 #' @author Gabor Csardi \email{csardi.gabor@@gmail.com}
 #' @keywords graphs
@@ -1019,7 +1043,11 @@ layout.reingold.tilford <- function(..., params = list()) {
 #' V(karate)$label.color <- membership(karate_groups)
 #' V(karate)$shape <- "none"
 #' plot(karate, layout = coords)
-layout_in_circle <- function(graph, order = V(graph)) {
+layout_in_circle <- function(graph, order = NULL) {
+  if (is.null(order)) {
+    order <- V(graph)
+  }
+
   # Use the _impl function
   layout_circle_impl(
     graph = graph,
@@ -1421,7 +1449,8 @@ layout.random <- function(..., params = list()) {
 #'   is not `NULL` then it should be an appropriate matrix of starting
 #'   coordinates.
 #' @param maxiter Number of iterations to perform in the first phase.
-#' @param fineiter Number of iterations in the fine tuning phase.
+#' @param fineiter Number of iterations in the fine tuning phase. The
+#'   default `NULL` uses `max(10, log2(vcount(graph)))`.
 #' @param cool.fact Cooling factor.
 #' @param weight.node.dist Weight for the node-node distances component of the
 #'   energy function.
@@ -1429,11 +1458,13 @@ layout.random <- function(..., params = list()) {
 #'   the energy function. It can be set to zero, if vertices are allowed to sit
 #'   on the border.
 #' @param weight.edge.lengths Weight for the edge length component of the
-#'   energy function.
+#'   energy function. The default `NULL` uses `edge_density(graph) / 10`.
 #' @param weight.edge.crossings Weight for the edge crossing component of the
-#'   energy function.
+#'   energy function. The default `NULL` uses
+#'   `1 - sqrt(edge_density(graph))`.
 #' @param weight.node.edge.dist Weight for the node-edge distance component of
-#'   the energy function.
+#'   the energy function. The default `NULL` uses
+#'   `0.2 * (1 - edge_density(graph))`.
 #' @return A matrix with two columns, containing the x and y coordinates
 #'   of the vertices:
 #'   \describe{
@@ -1510,13 +1541,13 @@ layout_with_dh <- function(
   ...,
   coords = NULL,
   maxiter = 10,
-  fineiter = max(10, log2(vcount(graph))),
+  fineiter = NULL,
   cool.fact = 0.75,
   weight.node.dist = 1.0,
   weight.border = 0.0,
-  weight.edge.lengths = edge_density(graph) / 10,
-  weight.edge.crossings = 1.0 - sqrt(edge_density(graph)),
-  weight.node.edge.dist = 0.2 * (1 - edge_density(graph))
+  weight.edge.lengths = NULL,
+  weight.edge.crossings = NULL,
+  weight.node.edge.dist = NULL
 ) {
   # BEGIN GENERATED ARG_HANDLE: layout_with_dh, do not edit, see tools/generate-migrations.R
   if (...length() > 0L) {
@@ -1580,13 +1611,13 @@ layout_with_dh <- function(
       defaults = list(
         coords = NULL,
         maxiter = 10,
-        fineiter = max(10, log2(vcount(graph))),
+        fineiter = NULL,
         cool.fact = 0.75,
         weight.node.dist = 1,
         weight.border = 0,
-        weight.edge.lengths = edge_density(graph) / 10,
-        weight.edge.crossings = 1 - sqrt(edge_density(graph)),
-        weight.node.edge.dist = 0.2 * (1 - edge_density(graph))
+        weight.edge.lengths = NULL,
+        weight.edge.crossings = NULL,
+        weight.node.edge.dist = NULL
       ),
       head_args = c("graph"),
       fn_name = "layout_with_dh"
@@ -1600,6 +1631,19 @@ layout_with_dh <- function(
   }
   # END GENERATED ARG_HANDLE
 
+  if (is.null(fineiter)) {
+    fineiter <- max(10, log2(vcount(graph)))
+  }
+  if (is.null(weight.edge.lengths)) {
+    weight.edge.lengths <- edge_density(graph) / 10
+  }
+  if (is.null(weight.edge.crossings)) {
+    weight.edge.crossings <- 1.0 - sqrt(edge_density(graph))
+  }
+  if (is.null(weight.node.edge.dist)) {
+    weight.node.edge.dist <- 0.2 * (1 - edge_density(graph))
+  }
+
   if (is.null(coords)) {
     coords <- matrix(NA_real_, ncol = 2, nrow = 0)
     use.seed <- FALSE
@@ -1651,7 +1695,8 @@ with_dh <- function(...) layout_spec(layout_with_dh, ...)
 #' @param niter Integer scalar, the number of iterations to perform.
 #' @param start.temp Real scalar, the start temperature. This is the maximum
 #'   amount of movement alloved along one axis, within one step, for a vertex.
-#'   Currently it is decreased linearly to zero during the iteration.
+#'   Currently it is decreased linearly to zero during the iteration. The
+#'   default `NULL` uses `sqrt(vcount(graph))`.
 #' @param grid Character scalar, whether to use the faster, but less accurate
 #'   grid based implementation of the algorithm. By default (\dQuote{auto}), the
 #'   grid-based implementation is used if the graph has more than one thousand
@@ -1716,7 +1761,7 @@ layout_with_fr <- function(
   coords = NULL,
   dim = c(2, 3),
   niter = 500,
-  start.temp = sqrt(vcount(graph)),
+  start.temp = NULL,
   grid = c("auto", "grid", "nogrid"),
   weights = NULL,
   minx = NULL,
@@ -1839,7 +1884,7 @@ layout_with_fr <- function(
         coords = NULL,
         dim = c(2, 3),
         niter = 500,
-        start.temp = sqrt(vcount(graph)),
+        start.temp = NULL,
         grid = c("auto", "grid", "nogrid"),
         weights = NULL,
         minx = NULL,
@@ -1868,6 +1913,10 @@ layout_with_fr <- function(
 
   # Argument checks
   ensure_igraph(graph)
+  if (is.null(start.temp)) {
+    start.temp <- sqrt(vcount(graph))
+  }
+
   coords[] <- as.numeric(coords)
   dim <- igraph_match_arg(dim)
   if (!missing(niter) && !missing(maxiter)) {
@@ -1997,16 +2046,16 @@ layout.fruchterman.reingold <- function(..., params = list()) {
 #'   depending on the `dim` argument.
 #'   Default: `NULL`.
 #' @param maxiter The maximum number of iterations to perform. Updating a
-#'   single vertex counts as an iteration.  A reasonable default is 40 * n * n,
+#'   single vertex counts as an iteration. The default `NULL` uses 40 * n * n,
 #'   where n is the number of vertices. The original paper suggests 4 * n * n,
 #'   but this usually only works if the other parameters are set up carefully.
-#' @param temp.max The maximum allowed local temperature. A reasonable default
-#'   is the number of vertices.
+#' @param temp.max The maximum allowed local temperature. The default `NULL`
+#'   uses the number of vertices.
 #' @param temp.min The global temperature at which the algorithm terminates
 #'   (even before reaching `maxiter` iterations). A reasonable default is
 #'   1/10.
-#' @param temp.init Initial local temperature of all vertices. A reasonable
-#'   default is the square root of the number of vertices.
+#' @param temp.init Initial local temperature of all vertices. The default
+#'   `NULL` uses the square root of the number of vertices.
 #' @return A numeric matrix with two columns, and as many rows as the number of
 #'   vertices.
 #' @author Gabor Csardi \email{csardi.gabor@@gmail.com}
@@ -2028,10 +2077,10 @@ layout_with_gem <- function(
   graph,
   ...,
   coords = NULL,
-  maxiter = 40 * vcount(graph)^2,
-  temp.max = max(vcount(graph), 1),
-  temp.min = 1 / 10,
-  temp.init = sqrt(max(vcount(graph), 1))
+  maxiter = NULL,
+  temp.max = NULL,
+  temp.min = 0.1,
+  temp.init = NULL
 ) {
   # BEGIN GENERATED ARG_HANDLE: layout_with_gem, do not edit, see tools/generate-migrations.R
   if (...length() > 0L) {
@@ -2050,10 +2099,10 @@ layout_with_gem <- function(
       match_to = c("coords", "maxiter", "temp.max", "temp.min", "temp.init"),
       defaults = list(
         coords = NULL,
-        maxiter = 40 * vcount(graph)^2,
-        temp.max = max(vcount(graph), 1),
-        temp.min = 1 / 10,
-        temp.init = sqrt(max(vcount(graph), 1))
+        maxiter = NULL,
+        temp.max = NULL,
+        temp.min = 0.1,
+        temp.init = NULL
       ),
       head_args = c("graph"),
       fn_name = "layout_with_gem"
@@ -2067,6 +2116,16 @@ layout_with_gem <- function(
   }
   # END GENERATED ARG_HANDLE
 
+  if (is.null(maxiter)) {
+    maxiter <- 40 * vcount(graph)^2
+  }
+  if (is.null(temp.max)) {
+    temp.max <- max(vcount(graph), 1)
+  }
+  if (is.null(temp.init)) {
+    temp.init <- sqrt(max(vcount(graph), 1))
+  }
+
   if (is.null(coords)) {
     coords <- matrix(NA_real_, ncol = 2, nrow = 0)
     use.seed <- FALSE
@@ -2267,13 +2326,14 @@ with_graphopt <- function(...) layout_spec(layout_with_graphopt, ...)
 #'   dimensional layouts are places on a plane, three dimensional ones in the 3d
 #'   space.
 #' @param maxiter The maximum number of iterations to perform. The algorithm
-#'   might terminate earlier, see the `epsilon` argument.
+#'   might terminate earlier, see the `epsilon` argument. The default `NULL`
+#'   uses `50 * vcount(graph)`.
 #' @param epsilon Numeric scalar, the algorithm terminates, if the maximal
 #'   delta is less than this. (See the reference below for what delta means.) If
 #'   you set this to zero, then the function always performs `maxiter`
 #'   iterations.
 #' @param kkconst Numeric scalar, the Kamada-Kawai vertex attraction constant.
-#'   Typical (and default) value is the number of vertices.
+#'   The default `NULL` uses the number of vertices.
 #' @param weights Edge weights, larger values will result in longer edges.
 #'   Note that this is the opposite of [layout_with_fr()], which produces
 #'   shorter edges for larger weights. Weights must be positive.
@@ -2315,9 +2375,9 @@ layout_with_kk <- function(
   ...,
   coords = NULL,
   dim = c(2, 3),
-  maxiter = 50 * vcount(graph),
+  maxiter = NULL,
   epsilon = 0.0,
-  kkconst = max(vcount(graph), 1),
+  kkconst = NULL,
   weights = NULL,
   minx = NULL,
   maxx = NULL,
@@ -2433,9 +2493,9 @@ layout_with_kk <- function(
       defaults = list(
         coords = NULL,
         dim = c(2, 3),
-        maxiter = 50 * vcount(graph),
+        maxiter = NULL,
         epsilon = 0,
-        kkconst = max(vcount(graph), 1),
+        kkconst = NULL,
         weights = NULL,
         minx = NULL,
         maxx = NULL,
@@ -2461,6 +2521,13 @@ layout_with_kk <- function(
   }
   # END GENERATED ARG_HANDLE
 
+  if (is.null(maxiter)) {
+    maxiter <- 50 * vcount(graph)
+  }
+  if (is.null(kkconst)) {
+    kkconst <- max(vcount(graph), 1)
+  }
+
   # Argument checks
   if (!missing(coords) && !missing(start)) {
     cli::cli_abort(c(
@@ -2595,17 +2662,17 @@ layout.kamada.kawai <- function(..., params = list()) {
 #' @param graph The input graph
 #' @inheritParams rlang::args_dots_empty
 #' @param maxiter The maximum number of iterations to perform (150).
-#' @param maxdelta The maximum change for a vertex during an iteration (the
-#'   number of vertices).
-#' @param area The area of the surface on which the vertices are placed (square
-#'   of the number of vertices).
+#' @param maxdelta The maximum change for a vertex during an iteration. The
+#'   default `NULL` uses the number of vertices.
+#' @param area The area of the surface on which the vertices are placed. The
+#'   default `NULL` uses the square of the number of vertices.
 #' @param coolexp The cooling exponent of the simulated annealing (1.5).
-#' @param repulserad Cancellation radius for the repulsion (the `area`
-#'   times the number of vertices).
+#' @param repulserad Cancellation radius for the repulsion. The default
+#'   `NULL` uses the `area` times the number of vertices.
 #' @param cellsize The size of the cells for the grid. When calculating the
 #'   repulsion forces between vertices only vertices in the same or neighboring
-#'   grid cells are taken into account (the fourth root of the number of
-#'   `area`.
+#'   grid cells are taken into account. The default `NULL` uses the square
+#'   root of the square root of the `area`.
 #' @param root The ID of the vertex to place at the middle of the layout. The
 #'   default value is -1 which means that a random vertex is selected.
 #' @return A numeric matrix with two columns and as many rows as vertices.
@@ -2617,11 +2684,11 @@ layout_with_lgl <- function(
   graph,
   ...,
   maxiter = 150,
-  maxdelta = vcount(graph),
-  area = vcount(graph)^2,
+  maxdelta = NULL,
+  area = NULL,
   coolexp = 1.5,
-  repulserad = area * vcount(graph),
-  cellsize = sqrt(sqrt(area)),
+  repulserad = NULL,
+  cellsize = NULL,
   root = NULL
 ) {
   # BEGIN GENERATED ARG_HANDLE: layout_with_lgl, do not edit, see tools/generate-migrations.R
@@ -2675,11 +2742,11 @@ layout_with_lgl <- function(
       ),
       defaults = list(
         maxiter = 150,
-        maxdelta = vcount(graph),
-        area = vcount(graph)^2,
+        maxdelta = NULL,
+        area = NULL,
         coolexp = 1.5,
-        repulserad = area * vcount(graph),
-        cellsize = sqrt(sqrt(area)),
+        repulserad = NULL,
+        cellsize = NULL,
         root = NULL
       ),
       head_args = c("graph"),
@@ -2695,6 +2762,19 @@ layout_with_lgl <- function(
   # END GENERATED ARG_HANDLE
 
   ensure_igraph(graph)
+  if (is.null(maxdelta)) {
+    maxdelta <- vcount(graph)
+  }
+  if (is.null(area)) {
+    area <- vcount(graph)^2
+  }
+  if (is.null(repulserad)) {
+    repulserad <- area * vcount(graph)
+  }
+  if (is.null(cellsize)) {
+    cellsize <- sqrt(sqrt(area))
+  }
+
   if (is.null(root)) {
     root <- -1
   } else {
@@ -3252,7 +3332,8 @@ with_sugiyama <- function(...) layout_spec(layout_with_sugiyama, ...)
 #' @inheritParams rlang::args_dots_empty
 #' @param method Character constant giving the method to use. Right now only
 #'   `dla` is implemented.
-#' @param layout A function object, the layout function to use.
+#' @param layout A function object, the layout function to use. The default
+#'   `NULL` uses `layout_with_kk`.
 #' @param \dots Additional arguments to pass to the `layout` layout
 #'   function.
 #' @return A matrix with two columns and as many lines as the total number of
@@ -3429,9 +3510,13 @@ norm_coords <- function(
 #' @rdname merge_coords
 #' @param graph The input graph.
 #' @export
-layout_components <- function(graph, layout = layout_with_kk, ...) {
+layout_components <- function(graph, layout = NULL, ...) {
   ensure_igraph(graph)
 
+  if (is.null(layout)) {
+    layout <- layout_with_kk
+  }
+
   V(graph)$id <- seq(vcount(graph))
   gl <- decompose(graph)
   ll <- lapply(gl, layout, ...)
@@ -3626,9 +3711,10 @@ layout.drl <- function(
 #' @param use.seed Logical, whether to use the coordinates given in the
 #'   `seed` argument as a starting point.
 #' @param seed A matrix with two columns, the starting coordinates for the
-#'   vertices is `use.seed` is `TRUE`. It is ignored otherwise.
+#'   vertices is `use.seed` is `TRUE`. It is ignored otherwise. The default
+#'   `NULL` draws uniformly random starting coordinates.
 #' @param options Options for the layout generator, a named list. See details
-#'   below.
+#'   below. The default `NULL` uses `drl_defaults$default`.
 #' @param weights The weights of the edges. It must be a positive numeric vector,
 #'   `NULL` or `NA`. If it is `NULL` and the input graph has a
 #'   \sQuote{weight} edge attribute, then that attribute will be used. If
@@ -3662,8 +3748,8 @@ layout_with_drl <- function(
   graph,
   ...,
   use.seed = FALSE,
-  seed = matrix(runif(vcount(graph) * 2), ncol = 2),
-  options = drl_defaults$default,
+  seed = NULL,
+  options = NULL,
   weights = NULL,
   dim = c(2, 3)
 ) {
@@ -3684,8 +3770,8 @@ layout_with_drl <- function(
       match_to = c("use.seed", "seed", "options", "weights", "dim"),
       defaults = list(
         use.seed = FALSE,
-        seed = matrix(runif(vcount(graph) * 2), ncol = 2),
-        options = drl_defaults$default,
+        seed = NULL,
+        options = NULL,
         weights = NULL,
         dim = c(2, 3)
       ),
@@ -3703,6 +3789,13 @@ layout_with_drl <- function(
 
   ensure_igraph(graph)
 
+  if (is.null(seed)) {
+    seed <- matrix(runif(vcount(graph) * 2), ncol = 2)
+  }
+  if (is.null(options)) {
+    options <- drl_defaults$default
+  }
+
   dim <- igraph_match_arg(dim)
 
   use.seed <- as.logical(use.seed)
diff --git a/R/make.R b/R/make.R
index 944ad66b39..fece9900e9 100644
--- a/R/make.R
+++ b/R/make.R
@@ -1393,7 +1393,8 @@ with_graph_ <- function(...) {
 #'   ignored (with a warning) if `edges` are symbolic vertex names. It
 #'   is also ignored if there is a bigger vertex ID in `edges`. This
 #'   means that for this function it is safe to supply zero here if the
-#'   vertex with the largest ID is not an isolate.
+#'   vertex with the largest ID is not an isolate. The default `NULL` uses
+#'   the largest vertex ID in `edges`.
 #' @param isolates Character vector, names of isolate vertices,
 #'   for symbolic edge lists. It is ignored for numeric edge lists.
 #' @param directed Whether to create a directed graph.
@@ -1427,14 +1428,14 @@ with_graph_ <- function(...) {
 make_graph <- function(
   edges,
   ...,
-  n = max(edges),
+  n = NULL,
   isolates = NULL,
   directed = TRUE,
-  dir = directed,
+  dir = NULL,
   simplify = TRUE
 ) {
   if (inherits(edges, "formula")) {
-    if (!missing(n)) {
+    if (!is.null(n)) {
       cli::cli_abort("{.arg n} should not be given for graph literals")
     }
     if (!missing(isolates)) {
@@ -1452,16 +1453,16 @@ make_graph <- function(
       cli::cli_abort("{.arg simplify} should only be used for graph literals")
     }
 
-    if (!missing(dir) && !missing(directed)) {
+    if (!is.null(dir) && !missing(directed)) {
       cli::cli_abort("Only give one of {.arg dir} and {.arg directed}")
     }
 
-    if (!missing(dir) && missing(directed)) {
+    if (!is.null(dir) && missing(directed)) {
       directed <- dir
     }
 
     if (is.character(edges) && length(edges) == 1) {
-      if (!missing(n)) {
+      if (!is.null(n)) {
         cli::cli_warn("{.arg n} is ignored for the {.str {edges}} graph.")
       }
       if (!missing(isolates)) {
@@ -1474,7 +1475,7 @@ make_graph <- function(
           "{.arg directed} is ignored for the {.str {edges}} graph."
         )
       }
-      if (!missing(dir)) {
+      if (!is.null(dir)) {
         cli::cli_warn("{.arg dir} is ignored for the {.str {edges}} graph.")
       }
       if (length(list(...))) {
@@ -1508,7 +1509,7 @@ make_graph <- function(
       }
 
       args <- list(edges, ...)
-      if (!missing(n)) {
+      if (!is.null(n)) {
         args <- c(args, list(n = n))
       }
       if (!missing(directed)) {
@@ -1517,7 +1518,7 @@ make_graph <- function(
 
       do.call(old_graph, args)
     } else if (is.character(edges)) {
-      if (!missing(n)) {
+      if (!is.null(n)) {
         cli::cli_warn("{.arg n} is ignored for edge list with vertex names.")
       }
       if (length(list(...))) {
@@ -1552,8 +1553,8 @@ make_famous_graph <- function(name) {
 
 #' @rdname make_graph
 #' @export
-make_directed_graph <- function(edges, n = max(edges)) {
-  if (missing(n)) {
+make_directed_graph <- function(edges, n = NULL) {
+  if (is.null(n)) {
     make_graph(edges, directed = TRUE)
   } else {
     make_graph(edges, n = n, directed = TRUE)
@@ -1562,8 +1563,8 @@ make_directed_graph <- function(edges, n = max(edges)) {
 
 #' @rdname make_graph
 #' @export
-make_undirected_graph <- function(edges, n = max(edges)) {
-  if (missing(n)) {
+make_undirected_graph <- function(edges, n = NULL) {
+  if (is.null(n)) {
     make_graph(edges, directed = FALSE)
   } else {
     make_graph(edges, n = n, directed = FALSE)
diff --git a/R/motifs.R b/R/motifs.R
index 509fae0079..4255817e69 100644
--- a/R/motifs.R
+++ b/R/motifs.R
@@ -331,7 +331,7 @@ sample_motifs <- function(
   graph,
   size = 3,
   ...,
-  cut.prob = rep(0, size),
+  cut.prob = NULL,
   sample.size = NULL,
   sample = NULL
 ) {
@@ -353,11 +353,7 @@ sample_motifs <- function(
       recover_old = c("cut.prob", "sample.size", "sample"),
       match_names = c("cut.prob", "sample.size", "sample"),
       match_to = c("cut.prob", "sample.size", "sample"),
-      defaults = list(
-        cut.prob = rep(0, size),
-        sample.size = NULL,
-        sample = NULL
-      ),
+      defaults = list(cut.prob = NULL, sample.size = NULL, sample = NULL),
       head_args = c("graph", "size"),
       fn_name = "sample_motifs"
     )
diff --git a/R/operators.R b/R/operators.R
index 4560514a94..439d9306b6 100644
--- a/R/operators.R
+++ b/R/operators.R
@@ -290,10 +290,10 @@ apply_one_combiner <- function(comb, x) {
 #' @param \dots Graph objects or lists of graph objects.
 #' @param x,y Graph objects.
 #' @param graph.attr.comb Specification for combining shared graph attributes.
-#'   Defaults to the `graph.attr.comb` igraph option (`"rename"` unless changed
-#'   via [igraph_options()]), which preserves the historical behaviour of
-#'   appending `_1`, `_2`, ... suffixes to clashing attribute names. See
-#'   [igraph-attribute-combination] for the available combiners.
+#'   The default `NULL` uses the `graph.attr.comb` igraph option (`"rename"`
+#'   unless changed via [igraph_options()]), which preserves the historical
+#'   behaviour of appending `_1`, `_2`, ... suffixes to clashing attribute
+#'   names. See [igraph-attribute-combination] for the available combiners.
 #' @return A new graph object.
 #' @author Gabor Csardi \email{csardi.gabor@@gmail.com}
 #' @export
@@ -309,8 +309,12 @@ apply_one_combiner <- function(comb, x) {
 #' @export
 disjoint_union <- function(
   ...,
-  graph.attr.comb = igraph_opt("graph.attr.comb")
+  graph.attr.comb = NULL
 ) {
+  if (is.null(graph.attr.comb)) {
+    graph.attr.comb <- igraph_opt("graph.attr.comb")
+  }
+
   graphs <- unlist(
     recursive = FALSE,
     lapply(list(...), function(l) {
@@ -1005,7 +1009,7 @@ compose <- function(
   g2,
   ...,
   byname = "auto",
-  graph.attr.comb = igraph_opt("graph.attr.comb"),
+  graph.attr.comb = NULL,
   vertex.attr.comb = "rename",
   edge.attr.comb = "rename"
 ) {
@@ -1045,7 +1049,7 @@ compose <- function(
       ),
       defaults = list(
         byname = "auto",
-        graph.attr.comb = igraph_opt("graph.attr.comb"),
+        graph.attr.comb = NULL,
         vertex.attr.comb = "rename",
         edge.attr.comb = "rename"
       ),
@@ -1064,6 +1068,10 @@ compose <- function(
   ensure_igraph(g1)
   ensure_igraph(g2)
 
+  if (is.null(graph.attr.comb)) {
+    graph.attr.comb <- igraph_opt("graph.attr.comb")
+  }
+
   if (byname != "auto" && !is.logical(byname)) {
     cli::cli_abort("{.arg bynam} must be \"auto\", or \"logical\".")
   }
@@ -1604,7 +1612,8 @@ rep.igraph <- function(x, n, mark = TRUE, ...) {
 #' all edges, this operation is also known as graph transpose.
 #'
 #' @param graph The input graph.
-#' @param eids The edge IDs of the edges to reverse.
+#' @param eids The edge IDs of the edges to reverse. The default `NULL`
+#'   reverses all edges.
 #' @return The result graph where the direction of the edges with the given
 #'   IDs are reversed
 #'
@@ -1614,7 +1623,11 @@ rep.igraph <- function(x, n, mark = TRUE, ...) {
 #' reverse_edges(g, 2)
 #' @family functions for manipulating graph structure
 #' @export
-reverse_edges <- function(graph, eids = E(graph)) {
+reverse_edges <- function(graph, eids = NULL) {
+  if (is.null(eids)) {
+    eids <- E(graph)
+  }
+
   reverse_edges_impl(
     graph = graph,
     eids = eids
diff --git a/R/paths.R b/R/paths.R
index 7931093f6e..24adaac043 100644
--- a/R/paths.R
+++ b/R/paths.R
@@ -85,7 +85,8 @@ is.dag <- function(graph) {
 #'
 #' @param graph The input graph.
 #' @param from The source vertex.
-#' @param to The target vertex of vertices. Defaults to all vertices.
+#' @param to The target vertex of vertices. The default `NULL` selects all
+#'   vertices.
 #' @inheritParams rlang::args_dots_empty
 #' @param mode Character constant, gives whether the shortest paths to or
 #'   from the given vertices should be calculated for directed graphs. If
@@ -110,7 +111,7 @@ is.dag <- function(graph) {
 all_simple_paths <- function(
   graph,
   from,
-  to = V(graph),
+  to = NULL,
   ...,
   mode = c("out", "in", "all", "total"),
   cutoff = -1
@@ -139,6 +140,9 @@ all_simple_paths <- function(
 
   ## Argument checks
   ensure_igraph(graph)
+  if (is.null(to)) {
+    to <- V(graph)
+  }
 
   ## Function call
   res <- with_igraph_opt(
diff --git a/R/plot.R b/R/plot.R
index 48d02cc0f0..eefa5a0151 100644
--- a/R/plot.R
+++ b/R/plot.R
@@ -49,10 +49,11 @@
 #'   used for the different vertex groups.
 #' @param mark.col A scalar or vector giving the colors of marking the
 #'   polygons, in any format accepted by [graphics::xspline()]; e.g.
-#'   numeric color IDs, symbolic color names, or colors in RGB.
+#'   numeric color IDs, symbolic color names, or colors in RGB. The default
+#'   `NULL` uses semi-transparent rainbow colors.
 #' @param mark.border A scalar or vector giving the colors of the borders of
 #'   the vertex group marking polygons. If it is `NA`, then no border is
-#'   drawn.
+#'   drawn. The default `NULL` uses rainbow colors.
 #' @param mark.expand A numeric scalar or vector, the size of the border around
 #'   the marked vertex groups. It is in the same units as the vertex sizes. If a
 #'   vector is given, then different values are used for the different vertex
@@ -92,9 +93,9 @@ plot.igraph <- function(
   xlim = NULL,
   ylim = NULL,
   mark.groups = list(),
-  mark.shape = 1 / 2,
-  mark.col = rainbow(length(mark.groups), alpha = 0.3),
-  mark.border = rainbow(length(mark.groups), alpha = 1),
+  mark.shape = 0.5,
+  mark.col = NULL,
+  mark.border = NULL,
   mark.expand = 15,
   mark.lwd = 1,
   loop.size = 1,
@@ -272,6 +273,12 @@ plot.igraph <- function(
   if (inherits(mark.groups, "communities")) {
     mark.groups <- communities(mark.groups)
   }
+  if (is.null(mark.col)) {
+    mark.col <- rainbow(length(mark.groups), alpha = 0.3)
+  }
+  if (is.null(mark.border)) {
+    mark.border <- rainbow(length(mark.groups), alpha = 1)
+  }
 
   mark.shape <- rep(mark.shape, length.out = length(mark.groups))
   mark.border <- rep(mark.border, length.out = length(mark.groups))
diff --git a/R/plot.shapes.R b/R/plot.shapes.R
index e9fd0a4ad1..927455849d 100644
--- a/R/plot.shapes.R
+++ b/R/plot.shapes.R
@@ -232,8 +232,10 @@ add.vertex.shape <- function(
 #' @param shape Character scalar, name of a vertex shape. If it is
 #'    `NULL` for `shapes()`, then the names of all defined
 #'    vertex shapes are returned.
-#' @param clip An R function object, the clipping function.
-#' @param plot An R function object, the plotting function.
+#' @param clip An R function object, the clipping function. The default
+#'   `NULL` uses `shape_noclip`.
+#' @param plot An R function object, the plotting function. The default
+#'   `NULL` uses `shape_noplot`.
 #' @param parameters Named list, additional plot/vertex/edge
 #'    parameters. The element named define the new parameters, and the
 #'    elements themselves define their default values.
@@ -374,8 +376,8 @@ shape_noplot <- function(coords, v = NULL, params) {
 add_shape <- function(
   shape,
   ...,
-  clip = shape_noclip,
-  plot = shape_noplot,
+  clip = NULL,
+  plot = NULL,
   parameters = list()
 ) {
   # BEGIN GENERATED ARG_HANDLE: add_shape, do not edit, see tools/generate-migrations.R
@@ -387,11 +389,7 @@ add_shape <- function(
       recover_old = c("clip", "plot", "parameters"),
       match_names = c("clip", "plot", "parameters"),
       match_to = c("clip", "plot", "parameters"),
-      defaults = list(
-        clip = shape_noclip,
-        plot = shape_noplot,
-        parameters = list()
-      ),
+      defaults = list(clip = NULL, plot = NULL, parameters = list()),
       head_args = c("shape"),
       fn_name = "add_shape"
     )
@@ -404,6 +402,13 @@ add_shape <- function(
   }
   # END GENERATED ARG_HANDLE
 
+  if (is.null(clip)) {
+    clip <- shape_noclip
+  }
+  if (is.null(plot)) {
+    plot <- shape_noplot
+  }
+
   if (!is.character(shape) || length(shape) != 1) {
     cli::cli_abort(c(
       "{.arg shape} must be a character of length 1.",
diff --git a/R/print.R b/R/print.R
index 20a9ce46e3..1c2afaf994 100644
--- a/R/print.R
+++ b/R/print.R
@@ -566,16 +566,21 @@ print_all <- function(object, ...) {
 #' @aliases print.igraph print_all summary.igraph str.igraph
 #' @param x The graph to print.
 #' @param full Logical, whether to print the graph structure itself as
-#'   well.
-#' @param graph.attributes Logical, whether to print graph attributes.
+#'   well. The default `NULL` uses the `print.full` igraph option.
+#' @param graph.attributes Logical, whether to print graph attributes. The
+#'   default `NULL` uses the `print.graph.attributes` igraph option.
 #' @param vertex.attributes Logical, whether to print vertex
-#'   attributes.
-#' @param edge.attributes Logical, whether to print edge attributes.
+#'   attributes. The default `NULL` uses the `print.vertex.attributes` igraph
+#'   option.
+#' @param edge.attributes Logical, whether to print edge attributes. The
+#'   default `NULL` uses the `print.edge.attributes` igraph option.
 #' @param names Logical, whether to print symbolic vertex names (i.e.
 #'   the `name` vertex attribute) or vertex IDs.
 #' @param max.lines The maximum number of lines to use. The rest of the
-#'   output will be truncated.
-#' @param id Whether to print the graph ID.
+#'   output will be truncated. If not given, the `auto.print.lines` igraph
+#'   option applies; `NULL` prints all lines.
+#' @param id Whether to print the graph ID. The default `NULL` uses the
+#'   `print.id` igraph option.
 #' @param object The graph of which the summary will be printed.
 #' @param \dots Additional agruments.
 #' @return All these functions return the graph invisibly.
@@ -593,17 +598,38 @@ print_all <- function(object, ...) {
 #'
 print.igraph <- function(
   x,
-  full = igraph_opt("print.full"),
-  graph.attributes = igraph_opt("print.graph.attributes"),
-  vertex.attributes = igraph_opt("print.vertex.attributes"),
-  edge.attributes = igraph_opt("print.edge.attributes"),
+  full = NULL,
+  graph.attributes = NULL,
+  vertex.attributes = NULL,
+  edge.attributes = NULL,
   names = TRUE,
-  max.lines = igraph_opt("auto.print.lines"),
-  id = igraph_opt("print.id"),
+  max.lines = NULL,
+  id = NULL,
   ...
 ) {
   ensure_igraph(x)
 
+  if (is.null(full)) {
+    full <- igraph_opt("print.full")
+  }
+  if (is.null(graph.attributes)) {
+    graph.attributes <- igraph_opt("print.graph.attributes")
+  }
+  if (is.null(vertex.attributes)) {
+    vertex.attributes <- igraph_opt("print.vertex.attributes")
+  }
+  if (is.null(edge.attributes)) {
+    edge.attributes <- igraph_opt("print.edge.attributes")
+  }
+  if (missing(max.lines)) {
+    # NULL is a legal value here (print all lines), so the option fallback
+    # applies only when the argument is not supplied at all.
+    max.lines <- igraph_opt("auto.print.lines")
+  }
+  if (is.null(id)) {
+    id <- igraph_opt("print.id")
+  }
+
   if (!is_cli_style()) {
     return(print_igraph_legacy(
       x,
diff --git a/R/printr.R b/R/printr.R
index dcc32ac622..7f5294859a 100644
--- a/R/printr.R
+++ b/R/printr.R
@@ -189,12 +189,17 @@ head_print_callback <- function(
 #'
 #' @param ... Passed to the printing function.
 #' @param .indent Character scalar, indent the printout with this.
-#' @param .printer The printing function, defaults to [print].
+#' @param .printer The printing function. The default `NULL` uses
+#'   [print].
 #' @return The first element in `...`, invisibly.
 #'
 #' @export
 
-indent_print <- function(..., .indent = " ", .printer = print) {
+indent_print <- function(..., .indent = " ", .printer = NULL) {
+  if (is.null(.printer)) {
+    .printer <- print
+  }
+
   if (length(.indent) != 1 || !is.character(.indent)) {
     indent <- .indent # cli literal cannot start with a dot
     cli::cli_abort(
diff --git a/R/similarity.R b/R/similarity.R
index 481ba60ff3..82a01219e3 100644
--- a/R/similarity.R
+++ b/R/similarity.R
@@ -26,7 +26,8 @@
 #' 25(3):211-230, 2003.
 #'
 #' @param graph The input graph.
-#' @param vids The vertex IDs for which the similarity is calculated.
+#' @param vids The vertex IDs for which the similarity is calculated. The
+#'   default `NULL` selects all vertices.
 #' @inheritParams rlang::args_dots_empty
 #' @param mode The type of neighboring vertices to use for the calculation,
 #'   possible values: \sQuote{`out`}, \sQuote{`in`},
@@ -52,7 +53,7 @@
 #' similarity(g, method = "jaccard")
 similarity <- function(
   graph,
-  vids = V(graph),
+  vids = NULL,
   ...,
   mode = c(
     "all",
@@ -93,6 +94,10 @@ similarity <- function(
   }
   # END GENERATED ARG_HANDLE
 
+  if (is.null(vids)) {
+    vids <- V(graph)
+  }
+
   method <- igraph_match_arg(method)
   if (method == "jaccard") {
     similarity_jaccard_impl(
diff --git a/R/simple.R b/R/simple.R
index dcc9120542..26ecd87a8b 100644
--- a/R/simple.R
+++ b/R/simple.R
@@ -71,7 +71,8 @@ is.simple <- function(graph) {
 #' @param edge.attr.comb Specifies what to do with edge attributes, if
 #'   `remove.multiple=TRUE`. In this case many edges might be mapped to a
 #'   single one in the new graph, and their attributes are combined. Please see
-#'   [attribute.combination()] for details on this.
+#'   [attribute.combination()] for details on this. The default `NULL` uses
+#'   the `edge.attr.comb` igraph option.
 #' @return a graph object with the loop and/or multiple edges removed; the
 #'   input graph is returned unchanged if it is already simple.
 #' @author Gabor Csardi \email{csardi.gabor@@gmail.com}
@@ -94,7 +95,7 @@ simplify <- function(
   graph,
   remove.multiple = TRUE,
   remove.loops = TRUE,
-  edge.attr.comb = igraph_opt("edge.attr.comb")
+  edge.attr.comb = NULL
 ) {
   # A graph that is already simple has no loops and no multiple edges, so
   # simplify_impl() would not change its structure regardless of the
@@ -104,6 +105,9 @@ simplify <- function(
   if (is_simple(graph)) {
     return(graph)
   }
+  if (is.null(edge.attr.comb)) {
+    edge.attr.comb <- igraph_opt("edge.attr.comb")
+  }
   simplify_impl(
     graph = graph,
     remove_multiple = remove.multiple,
diff --git a/R/stochastic_matrix.R b/R/stochastic_matrix.R
index 3d22332ebe..e91b98fc80 100644
--- a/R/stochastic_matrix.R
+++ b/R/stochastic_matrix.R
@@ -55,7 +55,8 @@ get.stochastic <- function(
 #' @param column.wise If `FALSE`, then the rows of the stochastic matrix
 #'   sum up to one; otherwise it is the columns.
 #' @param sparse Logical, whether to return a sparse matrix. The
-#'   `Matrix` package is needed for sparse matrices.
+#'   `Matrix` package is needed for sparse matrices. The default `NULL` uses
+#'   the `sparsematrices` igraph option.
 #' @return A regular matrix or a matrix of class `Matrix` if a
 #'   `sparse` argument was `TRUE`.
 #' @author Gabor Csardi \email{csardi.gabor@@gmail.com}
@@ -79,7 +80,7 @@ stochastic_matrix <- function(
   graph,
   ...,
   column.wise = FALSE,
-  sparse = igraph_opt("sparsematrices")
+  sparse = NULL
 ) {
   # BEGIN GENERATED ARG_HANDLE: stochastic_matrix, do not edit, see tools/generate-migrations.R
   if (...length() > 0L) {
@@ -90,10 +91,7 @@ stochastic_matrix <- function(
       recover_old = c("column.wise", "sparse"),
       match_names = c("column.wise", "sparse"),
       match_to = c("column.wise", "sparse"),
-      defaults = list(
-        column.wise = FALSE,
-        sparse = igraph_opt("sparsematrices")
-      ),
+      defaults = list(column.wise = FALSE, sparse = NULL),
       head_args = c("graph"),
       fn_name = "stochastic_matrix"
     )
@@ -108,6 +106,10 @@ stochastic_matrix <- function(
 
   ensure_igraph(graph)
 
+  if (is.null(sparse)) {
+    sparse <- igraph_opt("sparsematrices")
+  }
+
   column.wise <- as.logical(column.wise)
   if (length(column.wise) != 1) {
     cli::cli_abort(
diff --git a/R/structural-properties.R b/R/structural-properties.R
index 578eed4bbc..6c7053036e 100644
--- a/R/structural-properties.R
+++ b/R/structural-properties.R
@@ -993,6 +993,7 @@ mean_distance <- function(
 #'
 #' @param graph The graph to analyze.
 #' @param v The IDs of vertices of which the degree will be calculated.
+#'   The default `NULL` selects all vertices.
 #' @param mode Character string, \dQuote{out} for out-degree, \dQuote{in} for
 #'   in-degree or \dQuote{total} for the sum of the two. For undirected graphs
 #'   this argument is ignored. \dQuote{all} is a synonym of \dQuote{total}.
@@ -1030,7 +1031,7 @@ mean_distance <- function(
 #'
 degree <- function(
   graph,
-  v = V(graph),
+  v = NULL,
   ...,
   mode = c("all", "out", "in", "total"),
   loops = TRUE,
@@ -1063,6 +1064,9 @@ degree <- function(
   # END GENERATED ARG_HANDLE
 
   ensure_igraph(graph)
+  if (is.null(v)) {
+    v <- V(graph)
+  }
   v <- as_igraph_vs(graph, v)
   mode <- igraph_match_arg(mode)
 
@@ -1087,10 +1091,14 @@ degree <- function(
 max_degree <- function(
   graph,
   ...,
-  v = V(graph),
+  v = NULL,
   mode = c("all", "out", "in", "total"),
   loops = TRUE
 ) {
+  if (is.null(v)) {
+    v <- V(graph)
+  }
+
   maxdegree_impl(
     graph = graph,
     v = v,
@@ -1215,9 +1223,9 @@ degree_distribution <- function(graph, cumulative = FALSE, ...) {
 #'
 #' @param graph The graph to work on.
 #' @param v Numeric vector, the vertices from which the shortest paths will be
-#'   calculated.
+#'   calculated. The default `NULL` selects all vertices.
 #' @param to Numeric vector, the vertices to which the shortest paths will be
-#'   calculated. By default it includes all vertices. Note that for
+#'   calculated. The default `NULL` includes all vertices. Note that for
 #'   `distances()` every vertex must be included here at most once. (This
 #'   is not required for `shortest_paths()`.
 #' @inheritParams rlang::args_dots_empty
@@ -1360,8 +1368,8 @@ degree_distribution <- function(graph, cumulative = FALSE, ...) {
 #'
 distances <- function(
   graph,
-  v = V(graph),
-  to = V(graph),
+  v = NULL,
+  to = NULL,
   ...,
   mode = c("all", "out", "in"),
   weights = NULL,
@@ -1401,6 +1409,12 @@ distances <- function(
   # END GENERATED ARG_HANDLE
 
   ensure_igraph(graph)
+  if (is.null(v)) {
+    v <- V(graph)
+  }
+  if (is.null(to)) {
+    to <- V(graph)
+  }
 
   # make sure that the lower-level function in C gets mode == "out"
   # unconditionally when the graph is undirected; this is used for
@@ -1486,7 +1500,7 @@ distances <- function(
 shortest_paths <- function(
   graph,
   from,
-  to = V(graph),
+  to = NULL,
   ...,
   mode = c("out", "all", "in"),
   weights = NULL,
@@ -1560,6 +1574,9 @@ shortest_paths <- function(
   # END GENERATED ARG_HANDLE
 
   ensure_igraph(graph)
+  if (is.null(to)) {
+    to <- V(graph)
+  }
   mode <- igraph_match_arg(mode)
   mode <- switch(mode, "out" = 1, "in" = 2, "all" = 3)
   output <- igraph_match_arg(output)
@@ -1661,7 +1678,7 @@ shortest_paths <- function(
 all_shortest_paths <- function(
   graph,
   from,
-  to = V(graph),
+  to = NULL,
   ...,
   mode = c("out", "all", "in"),
   weights = NULL
@@ -1689,6 +1706,9 @@ all_shortest_paths <- function(
   # END GENERATED ARG_HANDLE
 
   ensure_igraph(graph)
+  if (is.null(to)) {
+    to <- V(graph)
+  }
 
   mode <- igraph_match_arg(mode)
 
@@ -2307,7 +2327,7 @@ transitivity <- function(
 #'
 #' @param graph A graph object, the input graph.
 #' @param nodes The vertices for which the constraint will be calculated.
-#'   Defaults to all vertices.
+#'   The default `NULL` selects all vertices.
 #' @inheritParams rlang::args_dots_empty
 #' @param weights The weights of the edges. If this is `NULL` and there is
 #'   a `weight` edge attribute this is used. If there is no such edge
@@ -2328,7 +2348,7 @@ transitivity <- function(
 #'
 constraint <- function(
   graph,
-  nodes = V(graph),
+  nodes = NULL,
   ...,
   weights = NULL
 ) {
@@ -2355,6 +2375,9 @@ constraint <- function(
   # END GENERATED ARG_HANDLE
 
   ensure_igraph(graph)
+  if (is.null(nodes)) {
+    nodes <- V(graph)
+  }
   nodes <- as_igraph_vs(graph, nodes)
 
   if (is.null(weights)) {
@@ -2522,7 +2545,7 @@ edge_density <- function(
 ego_size <- function(
   graph,
   order = 1,
-  nodes = V(graph),
+  nodes = NULL,
   ...,
   mode = c("all", "out", "in"),
   mindist = 0
@@ -2550,6 +2573,9 @@ ego_size <- function(
   # END GENERATED ARG_HANDLE
 
   ensure_igraph(graph)
+  if (is.null(nodes)) {
+    nodes <- V(graph)
+  }
   mode <- igraph_match_arg(mode)
   mode <- switch(mode, "out" = 1, "in" = 2, "all" = 3)
   mindist <- as.numeric(mindist)
@@ -2602,6 +2628,7 @@ neighborhood_size <- ego_size
 #' @param order Integer giving the order of the neighborhood. Negative values
 #'   indicate an infinite order.
 #' @param nodes The vertices for which the calculation is performed.
+#'   The default `NULL` selects all vertices.
 #' @inheritParams rlang::args_dots_empty
 #' @param mode Character constant, it specifies how to use the direction of
 #'   the edges if a directed graph is analyzed. For \sQuote{out} only the
@@ -2660,7 +2687,7 @@ neighborhood_size <- ego_size
 ego <- function(
   graph,
   order = 1,
-  nodes = V(graph),
+  nodes = NULL,
   ...,
   mode = c("all", "out", "in"),
   mindist = 0
@@ -2688,6 +2715,9 @@ ego <- function(
   # END GENERATED ARG_HANDLE
 
   ensure_igraph(graph)
+  if (is.null(nodes)) {
+    nodes <- V(graph)
+  }
   mode <- igraph_match_arg(mode)
   mode <- switch(mode, "out" = 1, "in" = 2, "all" = 3)
   mindist <- as.numeric(mindist)
@@ -2719,7 +2749,7 @@ neighborhood <- ego
 make_ego_graph <- function(
   graph,
   order = 1,
-  nodes = V(graph),
+  nodes = NULL,
   ...,
   mode = c("all", "out", "in"),
   mindist = 0
@@ -2747,6 +2777,9 @@ make_ego_graph <- function(
   # END GENERATED ARG_HANDLE
 
   ensure_igraph(graph)
+  if (is.null(nodes)) {
+    nodes <- V(graph)
+  }
   mode <- igraph_match_arg(mode)
   mode <- switch(mode, "out" = 1L, "in" = 2L, "all" = 3L)
   mindist <- as.numeric(mindist)
@@ -3163,8 +3196,8 @@ girth <- function(
 #' original multiplicity as an edge attribute.
 #'
 #' @param graph The input graph.
-#' @param eids The edges to which the query is restricted. By default this is
-#'   all edges in the graph.
+#' @param eids The edges to which the query is restricted. The default
+#'   `NULL` selects all edges.
 #' @return `any_loop()` and `any_multiple()` return a Logical.
 #'   `which_loop()` and `which_multiple()` return a logical vector.
 #'   `count_loops()` returns a numeric scalar with the total number of loop edges.
@@ -3201,7 +3234,11 @@ girth <- function(
 #' any(which_multiple(g))
 #' E(g)$weight
 #'
-which_multiple <- function(graph, eids = E(graph)) {
+which_multiple <- function(graph, eids = NULL) {
+  if (is.null(eids)) {
+    eids <- E(graph)
+  }
+
   is_multiple_impl(
     graph = graph,
     eids = eids
@@ -3216,7 +3253,11 @@ any_multiple <- function(graph) {
 }
 #' @rdname which_multiple
 #' @export
-count_multiple <- function(graph, eids = E(graph)) {
+count_multiple <- function(graph, eids = NULL) {
+  if (is.null(eids)) {
+    eids <- E(graph)
+  }
+
   count_multiple_impl(
     graph = graph,
     eids = eids
@@ -3224,7 +3265,11 @@ count_multiple <- function(graph, eids = E(graph)) {
 }
 #' @rdname which_multiple
 #' @export
-which_loop <- function(graph, eids = E(graph)) {
+which_loop <- function(graph, eids = NULL) {
+  if (is.null(eids)) {
+    eids <- E(graph)
+  }
+
   is_loop_impl(
     graph = graph,
     eids = eids
@@ -3307,6 +3352,7 @@ count_loops <- function(graph) {
 #'   Default: `NULL`.
 #' @param extra Additional argument to supply to the callback function.
 #' @param rho The environment in which the callback function is evaluated.
+#'   The default `NULL` uses the caller's environment.
 #' @param neimode `r lifecycle::badge("deprecated")` This argument is deprecated
 #'  from igraph 1.3.0; use `mode` instead.
 #' @inheritParams rlang::args_dots_empty
@@ -3401,7 +3447,7 @@ bfs <- function(
   dist = FALSE,
   callback = NULL,
   extra = NULL,
-  rho = parent.frame(),
+  rho = NULL,
   neimode = deprecated(),
   father = deprecated()
 ) {
@@ -3409,6 +3455,10 @@ bfs <- function(
 
   ensure_igraph(graph)
 
+  if (is.null(rho)) {
+    rho <- parent.frame()
+  }
+
   if (lifecycle::is_present(neimode)) {
     lifecycle::deprecate_stop(
       "1.3.0",
@@ -3586,6 +3636,7 @@ bfs <- function(
 #'   Default: `NULL`.
 #' @param extra Additional argument to supply to the callback function.
 #' @param rho The environment in which the callback function is evaluated.
+#'   The default `NULL` uses the caller's environment.
 #' @param neimode `r lifecycle::badge("deprecated")` This argument is deprecated from igraph 1.3.0; use
 #'   `mode` instead.
 #' @inheritParams rlang::args_dots_empty
@@ -3673,13 +3724,17 @@ dfs <- function(
   in.callback = NULL,
   out.callback = NULL,
   extra = NULL,
-  rho = parent.frame(),
+  rho = NULL,
   neimode = deprecated(),
   father = deprecated()
 ) {
   rlang::check_dots_empty()
 
   ensure_igraph(graph)
+  if (is.null(rho)) {
+    rho <- parent.frame()
+  }
+
   if (lifecycle::is_present(neimode)) {
     lifecycle::deprecate_stop(
       "1.3.0",
@@ -4274,9 +4329,9 @@ laplacian_matrix <- function(
 #' @param eps A small real number used in equality tests in the weighted
 #'   bipartite matching algorithm. Two real numbers are considered equal in
 #'   the algorithm if their difference is smaller than `eps`. This is
-#'   required to avoid the accumulation of numerical errors. By default it is
-#'   set to the smallest \eqn{x}, such that \eqn{1+x \ne 1}{1+x != 1}
-#'   holds. If you are running the algorithm with no weights, this argument
+#'   required to avoid the accumulation of numerical errors. The default
+#'   `NULL` stands for the smallest \eqn{x}, such that
+#'   \eqn{1+x \ne 1}{1+x != 1} holds (`.Machine$double.eps`). If you are running the algorithm with no weights, this argument
 #'   is ignored.
 #' @return `is_matching()` and `is_max_matching()` return a logical
 #'   scalar.
@@ -4364,7 +4419,7 @@ max_bipartite_match <- function(
   types = NULL,
   ...,
   weights = NULL,
-  eps = .Machine$double.eps
+  eps = NULL
 ) {
   # BEGIN GENERATED ARG_HANDLE: max_bipartite_match, do not edit, see tools/generate-migrations.R
   if (...length() > 0L) {
@@ -4375,7 +4430,7 @@ max_bipartite_match <- function(
       recover_old = c("weights", "eps"),
       match_names = c("weights", "eps"),
       match_to = c("weights", "eps"),
-      defaults = list(weights = NULL, eps = .Machine$double.eps),
+      defaults = list(weights = NULL, eps = NULL),
       head_args = c("graph", "types"),
       fn_name = "max_bipartite_match"
     )
@@ -4388,6 +4443,10 @@ max_bipartite_match <- function(
   }
   # END GENERATED ARG_HANDLE
 
+  if (is.null(eps)) {
+    eps <- .Machine$double.eps
+  }
+
   res <- maximum_bipartite_matching_impl(
     graph = graph,
     types = types,
@@ -4418,8 +4477,8 @@ max_bipartite_match <- function(
 #' Undirected graphs contain only mutual edges by definition.
 #'
 #' @param graph The input graph.
-#' @param eids Edge sequence, the edges that will be probed. By default is
-#'   includes all edges in the order of their IDs.
+#' @param eids Edge sequence, the edges that will be probed. The default
+#'   `NULL` includes all edges in the order of their IDs.
 #' @inheritParams rlang::args_dots_empty
 #' @param loops Logical, whether to consider directed self-loops to be mutual.
 #' @return A logical vector of the same length as the number of edges supplied.
@@ -4438,7 +4497,7 @@ max_bipartite_match <- function(
 #' @export
 which_mutual <- function(
   graph,
-  eids = E(graph),
+  eids = NULL,
   ...,
   loops = TRUE
 ) {
@@ -4464,6 +4523,10 @@ which_mutual <- function(
   }
   # END GENERATED ARG_HANDLE
 
+  if (is.null(eids)) {
+    eids <- E(graph)
+  }
+
   is_mutual_impl(
     graph = graph,
     eids = eids,
@@ -4492,8 +4555,8 @@ which_mutual <- function(
 #' and \eqn{k_v}{k_v} is the neighbors' degree, specified by `neighbor_degree_mode`.
 #'
 #' @param graph The input graph. It may be directed.
-#' @param vids The vertices for which the calculation is performed. Normally it
-#'   includes all vertices. Note, that if not all vertices are given here, then
+#' @param vids The vertices for which the calculation is performed.
+#'   The default `NULL` includes all vertices. Note, that if not all vertices are given here, then
 #'   both \sQuote{`knn`} and \sQuote{`knnk`} will be calculated based
 #'   on the given vertices only.
 #' @inheritParams rlang::args_dots_empty
@@ -4549,7 +4612,7 @@ which_mutual <- function(
 #' @export
 knn <- function(
   graph,
-  vids = V(graph),
+  vids = NULL,
   ...,
   mode = c("all", "out", "in", "total"),
   neighbor.degree.mode = c("all", "out", "in", "total"),
@@ -4585,6 +4648,10 @@ knn <- function(
   }
   # END GENERATED ARG_HANDLE
 
+  if (is.null(vids)) {
+    vids <- V(graph)
+  }
+
   avg_nearest_neighbor_degree_impl(
     graph = graph,
     vids = vids,
diff --git a/R/triangles.R b/R/triangles.R
index d9926f32aa..5a0e5ebb9d 100644
--- a/R/triangles.R
+++ b/R/triangles.R
@@ -55,9 +55,9 @@ adjacent.triangles <- function(graph, vids = V(graph)) {
 #' @aliases triangles
 #' @param graph The input graph. It might be directed, but edge directions are
 #'   ignored.
-#' @param vids The vertices to query, all of them by default. This might be a
-#'   vector of numeric IDs, or a character vector of symbolic vertex names for
-#'   named graphs.
+#' @param vids The vertices to query. This might be a vector of numeric IDs,
+#'   or a character vector of symbolic vertex names for named graphs. The
+#'   default `NULL` selects all vertices.
 #' @return For `triangles()` a numeric vector of vertex IDs, the first three
 #'   vertices belong to the first triangle found, etc.
 #'
@@ -96,7 +96,11 @@ triangles <- function(graph) {
 
 #' @export
 #' @rdname count_triangles
-count_triangles <- function(graph, vids = V(graph)) {
+count_triangles <- function(graph, vids = NULL) {
+  if (is.null(vids)) {
+    vids <- V(graph)
+  }
+
   count_adjacent_triangles_impl(
     graph = graph,
     vids = vids
diff --git a/man/add.vertex.shape.Rd b/man/add.vertex.shape.Rd
index 0a21bc5552..89adafd6ef 100644
--- a/man/add.vertex.shape.Rd
+++ b/man/add.vertex.shape.Rd
@@ -16,9 +16,11 @@ add.vertex.shape(
 \code{NULL} for \code{shapes()}, then the names of all defined
 vertex shapes are returned.}
 
-\item{clip}{An R function object, the clipping function.}
+\item{clip}{An R function object, the clipping function. The default
+\code{NULL} uses \code{shape_noclip}.}
 
-\item{plot}{An R function object, the plotting function.}
+\item{plot}{An R function object, the plotting function. The default
+\code{NULL} uses \code{shape_noplot}.}
 
 \item{parameters}{Named list, additional plot/vertex/edge
 parameters. The element named define the new parameters, and the
diff --git a/man/adjacent.triangles.Rd b/man/adjacent.triangles.Rd
index 5357c0dfc1..88abc2cda0 100644
--- a/man/adjacent.triangles.Rd
+++ b/man/adjacent.triangles.Rd
@@ -10,9 +10,9 @@ adjacent.triangles(graph, vids = V(graph))
 \item{graph}{The input graph. It might be directed, but edge directions are
 ignored.}
 
-\item{vids}{The vertices to query, all of them by default. This might be a
-vector of numeric IDs, or a character vector of symbolic vertex names for
-named graphs.}
+\item{vids}{The vertices to query. This might be a vector of numeric IDs,
+or a character vector of symbolic vertex names for named graphs. The
+default \code{NULL} selects all vertices.}
 }
 \description{
 \ifelse{html}{\href{https://lifecycle.r-lib.org/articles/stages.html#deprecated}{\figure{lifecycle-deprecated.svg}{options: alt='[Deprecated]'}}}{\strong{[Deprecated]}}
diff --git a/man/all_simple_paths.Rd b/man/all_simple_paths.Rd
index e406597517..538fc0756b 100644
--- a/man/all_simple_paths.Rd
+++ b/man/all_simple_paths.Rd
@@ -7,7 +7,7 @@
 all_simple_paths(
   graph,
   from,
-  to = V(graph),
+  to = NULL,
   ...,
   mode = c("out", "in", "all", "total"),
   cutoff = -1
@@ -18,7 +18,8 @@ all_simple_paths(
 
 \item{from}{The source vertex.}
 
-\item{to}{The target vertex of vertices. Defaults to all vertices.}
+\item{to}{The target vertex of vertices. The default \code{NULL} selects all
+vertices.}
 
 \item{...}{These dots are for future extensions and must be empty.}
 
diff --git a/man/alpha.centrality.Rd b/man/alpha.centrality.Rd
index 4cda7bcae5..fdff3c9a1f 100644
--- a/man/alpha.centrality.Rd
+++ b/man/alpha.centrality.Rd
@@ -20,8 +20,8 @@ alpha.centrality(
 graphs, edges are treated as if they were reciprocal directed ones.}
 
 \item{nodes}{Vertex sequence, the vertices for which the alpha centrality
-values are returned. (For technical reasons they will be calculated for all
-vertices, anyway.)}
+values are returned. The default \code{NULL} selects all vertices.
+(For technical reasons they will be calculated for all vertices, anyway.)}
 
 \item{alpha}{Parameter specifying the relative importance of endogenous
 versus exogenous factors in the determination of centrality. See details
diff --git a/man/alpha_centrality.Rd b/man/alpha_centrality.Rd
index 0c3050dedc..2b90125c92 100644
--- a/man/alpha_centrality.Rd
+++ b/man/alpha_centrality.Rd
@@ -6,7 +6,7 @@
 \usage{
 alpha_centrality(
   graph,
-  nodes = V(graph),
+  nodes = NULL,
   ...,
   alpha = 1,
   loops = FALSE,
@@ -21,8 +21,8 @@ alpha_centrality(
 graphs, edges are treated as if they were reciprocal directed ones.}
 
 \item{nodes}{Vertex sequence, the vertices for which the alpha centrality
-values are returned. (For technical reasons they will be calculated for all
-vertices, anyway.)}
+values are returned. The default \code{NULL} selects all vertices.
+(For technical reasons they will be calculated for all vertices, anyway.)}
 
 \item{...}{These dots are for future extensions and must be empty.}
 
diff --git a/man/as.undirected.Rd b/man/as.undirected.Rd
index 1a0ae66f3f..63476faf18 100644
--- a/man/as.undirected.Rd
+++ b/man/as.undirected.Rd
@@ -22,7 +22,7 @@ as.undirected(
 \code{mode="collapse"} or \code{mode="mutual"}.  In these cases many edges
 might be mapped to a single one in the new graph, and their attributes are
 combined. Please see \code{\link[=attribute.combination]{attribute.combination()}} for details on
-this.}
+this. The default \code{NULL} uses the \code{edge.attr.comb} igraph option.}
 }
 \description{
 \ifelse{html}{\href{https://lifecycle.r-lib.org/articles/stages.html#deprecated}{\figure{lifecycle-deprecated.svg}{options: alt='[Deprecated]'}}}{\strong{[Deprecated]}}
diff --git a/man/as_adj.Rd b/man/as_adj.Rd
index 211d5c1b0d..c729a32d7f 100644
--- a/man/as_adj.Rd
+++ b/man/as_adj.Rd
@@ -49,7 +49,7 @@ is present in the graph.}
 
 \item{sparse}{Logical, whether to create a sparse matrix. The
 \sQuote{\code{Matrix}} package must be installed for creating sparse
-matrices.}
+matrices. The default \code{NULL} uses the \code{sparsematrices} igraph option.}
 }
 \description{
 \ifelse{html}{\href{https://lifecycle.r-lib.org/articles/stages.html#deprecated}{\figure{lifecycle-deprecated.svg}{options: alt='[Deprecated]'}}}{\strong{[Deprecated]}}
diff --git a/man/as_adjacency_matrix.Rd b/man/as_adjacency_matrix.Rd
index e89b439556..983751cac3 100644
--- a/man/as_adjacency_matrix.Rd
+++ b/man/as_adjacency_matrix.Rd
@@ -10,7 +10,7 @@ as_adjacency_matrix(
   ...,
   weights = NULL,
   names = TRUE,
-  sparse = igraph_opt("sparsematrices"),
+  sparse = NULL,
   edges = deprecated(),
   attr = deprecated()
 )
@@ -45,7 +45,7 @@ is present in the graph.}
 
 \item{sparse}{Logical, whether to create a sparse matrix. The
 \sQuote{\code{Matrix}} package must be installed for creating sparse
-matrices.}
+matrices. The default \code{NULL} uses the \code{sparsematrices} igraph option.}
 
 \item{edges}{\ifelse{html}{\href{https://lifecycle.r-lib.org/articles/stages.html#deprecated}{\figure{lifecycle-deprecated.svg}{options: alt='[Deprecated]'}}}{\strong{[Deprecated]}} Logical, whether to return the edge IDs in the matrix.
 For non-existant edges zero is returned.}
diff --git a/man/as_directed.Rd b/man/as_directed.Rd
index f4032e7956..ca83689eb0 100644
--- a/man/as_directed.Rd
+++ b/man/as_directed.Rd
@@ -10,7 +10,7 @@ as_directed(graph, ..., mode = c("mutual", "arbitrary", "random", "acyclic"))
 as_undirected(
   graph,
   mode = c("collapse", "each", "mutual"),
-  edge.attr.comb = igraph_opt("edge.attr.comb")
+  edge.attr.comb = NULL
 )
 }
 \arguments{
@@ -27,7 +27,7 @@ as_undirected(
 \code{mode="collapse"} or \code{mode="mutual"}.  In these cases many edges
 might be mapped to a single one in the new graph, and their attributes are
 combined. Please see \code{\link[=attribute.combination]{attribute.combination()}} for details on
-this.}
+this. The default \code{NULL} uses the \code{edge.attr.comb} igraph option.}
 }
 \value{
 A new graph object.
diff --git a/man/asymmetric.preference.game.Rd b/man/asymmetric.preference.game.Rd
index 41ed134280..a25892561c 100644
--- a/man/asymmetric.preference.game.Rd
+++ b/man/asymmetric.preference.game.Rd
@@ -18,11 +18,12 @@ asymmetric.preference.game(
 \item{types}{The number of different vertex types.}
 
 \item{type.dist.matrix}{The joint distribution of the in- and out-vertex
-types.}
+types. The default \code{NULL} gives a uniform distribution.}
 
 \item{pref.matrix}{A square matrix giving the preferences of the vertex
 types. The matrix has \sQuote{types} rows and columns. When generating
-an undirected graph, it must be symmetric.}
+an undirected graph, it must be symmetric. The default \code{NULL} sets all
+preferences to one.}
 
 \item{loops}{Logical, whether self-loops are allowed in the graph.}
 }
diff --git a/man/betweenness.Rd b/man/betweenness.Rd
index 119539bf6f..f8dbd6d4a3 100644
--- a/man/betweenness.Rd
+++ b/man/betweenness.Rd
@@ -9,7 +9,7 @@
 \usage{
 betweenness(
   graph,
-  v = V(graph),
+  v = NULL,
   ...,
   directed = TRUE,
   weights = NULL,
@@ -19,7 +19,7 @@ betweenness(
 
 edge_betweenness(
   graph,
-  e = E(graph),
+  e = NULL,
   ...,
   directed = TRUE,
   weights = NULL,
@@ -29,7 +29,8 @@ edge_betweenness(
 \arguments{
 \item{graph}{The graph to analyze.}
 
-\item{v}{The vertices for which the vertex betweenness will be calculated.}
+\item{v}{The vertices for which the vertex betweenness will be calculated.
+The default \code{NULL} selects all vertices.}
 
 \item{...}{These dots are for future extensions and must be empty.}
 
@@ -56,7 +57,8 @@ from each other may be less than \eqn{(n-1)(n-2)/2}.}
 \item{cutoff}{The maximum shortest path length to consider when calculating
 betweenness. If negative, then there is no such limit.}
 
-\item{e}{The edges for which the edge betweenness will be calculated.}
+\item{e}{The edges for which the edge betweenness will be calculated.
+The default \code{NULL} selects all edges.}
 }
 \value{
 A numeric vector with the betweenness score for each vertex in
diff --git a/man/bfs.Rd b/man/bfs.Rd
index 74d1db1bf6..b18e9a28f9 100644
--- a/man/bfs.Rd
+++ b/man/bfs.Rd
@@ -19,7 +19,7 @@ bfs(
   dist = FALSE,
   callback = NULL,
   extra = NULL,
-  rho = parent.frame(),
+  rho = NULL,
   neimode = deprecated(),
   father = deprecated()
 )
@@ -71,7 +71,8 @@ Default: \code{NULL}.}
 
 \item{extra}{Additional argument to supply to the callback function.}
 
-\item{rho}{The environment in which the callback function is evaluated.}
+\item{rho}{The environment in which the callback function is evaluated.
+The default \code{NULL} uses the caller's environment.}
 
 \item{neimode}{\ifelse{html}{\href{https://lifecycle.r-lib.org/articles/stages.html#deprecated}{\figure{lifecycle-deprecated.svg}{options: alt='[Deprecated]'}}}{\strong{[Deprecated]}} This argument is deprecated
 from igraph 1.3.0; use \code{mode} instead.}
diff --git a/man/bonpow.Rd b/man/bonpow.Rd
index 33cb759d42..d4745d8019 100644
--- a/man/bonpow.Rd
+++ b/man/bonpow.Rd
@@ -18,7 +18,7 @@ bonpow(
 \item{graph}{the input graph.}
 
 \item{nodes}{vertex sequence indicating which vertices are to be included in
-the calculation.  By default, all vertices are included.}
+the calculation. The default \code{NULL} selects all vertices.}
 
 \item{loops}{Logical indicating whether or not the diagonal should be
 treated as valid data.  Set this true if and only if the data can contain
diff --git a/man/callaway.traits.game.Rd b/man/callaway.traits.game.Rd
index b4abb17313..487c885599 100644
--- a/man/callaway.traits.game.Rd
+++ b/man/callaway.traits.game.Rd
@@ -21,10 +21,11 @@ callaway.traits.game(
 \item{edge.per.step}{The number of edges to add to the graph per time step.}
 
 \item{type.dist}{The distribution of the vertex types. This is assumed to be
-stationary in time.}
+stationary in time. The default \code{NULL} gives a uniform distribution.}
 
 \item{pref.matrix}{A matrix giving the preferences of the given vertex
-types. These should be probabilities, i.e. numbers between zero and one.}
+types. These should be probabilities, i.e. numbers between zero and one.
+The default \code{NULL} sets all preferences to one.}
 
 \item{directed}{Logical, whether to generate directed graphs.}
 }
diff --git a/man/cited.type.game.Rd b/man/cited.type.game.Rd
index 8af960e4d8..36274fe108 100644
--- a/man/cited.type.game.Rd
+++ b/man/cited.type.game.Rd
@@ -19,11 +19,14 @@ cited.type.game(
 \item{edges}{Number of edges per step.}
 
 \item{types}{Vector of length \sQuote{\code{n}}, the types of the vertices.
-Types are numbered from zero.}
+Types are numbered from zero. The default \code{NULL} gives all vertices
+type zero.}
 
 \item{pref}{Vector (\code{sample_last_cit()} and \code{sample_cit_types()} or
 matrix (\code{sample_cit_cit_types()}) giving the (unnormalized) citation
-probabilities for the different vertex types.}
+probabilities for the different vertex types. The default \code{NULL} uses
+\code{(1:(agebins + 1))^-3} for \code{sample_last_cit()} and all-one probabilities
+for the other two.}
 
 \item{directed}{Logical, whether to generate directed networks.}
 
diff --git a/man/citing.cited.type.game.Rd b/man/citing.cited.type.game.Rd
index 34df098f14..151924096c 100644
--- a/man/citing.cited.type.game.Rd
+++ b/man/citing.cited.type.game.Rd
@@ -19,11 +19,14 @@ citing.cited.type.game(
 \item{edges}{Number of edges per step.}
 
 \item{types}{Vector of length \sQuote{\code{n}}, the types of the vertices.
-Types are numbered from zero.}
+Types are numbered from zero. The default \code{NULL} gives all vertices
+type zero.}
 
 \item{pref}{Vector (\code{sample_last_cit()} and \code{sample_cit_types()} or
 matrix (\code{sample_cit_cit_types()}) giving the (unnormalized) citation
-probabilities for the different vertex types.}
+probabilities for the different vertex types. The default \code{NULL} uses
+\code{(1:(agebins + 1))^-3} for \code{sample_last_cit()} and all-one probabilities
+for the other two.}
 
 \item{directed}{Logical, whether to generate directed networks.}
 
diff --git a/man/closeness.Rd b/man/closeness.Rd
index ca5bb3b4ba..3103521ed9 100644
--- a/man/closeness.Rd
+++ b/man/closeness.Rd
@@ -7,7 +7,7 @@
 \usage{
 closeness(
   graph,
-  vids = V(graph),
+  vids = NULL,
   ...,
   mode = c("out", "in", "all", "total"),
   weights = NULL,
@@ -18,7 +18,8 @@ closeness(
 \arguments{
 \item{graph}{The graph to analyze.}
 
-\item{vids}{The vertices for which closeness will be calculated.}
+\item{vids}{The vertices for which closeness will be calculated.
+The default \code{NULL} selects all vertices.}
 
 \item{...}{These dots are for future extensions and must be empty.}
 
diff --git a/man/cocitation.Rd b/man/cocitation.Rd
index 727d71e807..e148aecbf8 100644
--- a/man/cocitation.Rd
+++ b/man/cocitation.Rd
@@ -5,16 +5,16 @@
 \alias{bibcoupling}
 \title{Cocitation coupling}
 \usage{
-cocitation(graph, v = V(graph))
+cocitation(graph, v = NULL)
 
-bibcoupling(graph, v = V(graph))
+bibcoupling(graph, v = NULL)
 }
 \arguments{
 \item{graph}{The graph object to analyze}
 
 \item{v}{Vertex sequence or numeric vector, the vertex IDs for which the
 cocitation or bibliographic coupling values we want to calculate. The
-default is all vertices.}
+default \code{NULL} selects all vertices.}
 }
 \value{
 A numeric matrix with \code{length(v)} lines and
diff --git a/man/cohesive_blocks.Rd b/man/cohesive_blocks.Rd
index 4c80d62bd1..e03fc6bf35 100644
--- a/man/cohesive_blocks.Rd
+++ b/man/cohesive_blocks.Rd
@@ -44,11 +44,7 @@ parent(blocks)
   ...
 )
 
-plot_hierarchy(
-  blocks,
-  layout = layout_as_tree(hierarchy(blocks), root = 1),
-  ...
-)
+plot_hierarchy(blocks, layout = NULL, ...)
 
 export_pajek(blocks, graph, file, ..., project.file = TRUE)
 
@@ -92,8 +88,8 @@ them. By default all cohesive blocks are marked, except the one
 corresponding to the all vertices.}
 
 \item{layout}{The layout of a plot, it is simply passed on to
-\code{plot.igraph()}, see the possible formats there. By default the
-Reingold-Tilford layout generator is used.}
+\code{plot.igraph()}, see the possible formats there. The default \code{NULL} uses
+the Reingold-Tilford layout generator.}
 
 \item{file}{Defines the file (or connection) the Pajek file is written to.
 
diff --git a/man/compose.Rd b/man/compose.Rd
index ea67a1050c..ff5e5adcf7 100644
--- a/man/compose.Rd
+++ b/man/compose.Rd
@@ -10,7 +10,7 @@ compose(
   g2,
   ...,
   byname = "auto",
-  graph.attr.comb = igraph_opt("graph.attr.comb"),
+  graph.attr.comb = NULL,
   vertex.attr.comb = "rename",
   edge.attr.comb = "rename"
 )
@@ -81,7 +81,7 @@ g1 and g2, respectively, then (a,a) is included in the result. See
 \code{\link[=simplify]{simplify()}} if you want to get rid of the self-loops.
 }
 \section{Related documentation in the C library}{
-\href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_vcount}{\code{vcount()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Isomorphism.html#igraph_permute_vertices}{\code{permute_vertices()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_edges}{\code{edges()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_get_eids}{\code{get_eids()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_ecount}{\code{ecount()}}
+\href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_vcount}{\code{vcount()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Isomorphism.html#igraph_permute_vertices}{\code{permute_vertices()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_get_eids}{\code{get_eids()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_edges}{\code{edges()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_ecount}{\code{ecount()}}
 }
 
 \examples{
diff --git a/man/constraint.Rd b/man/constraint.Rd
index 2d4e7afbd2..3dd16200b3 100644
--- a/man/constraint.Rd
+++ b/man/constraint.Rd
@@ -4,13 +4,13 @@
 \alias{constraint}
 \title{Burt's constraint}
 \usage{
-constraint(graph, nodes = V(graph), ..., weights = NULL)
+constraint(graph, nodes = NULL, ..., weights = NULL)
 }
 \arguments{
 \item{graph}{A graph object, the input graph.}
 
 \item{nodes}{The vertices for which the constraint will be calculated.
-Defaults to all vertices.}
+The default \code{NULL} selects all vertices.}
 
 \item{...}{These dots are for future extensions and must be empty.}
 
diff --git a/man/contract.Rd b/man/contract.Rd
index 677567c3d0..aa11b59ece 100644
--- a/man/contract.Rd
+++ b/man/contract.Rd
@@ -4,7 +4,7 @@
 \alias{contract}
 \title{Contract several vertices into a single one}
 \usage{
-contract(graph, mapping, vertex.attr.comb = igraph_opt("vertex.attr.comb"))
+contract(graph, mapping, vertex.attr.comb = NULL)
 }
 \arguments{
 \item{graph}{The input graph, it can be directed or undirected.}
@@ -14,7 +14,8 @@ correspond to the vertices, and for each element the ID in the new graph is
 given.}
 
 \item{vertex.attr.comb}{Specifies how to combine the vertex attributes in
-the new graph. Please see \code{\link[=attribute.combination]{attribute.combination()}} for details.}
+the new graph. Please see \code{\link[=attribute.combination]{attribute.combination()}} for details. The
+default \code{NULL} uses the \code{vertex.attr.comb} igraph option.}
 }
 \value{
 A new graph object.
diff --git a/man/contract.vertices.Rd b/man/contract.vertices.Rd
index 41c8703202..b527e63f73 100644
--- a/man/contract.vertices.Rd
+++ b/man/contract.vertices.Rd
@@ -18,7 +18,8 @@ correspond to the vertices, and for each element the ID in the new graph is
 given.}
 
 \item{vertex.attr.comb}{Specifies how to combine the vertex attributes in
-the new graph. Please see \code{\link[=attribute.combination]{attribute.combination()}} for details.}
+the new graph. Please see \code{\link[=attribute.combination]{attribute.combination()}} for details. The
+default \code{NULL} uses the \code{vertex.attr.comb} igraph option.}
 }
 \description{
 \ifelse{html}{\href{https://lifecycle.r-lib.org/articles/stages.html#deprecated}{\figure{lifecycle-deprecated.svg}{options: alt='[Deprecated]'}}}{\strong{[Deprecated]}}
diff --git a/man/count.multiple.Rd b/man/count.multiple.Rd
index 0f20b5b875..a4e624fe0c 100644
--- a/man/count.multiple.Rd
+++ b/man/count.multiple.Rd
@@ -9,8 +9,8 @@ count.multiple(graph, eids = E(graph))
 \arguments{
 \item{graph}{The input graph.}
 
-\item{eids}{The edges to which the query is restricted. By default this is
-all edges in the graph.}
+\item{eids}{The edges to which the query is restricted. The default
+\code{NULL} selects all edges.}
 }
 \description{
 \ifelse{html}{\href{https://lifecycle.r-lib.org/articles/stages.html#deprecated}{\figure{lifecycle-deprecated.svg}{options: alt='[Deprecated]'}}}{\strong{[Deprecated]}}
diff --git a/man/count_triangles.Rd b/man/count_triangles.Rd
index 85bd5504cf..99af82f07d 100644
--- a/man/count_triangles.Rd
+++ b/man/count_triangles.Rd
@@ -7,15 +7,15 @@
 \usage{
 triangles(graph)
 
-count_triangles(graph, vids = V(graph))
+count_triangles(graph, vids = NULL)
 }
 \arguments{
 \item{graph}{The input graph. It might be directed, but edge directions are
 ignored.}
 
-\item{vids}{The vertices to query, all of them by default. This might be a
-vector of numeric IDs, or a character vector of symbolic vertex names for
-named graphs.}
+\item{vids}{The vertices to query. This might be a vector of numeric IDs,
+or a character vector of symbolic vertex names for named graphs. The
+default \code{NULL} selects all vertices.}
 }
 \value{
 For \code{triangles()} a numeric vector of vertex IDs, the first three
diff --git a/man/degree.Rd b/man/degree.Rd
index 5c26929cc4..ec7c9277b4 100644
--- a/man/degree.Rd
+++ b/man/degree.Rd
@@ -9,7 +9,7 @@
 \usage{
 degree(
   graph,
-  v = V(graph),
+  v = NULL,
   ...,
   mode = c("all", "out", "in", "total"),
   loops = TRUE,
@@ -19,7 +19,7 @@ degree(
 max_degree(
   graph,
   ...,
-  v = V(graph),
+  v = NULL,
   mode = c("all", "out", "in", "total"),
   loops = TRUE
 )
@@ -31,7 +31,8 @@ degree_distribution(graph, cumulative = FALSE, ...)
 \arguments{
 \item{graph}{The graph to analyze.}
 
-\item{v}{The IDs of vertices of which the degree will be calculated.}
+\item{v}{The IDs of vertices of which the degree will be calculated.
+The default \code{NULL} selects all vertices.}
 
 \item{...}{These dots are for future extensions and must be empty.}
 
diff --git a/man/dendPlot.Rd b/man/dendPlot.Rd
index 20732492af..0333797f58 100644
--- a/man/dendPlot.Rd
+++ b/man/dendPlot.Rd
@@ -10,7 +10,8 @@ dendPlot(x, mode = igraph_opt("dend.plot.type"), ...)
 \item{x}{An object containing the community structure of a graph. See
 \code{\link[=communities]{communities()}} for details.}
 
-\item{mode}{Which dendrogram plotting function to use. See details below.}
+\item{mode}{Which dendrogram plotting function to use. See details below.
+The default \code{NULL} uses the \code{dend.plot.type} igraph option.}
 
 \item{...}{Additional arguments to supply to the dendrogram plotting
 function.}
diff --git a/man/dfs.Rd b/man/dfs.Rd
index 05d1cb6391..5364bfd860 100644
--- a/man/dfs.Rd
+++ b/man/dfs.Rd
@@ -17,7 +17,7 @@ dfs(
   in.callback = NULL,
   out.callback = NULL,
   extra = NULL,
-  rho = parent.frame(),
+  rho = NULL,
   neimode = deprecated(),
   father = deprecated()
 )
@@ -61,7 +61,8 @@ Default: \code{NULL}.}
 
 \item{extra}{Additional argument to supply to the callback function.}
 
-\item{rho}{The environment in which the callback function is evaluated.}
+\item{rho}{The environment in which the callback function is evaluated.
+The default \code{NULL} uses the caller's environment.}
 
 \item{neimode}{\ifelse{html}{\href{https://lifecycle.r-lib.org/articles/stages.html#deprecated}{\figure{lifecycle-deprecated.svg}{options: alt='[Deprecated]'}}}{\strong{[Deprecated]}} This argument is deprecated from igraph 1.3.0; use
 \code{mode} instead.}
diff --git a/man/disjoint_union.Rd b/man/disjoint_union.Rd
index 7b0b040da9..c0ae54327c 100644
--- a/man/disjoint_union.Rd
+++ b/man/disjoint_union.Rd
@@ -5,7 +5,7 @@
 \alias{\%du\%}
 \title{Disjoint union of graphs}
 \usage{
-disjoint_union(..., graph.attr.comb = igraph_opt("graph.attr.comb"))
+disjoint_union(..., graph.attr.comb = NULL)
 
 x \%du\% y
 }
@@ -13,10 +13,10 @@ x \%du\% y
 \item{\dots}{Graph objects or lists of graph objects.}
 
 \item{graph.attr.comb}{Specification for combining shared graph attributes.
-Defaults to the \code{graph.attr.comb} igraph option (\code{"rename"} unless changed
-via \code{\link[=igraph_options]{igraph_options()}}), which preserves the historical behaviour of
-appending \verb{_1}, \verb{_2}, ... suffixes to clashing attribute names. See
-\link{igraph-attribute-combination} for the available combiners.}
+The default \code{NULL} uses the \code{graph.attr.comb} igraph option (\code{"rename"}
+unless changed via \code{\link[=igraph_options]{igraph_options()}}), which preserves the historical
+behaviour of appending \verb{_1}, \verb{_2}, ... suffixes to clashing attribute
+names. See \link{igraph-attribute-combination} for the available combiners.}
 
 \item{x, y}{Graph objects.}
 }
@@ -50,7 +50,7 @@ An error is generated if some input graphs are directed and others are
 undirected.
 }
 \section{Related documentation in the C library}{
-\href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_vcount}{\code{vcount()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_edges}{\code{edges()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_get_eids}{\code{get_eids()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_ecount}{\code{ecount()}}
+\href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_vcount}{\code{vcount()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_get_eids}{\code{get_eids()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_edges}{\code{edges()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_ecount}{\code{ecount()}}
 }
 
 \examples{
diff --git a/man/distances.Rd b/man/distances.Rd
index e52213e5b5..2e84b17143 100644
--- a/man/distances.Rd
+++ b/man/distances.Rd
@@ -21,8 +21,8 @@ mean_distance(
 
 distances(
   graph,
-  v = V(graph),
-  to = V(graph),
+  v = NULL,
+  to = NULL,
   ...,
   mode = c("all", "out", "in"),
   weights = NULL,
@@ -33,7 +33,7 @@ distances(
 shortest_paths(
   graph,
   from,
-  to = V(graph),
+  to = NULL,
   ...,
   mode = c("out", "all", "in"),
   weights = NULL,
@@ -46,7 +46,7 @@ shortest_paths(
 all_shortest_paths(
   graph,
   from,
-  to = V(graph),
+  to = NULL,
   ...,
   mode = c("out", "all", "in"),
   weights = NULL
@@ -78,10 +78,10 @@ additional information like the number of disconnected vertex pairs in
 the result when this parameter is set to \code{TRUE}.}
 
 \item{v}{Numeric vector, the vertices from which the shortest paths will be
-calculated.}
+calculated. The default \code{NULL} selects all vertices.}
 
 \item{to}{Numeric vector, the vertices to which the shortest paths will be
-calculated. By default it includes all vertices. Note that for
+calculated. The default \code{NULL} includes all vertices. Note that for
 \code{distances()} every vertex must be included here at most once. (This
 is not required for \code{shortest_paths()}.}
 
diff --git a/man/diversity.Rd b/man/diversity.Rd
index 8c614717ce..0fe5d21dfd 100644
--- a/man/diversity.Rd
+++ b/man/diversity.Rd
@@ -4,7 +4,7 @@
 \alias{diversity}
 \title{Graph diversity}
 \usage{
-diversity(graph, ..., weights = NULL, vids = V(graph))
+diversity(graph, ..., weights = NULL, vids = NULL)
 }
 \arguments{
 \item{graph}{The input graph. Edge directions are ignored.}
@@ -15,7 +15,8 @@ diversity(graph, ..., weights = NULL, vids = V(graph))
 computation. If \code{NULL}, then the \sQuote{weight} attibute is used. Note
 that this measure is not defined for unweighted graphs.}
 
-\item{vids}{The vertex IDs for which to calculate the measure.}
+\item{vids}{The vertex IDs for which to calculate the measure.
+The default \code{NULL} selects all vertices.}
 }
 \value{
 A numeric vector, its length is the number of vertices.
diff --git a/man/dot-apply_modifiers.Rd b/man/dot-apply_modifiers.Rd
index 99caccbb44..18e9b439e0 100644
--- a/man/dot-apply_modifiers.Rd
+++ b/man/dot-apply_modifiers.Rd
@@ -19,7 +19,7 @@ This is a helper function for the common parts of \code{make_()} and
 \code{sample_()}.
 }
 \section{Related documentation in the C library}{
-\href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_vcount}{\code{vcount()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Operators.html#igraph_simplify}{\code{simplify()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Structural.html#igraph_is_simple}{\code{is_simple()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_edges}{\code{edges()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_get_eids}{\code{get_eids()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_ecount}{\code{ecount()}}
+\href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_vcount}{\code{vcount()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Operators.html#igraph_simplify}{\code{simplify()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Structural.html#igraph_is_simple}{\code{is_simple()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_get_eids}{\code{get_eids()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_edges}{\code{edges()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_ecount}{\code{ecount()}}
 }
 
 \keyword{internal}
diff --git a/man/edge.betweenness.Rd b/man/edge.betweenness.Rd
index 7084de3a29..a98737cf18 100644
--- a/man/edge.betweenness.Rd
+++ b/man/edge.betweenness.Rd
@@ -15,7 +15,8 @@ edge.betweenness(
 \arguments{
 \item{graph}{The graph to analyze.}
 
-\item{e}{The edges for which the edge betweenness will be calculated.}
+\item{e}{The edges for which the edge betweenness will be calculated.
+The default \code{NULL} selects all edges.}
 
 \item{directed}{Logical, whether directed paths should be considered while
 determining the shortest paths.}
diff --git a/man/edge_attr-set.Rd b/man/edge_attr-set.Rd
index 229c67d280..e25638f33e 100644
--- a/man/edge_attr-set.Rd
+++ b/man/edge_attr-set.Rd
@@ -5,7 +5,7 @@
 \alias{edge.attributes<-}
 \title{Set one or more edge attributes}
 \usage{
-edge_attr(graph, name, index = E(graph)) <- value
+edge_attr(graph, name, index = NULL) <- value
 }
 \arguments{
 \item{graph}{The graph.}
@@ -15,7 +15,7 @@ then \code{value} must be a named list, and its entries are
 set as edge attributes.}
 
 \item{index}{An optional edge sequence to set the attributes
-of a subset of edges.}
+of a subset of edges. The default \code{NULL} selects all edges.}
 
 \item{value}{The new value of the attribute(s) for all
 (or \code{index}) edges.}
diff --git a/man/edge_attr.Rd b/man/edge_attr.Rd
index a56595ace8..d1a412dc8d 100644
--- a/man/edge_attr.Rd
+++ b/man/edge_attr.Rd
@@ -5,7 +5,7 @@
 \alias{edge.attributes}
 \title{Query edge attributes of a graph}
 \usage{
-edge_attr(graph, name, index = E(graph))
+edge_attr(graph, name, index = NULL)
 }
 \arguments{
 \item{graph}{The graph}
@@ -14,7 +14,7 @@ edge_attr(graph, name, index = E(graph))
 all edge attributes are returned in a list.}
 
 \item{index}{An optional edge sequence to query edge attributes
-for a subset of edges.}
+for a subset of edges. The default \code{NULL} selects all edges.}
 }
 \value{
 The value of the edge attribute, or the list of all
diff --git a/man/ego.Rd b/man/ego.Rd
index 0a92929bb2..67ddfceda0 100644
--- a/man/ego.Rd
+++ b/man/ego.Rd
@@ -16,7 +16,7 @@ connect(graph, order, ..., mode = c("all", "out", "in", "total"))
 ego_size(
   graph,
   order = 1,
-  nodes = V(graph),
+  nodes = NULL,
   ...,
   mode = c("all", "out", "in"),
   mindist = 0
@@ -25,7 +25,7 @@ ego_size(
 neighborhood_size(
   graph,
   order = 1,
-  nodes = V(graph),
+  nodes = NULL,
   ...,
   mode = c("all", "out", "in"),
   mindist = 0
@@ -34,7 +34,7 @@ neighborhood_size(
 ego(
   graph,
   order = 1,
-  nodes = V(graph),
+  nodes = NULL,
   ...,
   mode = c("all", "out", "in"),
   mindist = 0
@@ -43,7 +43,7 @@ ego(
 neighborhood(
   graph,
   order = 1,
-  nodes = V(graph),
+  nodes = NULL,
   ...,
   mode = c("all", "out", "in"),
   mindist = 0
@@ -52,7 +52,7 @@ neighborhood(
 make_ego_graph(
   graph,
   order = 1,
-  nodes = V(graph),
+  nodes = NULL,
   ...,
   mode = c("all", "out", "in"),
   mindist = 0
@@ -61,7 +61,7 @@ make_ego_graph(
 make_neighborhood_graph(
   graph,
   order = 1,
-  nodes = V(graph),
+  nodes = NULL,
   ...,
   mode = c("all", "out", "in"),
   mindist = 0
@@ -83,7 +83,8 @@ vertices from which the source vertex is reachable in at most \code{order}
 steps are counted. \sQuote{"all"} ignores the direction of the edges. This
 argument is ignored for undirected graphs.}
 
-\item{nodes}{The vertices for which the calculation is performed.}
+\item{nodes}{The vertices for which the calculation is performed.
+The default \code{NULL} selects all vertices.}
 
 \item{mindist}{The minimum distance to include the vertex in the result.}
 }
diff --git a/man/embed_adjacency_matrix.Rd b/man/embed_adjacency_matrix.Rd
index f7c3abd2e9..b5071c1785 100644
--- a/man/embed_adjacency_matrix.Rd
+++ b/man/embed_adjacency_matrix.Rd
@@ -11,8 +11,8 @@ embed_adjacency_matrix(
   weights = NULL,
   which = c("lm", "la", "sa"),
   scaled = TRUE,
-  cvec = strength(graph, weights = weights)/(vcount(graph) - 1),
-  options = arpack_defaults()
+  cvec = NULL,
+  options = NULL
 )
 }
 \arguments{
@@ -41,11 +41,13 @@ are used for the ordering.}
 returned instead of \eqn{X} and \eqn{Y}.}
 
 \item{cvec}{A numeric vector, its length is the number vertices in the
-graph. This vector is added to the diagonal of the adjacency matrix.}
+graph. This vector is added to the diagonal of the adjacency matrix. The
+default \code{NULL} uses
+\code{strength(graph, weights = weights) / (vcount(graph) - 1)}.}
 
 \item{options}{A named list containing the parameters for the SVD
-computation algorithm in ARPACK. By default, the list of values is assigned
-the values given by \code{\link[=arpack_defaults]{arpack_defaults()}}.}
+computation algorithm in ARPACK. The default \code{NULL} uses the values given
+by \code{\link[=arpack_defaults]{arpack_defaults()}}.}
 }
 \value{
 A list containing with entries:
diff --git a/man/embed_laplacian_matrix.Rd b/man/embed_laplacian_matrix.Rd
index f857c1da76..ae89843dbd 100644
--- a/man/embed_laplacian_matrix.Rd
+++ b/man/embed_laplacian_matrix.Rd
@@ -12,7 +12,7 @@ embed_laplacian_matrix(
   which = c("lm", "la", "sa"),
   type = c("default", "D-A", "DAD", "I-DAD", "OAP"),
   scaled = TRUE,
-  options = arpack_defaults()
+  options = NULL
 )
 }
 \arguments{
@@ -61,8 +61,8 @@ graphs and \code{OAP} for directed graphs.}
 returned instead of \eqn{X} and \eqn{Y}.}
 
 \item{options}{A named list containing the parameters for the SVD
-computation algorithm in ARPACK. By default, the list of values is assigned
-the values given by \code{\link[=arpack_defaults]{arpack_defaults()}}.}
+computation algorithm in ARPACK. The default \code{NULL} uses the values given
+by \code{\link[=arpack_defaults]{arpack_defaults()}}.}
 }
 \value{
 A list containing with entries:
diff --git a/man/establishment.game.Rd b/man/establishment.game.Rd
index fab6da8bb3..c27ce60c40 100644
--- a/man/establishment.game.Rd
+++ b/man/establishment.game.Rd
@@ -21,10 +21,11 @@ establishment.game(
 \item{k}{The number of trials per time step, see details below.}
 
 \item{type.dist}{The distribution of the vertex types. This is assumed to be
-stationary in time.}
+stationary in time. The default \code{NULL} gives a uniform distribution.}
 
 \item{pref.matrix}{A matrix giving the preferences of the given vertex
-types. These should be probabilities, i.e. numbers between zero and one.}
+types. These should be probabilities, i.e. numbers between zero and one.
+The default \code{NULL} sets all preferences to one.}
 
 \item{directed}{Logical, whether to generate directed graphs.}
 }
diff --git a/man/estimate_closeness.Rd b/man/estimate_closeness.Rd
index 0f71008210..2b7d031524 100644
--- a/man/estimate_closeness.Rd
+++ b/man/estimate_closeness.Rd
@@ -16,7 +16,8 @@ estimate_closeness(
 \arguments{
 \item{graph}{The graph to analyze.}
 
-\item{vids}{The vertices for which closeness will be calculated.}
+\item{vids}{The vertices for which closeness will be calculated.
+The default \code{NULL} selects all vertices.}
 
 \item{mode}{Character string, defined the types of the paths used for
 measuring the distance in directed graphs. \dQuote{in} measures the paths
diff --git a/man/estimate_edge_betweenness.Rd b/man/estimate_edge_betweenness.Rd
index b5ebe0531d..acd055427c 100644
--- a/man/estimate_edge_betweenness.Rd
+++ b/man/estimate_edge_betweenness.Rd
@@ -15,7 +15,8 @@ estimate_edge_betweenness(
 \arguments{
 \item{graph}{The graph to analyze.}
 
-\item{e}{The edges for which the edge betweenness will be calculated.}
+\item{e}{The edges for which the edge betweenness will be calculated.
+The default \code{NULL} selects all edges.}
 
 \item{directed}{Logical, whether directed paths should be considered while
 determining the shortest paths.}
diff --git a/man/get.adjacency.Rd b/man/get.adjacency.Rd
index 3fa07e6a9e..c514ab5160 100644
--- a/man/get.adjacency.Rd
+++ b/man/get.adjacency.Rd
@@ -35,7 +35,7 @@ is present in the graph.}
 
 \item{sparse}{Logical, whether to create a sparse matrix. The
 \sQuote{\code{Matrix}} package must be installed for creating sparse
-matrices.}
+matrices. The default \code{NULL} uses the \code{sparsematrices} igraph option.}
 }
 \description{
 \ifelse{html}{\href{https://lifecycle.r-lib.org/articles/stages.html#deprecated}{\figure{lifecycle-deprecated.svg}{options: alt='[Deprecated]'}}}{\strong{[Deprecated]}}
diff --git a/man/get.all.shortest.paths.Rd b/man/get.all.shortest.paths.Rd
index 8710387e65..ac517470fc 100644
--- a/man/get.all.shortest.paths.Rd
+++ b/man/get.all.shortest.paths.Rd
@@ -20,7 +20,7 @@ be calculated. Note that right now this is not a vector of vertex IDs, but
 only a single vertex.}
 
 \item{to}{Numeric vector, the vertices to which the shortest paths will be
-calculated. By default it includes all vertices. Note that for
+calculated. The default \code{NULL} includes all vertices. Note that for
 \code{distances()} every vertex must be included here at most once. (This
 is not required for \code{shortest_paths()}.}
 
diff --git a/man/get.edge.attribute.Rd b/man/get.edge.attribute.Rd
index 2a8249a075..5fc746f899 100644
--- a/man/get.edge.attribute.Rd
+++ b/man/get.edge.attribute.Rd
@@ -13,7 +13,7 @@ get.edge.attribute(graph, name, index = E(graph))
 all edge attributes are returned in a list.}
 
 \item{index}{An optional edge sequence to query edge attributes
-for a subset of edges.}
+for a subset of edges. The default \code{NULL} selects all edges.}
 }
 \description{
 \ifelse{html}{\href{https://lifecycle.r-lib.org/articles/stages.html#deprecated}{\figure{lifecycle-deprecated.svg}{options: alt='[Deprecated]'}}}{\strong{[Deprecated]}}
diff --git a/man/get.shortest.paths.Rd b/man/get.shortest.paths.Rd
index f631dd02c8..d71de3f3a7 100644
--- a/man/get.shortest.paths.Rd
+++ b/man/get.shortest.paths.Rd
@@ -24,7 +24,7 @@ be calculated. Note that right now this is not a vector of vertex IDs, but
 only a single vertex.}
 
 \item{to}{Numeric vector, the vertices to which the shortest paths will be
-calculated. By default it includes all vertices. Note that for
+calculated. The default \code{NULL} includes all vertices. Note that for
 \code{distances()} every vertex must be included here at most once. (This
 is not required for \code{shortest_paths()}.}
 
diff --git a/man/get.stochastic.Rd b/man/get.stochastic.Rd
index 9a0c834553..d5b47e6884 100644
--- a/man/get.stochastic.Rd
+++ b/man/get.stochastic.Rd
@@ -17,7 +17,8 @@ get.stochastic(
 sum up to one; otherwise it is the columns.}
 
 \item{sparse}{Logical, whether to return a sparse matrix. The
-\code{Matrix} package is needed for sparse matrices.}
+\code{Matrix} package is needed for sparse matrices. The default \code{NULL} uses
+the \code{sparsematrices} igraph option.}
 }
 \description{
 \ifelse{html}{\href{https://lifecycle.r-lib.org/articles/stages.html#deprecated}{\figure{lifecycle-deprecated.svg}{options: alt='[Deprecated]'}}}{\strong{[Deprecated]}}
diff --git a/man/get.vertex.attribute.Rd b/man/get.vertex.attribute.Rd
index 77bad90850..b13e8f4446 100644
--- a/man/get.vertex.attribute.Rd
+++ b/man/get.vertex.attribute.Rd
@@ -13,7 +13,7 @@ get.vertex.attribute(graph, name, index = V(graph))
 all vertex attributes are returned in a list.}
 
 \item{index}{An optional vertex sequence to query the attribute only
-for these vertices.}
+for these vertices. The default \code{NULL} selects all vertices.}
 }
 \description{
 \ifelse{html}{\href{https://lifecycle.r-lib.org/articles/stages.html#deprecated}{\figure{lifecycle-deprecated.svg}{options: alt='[Deprecated]'}}}{\strong{[Deprecated]}}
diff --git a/man/global_efficiency.Rd b/man/global_efficiency.Rd
index b24157c080..2a390554fa 100644
--- a/man/global_efficiency.Rd
+++ b/man/global_efficiency.Rd
@@ -10,7 +10,7 @@ global_efficiency(graph, ..., weights = NULL, directed = TRUE)
 
 local_efficiency(
   graph,
-  vids = V(graph),
+  vids = NULL,
   ...,
   weights = NULL,
   directed = TRUE,
@@ -38,7 +38,8 @@ and the graph has a \code{weight} edge attribute, then it is used automatically.
 for undirected graphs.}
 
 \item{vids}{The vertex IDs of the vertices for which the calculation will be done.
-Applies to the local efficiency calculation only.}
+Applies to the local efficiency calculation only. The default \code{NULL}
+selects all vertices.}
 
 \item{mode}{Specifies how to define the local neighborhood of a vertex in
 directed graphs. \dQuote{out} considers out-neighbors only, \dQuote{in}
diff --git a/man/graph.Rd b/man/graph.Rd
index 1502dd379a..25e0b0c35e 100644
--- a/man/graph.Rd
+++ b/man/graph.Rd
@@ -40,7 +40,8 @@ Passed to \code{make_directed_graph()} or \code{make_undirected_graph()}.}
 ignored (with a warning) if \code{edges} are symbolic vertex names. It
 is also ignored if there is a bigger vertex ID in \code{edges}. This
 means that for this function it is safe to supply zero here if the
-vertex with the largest ID is not an isolate.}
+vertex with the largest ID is not an isolate. The default \code{NULL} uses
+the largest vertex ID in \code{edges}.}
 
 \item{isolates}{Character vector, names of isolate vertices,
 for symbolic edge lists. It is ignored for numeric edge lists.}
diff --git a/man/graph.bfs.Rd b/man/graph.bfs.Rd
index cfd41d968d..56642407ca 100644
--- a/man/graph.bfs.Rd
+++ b/man/graph.bfs.Rd
@@ -67,7 +67,8 @@ Default: \code{NULL}.}
 
 \item{extra}{Additional argument to supply to the callback function.}
 
-\item{rho}{The environment in which the callback function is evaluated.}
+\item{rho}{The environment in which the callback function is evaluated.
+The default \code{NULL} uses the caller's environment.}
 
 \item{neimode}{\ifelse{html}{\href{https://lifecycle.r-lib.org/articles/stages.html#deprecated}{\figure{lifecycle-deprecated.svg}{options: alt='[Deprecated]'}}}{\strong{[Deprecated]}} This argument is deprecated
 from igraph 1.3.0; use \code{mode} instead.}
diff --git a/man/graph.compose.Rd b/man/graph.compose.Rd
index b033a85150..7b22c64634 100644
--- a/man/graph.compose.Rd
+++ b/man/graph.compose.Rd
@@ -24,7 +24,7 @@ but not both graphs are named.}
 consistent API.
 }
 \section{Related documentation in the C library}{
-\href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_vcount}{\code{vcount()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Isomorphism.html#igraph_permute_vertices}{\code{permute_vertices()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_edges}{\code{edges()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_get_eids}{\code{get_eids()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_ecount}{\code{ecount()}}
+\href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_vcount}{\code{vcount()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Isomorphism.html#igraph_permute_vertices}{\code{permute_vertices()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_get_eids}{\code{get_eids()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_edges}{\code{edges()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_ecount}{\code{ecount()}}
 }
 
 \keyword{internal}
diff --git a/man/graph.dfs.Rd b/man/graph.dfs.Rd
index 6af1a07a25..cdb45188a1 100644
--- a/man/graph.dfs.Rd
+++ b/man/graph.dfs.Rd
@@ -57,7 +57,8 @@ Default: \code{NULL}.}
 
 \item{extra}{Additional argument to supply to the callback function.}
 
-\item{rho}{The environment in which the callback function is evaluated.}
+\item{rho}{The environment in which the callback function is evaluated.
+The default \code{NULL} uses the caller's environment.}
 
 \item{neimode}{\ifelse{html}{\href{https://lifecycle.r-lib.org/articles/stages.html#deprecated}{\figure{lifecycle-deprecated.svg}{options: alt='[Deprecated]'}}}{\strong{[Deprecated]}} This argument is deprecated from igraph 1.3.0; use
 \code{mode} instead.}
diff --git a/man/graph.disjoint.union.Rd b/man/graph.disjoint.union.Rd
index 860eec15e5..fcd7fbb890 100644
--- a/man/graph.disjoint.union.Rd
+++ b/man/graph.disjoint.union.Rd
@@ -16,7 +16,7 @@ graph.disjoint.union(...)
 consistent API.
 }
 \section{Related documentation in the C library}{
-\href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_vcount}{\code{vcount()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_edges}{\code{edges()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_get_eids}{\code{get_eids()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_ecount}{\code{ecount()}}
+\href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_vcount}{\code{vcount()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_get_eids}{\code{get_eids()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_edges}{\code{edges()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_ecount}{\code{ecount()}}
 }
 
 \keyword{internal}
diff --git a/man/graph.diversity.Rd b/man/graph.diversity.Rd
index 552c78a974..bd1ca92af5 100644
--- a/man/graph.diversity.Rd
+++ b/man/graph.diversity.Rd
@@ -13,7 +13,8 @@ graph.diversity(graph, weights = NULL, vids = V(graph))
 computation. If \code{NULL}, then the \sQuote{weight} attibute is used. Note
 that this measure is not defined for unweighted graphs.}
 
-\item{vids}{The vertex IDs for which to calculate the measure.}
+\item{vids}{The vertex IDs for which to calculate the measure.
+The default \code{NULL} selects all vertices.}
 }
 \description{
 \ifelse{html}{\href{https://lifecycle.r-lib.org/articles/stages.html#deprecated}{\figure{lifecycle-deprecated.svg}{options: alt='[Deprecated]'}}}{\strong{[Deprecated]}}
diff --git a/man/graph.famous.Rd b/man/graph.famous.Rd
index 52c5eff3fb..773e87321a 100644
--- a/man/graph.famous.Rd
+++ b/man/graph.famous.Rd
@@ -40,7 +40,8 @@ Passed to \code{make_directed_graph()} or \code{make_undirected_graph()}.}
 ignored (with a warning) if \code{edges} are symbolic vertex names. It
 is also ignored if there is a bigger vertex ID in \code{edges}. This
 means that for this function it is safe to supply zero here if the
-vertex with the largest ID is not an isolate.}
+vertex with the largest ID is not an isolate. The default \code{NULL} uses
+the largest vertex ID in \code{edges}.}
 
 \item{isolates}{Character vector, names of isolate vertices,
 for symbolic edge lists. It is ignored for numeric edge lists.}
diff --git a/man/graph.knn.Rd b/man/graph.knn.Rd
index 2b4a24c363..70c502f64e 100644
--- a/man/graph.knn.Rd
+++ b/man/graph.knn.Rd
@@ -15,8 +15,8 @@ graph.knn(
 \arguments{
 \item{graph}{The input graph. It may be directed.}
 
-\item{vids}{The vertices for which the calculation is performed. Normally it
-includes all vertices. Note, that if not all vertices are given here, then
+\item{vids}{The vertices for which the calculation is performed.
+The default \code{NULL} includes all vertices. Note, that if not all vertices are given here, then
 both \sQuote{\code{knn}} and \sQuote{\code{knnk}} will be calculated based
 on the given vertices only.}
 
diff --git a/man/graph.neighborhood.Rd b/man/graph.neighborhood.Rd
index 3a52ea1951..2ed769672d 100644
--- a/man/graph.neighborhood.Rd
+++ b/man/graph.neighborhood.Rd
@@ -18,7 +18,8 @@ graph.neighborhood(
 \item{order}{Integer giving the order of the neighborhood. Negative values
 indicate an infinite order.}
 
-\item{nodes}{The vertices for which the calculation is performed.}
+\item{nodes}{The vertices for which the calculation is performed.
+The default \code{NULL} selects all vertices.}
 
 \item{mode}{Character constant, it specifies how to use the direction of
 the edges if a directed graph is analyzed. For \sQuote{out} only the
diff --git a/man/graph.strength.Rd b/man/graph.strength.Rd
index d4cc8e6242..daf41c663f 100644
--- a/man/graph.strength.Rd
+++ b/man/graph.strength.Rd
@@ -15,7 +15,8 @@ graph.strength(
 \arguments{
 \item{graph}{The input graph.}
 
-\item{vids}{The vertices for which the strength will be calculated.}
+\item{vids}{The vertices for which the strength will be calculated.
+The default \code{NULL} selects all vertices.}
 
 \item{mode}{Character string, \dQuote{out} for out-degree, \dQuote{in} for
 in-degree or \dQuote{all} for the sum of the two. For undirected graphs this
diff --git a/man/graph.union.Rd b/man/graph.union.Rd
index 5598a09fa6..a87f5e1a2e 100644
--- a/man/graph.union.Rd
+++ b/man/graph.union.Rd
@@ -22,7 +22,7 @@ graphs are named.}
 consistent API.
 }
 \section{Related documentation in the C library}{
-\href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_vcount}{\code{vcount()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Isomorphism.html#igraph_permute_vertices}{\code{permute_vertices()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_edges}{\code{edges()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_get_eids}{\code{get_eids()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_ecount}{\code{ecount()}}
+\href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_vcount}{\code{vcount()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Isomorphism.html#igraph_permute_vertices}{\code{permute_vertices()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_get_eids}{\code{get_eids()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_edges}{\code{edges()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_ecount}{\code{ecount()}}
 }
 
 \keyword{internal}
diff --git a/man/graph_.Rd b/man/graph_.Rd
index 823d7aab3a..287c1b7b63 100644
--- a/man/graph_.Rd
+++ b/man/graph_.Rd
@@ -16,7 +16,7 @@ This is a generic function to convert R objects to igraph graphs.
 TODO
 }
 \section{Related documentation in the C library}{
-\href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_vcount}{\code{vcount()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Operators.html#igraph_simplify}{\code{simplify()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Structural.html#igraph_is_simple}{\code{is_simple()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_edges}{\code{edges()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_get_eids}{\code{get_eids()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_ecount}{\code{ecount()}}
+\href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_vcount}{\code{vcount()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Operators.html#igraph_simplify}{\code{simplify()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Structural.html#igraph_is_simple}{\code{is_simple()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_get_eids}{\code{get_eids()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_edges}{\code{edges()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_ecount}{\code{ecount()}}
 }
 
 \examples{
diff --git a/man/graphlet_basis.Rd b/man/graphlet_basis.Rd
index 1fd9f5783d..a6e9708624 100644
--- a/man/graphlet_basis.Rd
+++ b/man/graphlet_basis.Rd
@@ -8,14 +8,7 @@
 \usage{
 graphlet_basis(graph, ..., weights = NULL)
 
-graphlet_proj(
-  graph,
-  ...,
-  weights = NULL,
-  cliques,
-  niter = 1000,
-  Mu = rep(1, length(cliques))
-)
+graphlet_proj(graph, ..., weights = NULL, cliques, niter = 1000, Mu = NULL)
 
 graphlets(graph, ..., weights = NULL, niter = 1000)
 }
@@ -34,7 +27,8 @@ projection.}
 
 \item{niter}{Integer scalar, the number of iterations to perform.}
 
-\item{Mu}{Starting weights for the projection.}
+\item{Mu}{Starting weights for the projection. The default \code{NULL} uses a
+weight of one for each clique.}
 }
 \value{
 \code{graphlets()} returns a list with two members:
diff --git a/man/graphlets.project.Rd b/man/graphlets.project.Rd
index dce325e0ea..535c2c5e9c 100644
--- a/man/graphlets.project.Rd
+++ b/man/graphlets.project.Rd
@@ -25,7 +25,8 @@ projection.}
 
 \item{niter}{Integer scalar, the number of iterations to perform.}
 
-\item{Mu}{Starting weights for the projection.}
+\item{Mu}{Starting weights for the projection. The default \code{NULL} uses a
+weight of one for each clique.}
 }
 \description{
 \ifelse{html}{\href{https://lifecycle.r-lib.org/articles/stages.html#deprecated}{\figure{lifecycle-deprecated.svg}{options: alt='[Deprecated]'}}}{\strong{[Deprecated]}}
diff --git a/man/harmonic_centrality.Rd b/man/harmonic_centrality.Rd
index e45621660c..1c93bc61ca 100644
--- a/man/harmonic_centrality.Rd
+++ b/man/harmonic_centrality.Rd
@@ -6,7 +6,7 @@
 \usage{
 harmonic_centrality(
   graph,
-  vids = V(graph),
+  vids = NULL,
   ...,
   mode = c("out", "in", "all", "total"),
   weights = NULL,
@@ -17,7 +17,8 @@ harmonic_centrality(
 \arguments{
 \item{graph}{The graph to analyze.}
 
-\item{vids}{The vertices for which harmonic centrality will be calculated.}
+\item{vids}{The vertices for which harmonic centrality will be calculated.
+The default \code{NULL} selects all vertices.}
 
 \item{...}{These dots are for future extensions and must be empty.}
 
diff --git a/man/hits_scores.Rd b/man/hits_scores.Rd
index 83db1d6116..ee1440b68c 100644
--- a/man/hits_scores.Rd
+++ b/man/hits_scores.Rd
@@ -4,13 +4,7 @@
 \alias{hits_scores}
 \title{Kleinberg's hub and authority centrality scores.}
 \usage{
-hits_scores(
-  graph,
-  ...,
-  scale = TRUE,
-  weights = NULL,
-  options = arpack_defaults()
-)
+hits_scores(graph, ..., scale = TRUE, weights = NULL, options = NULL)
 }
 \arguments{
 \item{graph}{The input graph.}
@@ -28,7 +22,7 @@ interprets edge weights as connection strengths. The weights of parallel
 edges are effectively added up.}
 
 \item{options}{A named list, to override some ARPACK options. See
-\code{\link[=arpack]{arpack()}} for details.}
+\code{\link[=arpack]{arpack()}} for details. The default \code{NULL} uses \code{\link[=arpack_defaults]{arpack_defaults()}}.}
 }
 \value{
 A named list with members:
diff --git a/man/indent_print.Rd b/man/indent_print.Rd
index 44dc63c544..abb8662347 100644
--- a/man/indent_print.Rd
+++ b/man/indent_print.Rd
@@ -4,14 +4,15 @@
 \alias{indent_print}
 \title{Indent a printout}
 \usage{
-indent_print(..., .indent = " ", .printer = print)
+indent_print(..., .indent = " ", .printer = NULL)
 }
 \arguments{
 \item{...}{Passed to the printing function.}
 
 \item{.indent}{Character scalar, indent the printout with this.}
 
-\item{.printer}{The printing function, defaults to \link{print}.}
+\item{.printer}{The printing function. The default \code{NULL} uses
+\link{print}.}
 }
 \value{
 The first element in \code{...}, invisibly.
diff --git a/man/intersection.igraph.Rd b/man/intersection.igraph.Rd
index 5dffbafe55..6a07aae79f 100644
--- a/man/intersection.igraph.Rd
+++ b/man/intersection.igraph.Rd
@@ -65,7 +65,7 @@ An error is generated if some input graphs are directed and others are
 undirected.
 }
 \section{Related documentation in the C library}{
-\href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_vcount}{\code{vcount()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Isomorphism.html#igraph_permute_vertices}{\code{permute_vertices()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_edges}{\code{edges()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_get_eids}{\code{get_eids()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_ecount}{\code{ecount()}}
+\href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_vcount}{\code{vcount()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Isomorphism.html#igraph_permute_vertices}{\code{permute_vertices()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_get_eids}{\code{get_eids()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_edges}{\code{edges()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_ecount}{\code{ecount()}}
 }
 
 \examples{
diff --git a/man/is.loop.Rd b/man/is.loop.Rd
index afbc31e9cc..ce99c88946 100644
--- a/man/is.loop.Rd
+++ b/man/is.loop.Rd
@@ -9,8 +9,8 @@ is.loop(graph, eids = E(graph))
 \arguments{
 \item{graph}{The input graph.}
 
-\item{eids}{The edges to which the query is restricted. By default this is
-all edges in the graph.}
+\item{eids}{The edges to which the query is restricted. The default
+\code{NULL} selects all edges.}
 }
 \description{
 \ifelse{html}{\href{https://lifecycle.r-lib.org/articles/stages.html#deprecated}{\figure{lifecycle-deprecated.svg}{options: alt='[Deprecated]'}}}{\strong{[Deprecated]}}
diff --git a/man/is.multiple.Rd b/man/is.multiple.Rd
index 267db47fed..8ea9bd2d76 100644
--- a/man/is.multiple.Rd
+++ b/man/is.multiple.Rd
@@ -9,8 +9,8 @@ is.multiple(graph, eids = E(graph))
 \arguments{
 \item{graph}{The input graph.}
 
-\item{eids}{The edges to which the query is restricted. By default this is
-all edges in the graph.}
+\item{eids}{The edges to which the query is restricted. The default
+\code{NULL} selects all edges.}
 }
 \description{
 \ifelse{html}{\href{https://lifecycle.r-lib.org/articles/stages.html#deprecated}{\figure{lifecycle-deprecated.svg}{options: alt='[Deprecated]'}}}{\strong{[Deprecated]}}
diff --git a/man/is.mutual.Rd b/man/is.mutual.Rd
index 099975f9e2..321108ac67 100644
--- a/man/is.mutual.Rd
+++ b/man/is.mutual.Rd
@@ -9,8 +9,8 @@ is.mutual(graph, eids = E(graph), loops = TRUE)
 \arguments{
 \item{graph}{The input graph.}
 
-\item{eids}{Edge sequence, the edges that will be probed. By default is
-includes all edges in the order of their IDs.}
+\item{eids}{Edge sequence, the edges that will be probed. The default
+\code{NULL} includes all edges in the order of their IDs.}
 
 \item{loops}{Logical, whether to consider directed self-loops to be mutual.}
 }
diff --git a/man/knn.Rd b/man/knn.Rd
index fb25f706c9..96ba5158ec 100644
--- a/man/knn.Rd
+++ b/man/knn.Rd
@@ -6,7 +6,7 @@
 \usage{
 knn(
   graph,
-  vids = V(graph),
+  vids = NULL,
   ...,
   mode = c("all", "out", "in", "total"),
   neighbor.degree.mode = c("all", "out", "in", "total"),
@@ -16,8 +16,8 @@ knn(
 \arguments{
 \item{graph}{The input graph. It may be directed.}
 
-\item{vids}{The vertices for which the calculation is performed. Normally it
-includes all vertices. Note, that if not all vertices are given here, then
+\item{vids}{The vertices for which the calculation is performed.
+The default \code{NULL} includes all vertices. Note, that if not all vertices are given here, then
 both \sQuote{\code{knn}} and \sQuote{\code{knnk}} will be calculated based
 on the given vertices only.}
 
diff --git a/man/lastcit.game.Rd b/man/lastcit.game.Rd
index ec2be85128..538998802c 100644
--- a/man/lastcit.game.Rd
+++ b/man/lastcit.game.Rd
@@ -17,11 +17,13 @@ lastcit.game(
 
 \item{edges}{Number of edges per step.}
 
-\item{agebins}{Number of aging bins.}
+\item{agebins}{Number of aging bins. The default \code{NULL} uses \code{n / 7100}.}
 
 \item{pref}{Vector (\code{sample_last_cit()} and \code{sample_cit_types()} or
 matrix (\code{sample_cit_cit_types()}) giving the (unnormalized) citation
-probabilities for the different vertex types.}
+probabilities for the different vertex types. The default \code{NULL} uses
+\code{(1:(agebins + 1))^-3} for \code{sample_last_cit()} and all-one probabilities
+for the other two.}
 
 \item{directed}{Logical, whether to generate directed networks.}
 }
diff --git a/man/layout.davidson.harel.Rd b/man/layout.davidson.harel.Rd
index 28416461c1..2df57ce460 100644
--- a/man/layout.davidson.harel.Rd
+++ b/man/layout.davidson.harel.Rd
@@ -26,7 +26,8 @@ coordinates.}
 
 \item{maxiter}{Number of iterations to perform in the first phase.}
 
-\item{fineiter}{Number of iterations in the fine tuning phase.}
+\item{fineiter}{Number of iterations in the fine tuning phase. The
+default \code{NULL} uses \code{max(10, log2(vcount(graph)))}.}
 
 \item{cool.fact}{Cooling factor.}
 
@@ -38,13 +39,15 @@ the energy function. It can be set to zero, if vertices are allowed to sit
 on the border.}
 
 \item{weight.edge.lengths}{Weight for the edge length component of the
-energy function.}
+energy function. The default \code{NULL} uses \code{edge_density(graph) / 10}.}
 
 \item{weight.edge.crossings}{Weight for the edge crossing component of the
-energy function.}
+energy function. The default \code{NULL} uses
+\code{1 - sqrt(edge_density(graph))}.}
 
 \item{weight.node.edge.dist}{Weight for the node-edge distance component of
-the energy function.}
+the energy function. The default \code{NULL} uses
+\code{0.2 * (1 - edge_density(graph))}.}
 }
 \description{
 \ifelse{html}{\href{https://lifecycle.r-lib.org/articles/stages.html#deprecated}{\figure{lifecycle-deprecated.svg}{options: alt='[Deprecated]'}}}{\strong{[Deprecated]}}
diff --git a/man/layout.drl.Rd b/man/layout.drl.Rd
index f8c21417d8..2a6be6f177 100644
--- a/man/layout.drl.Rd
+++ b/man/layout.drl.Rd
@@ -20,10 +20,11 @@ layout.drl(
 \code{seed} argument as a starting point.}
 
 \item{seed}{A matrix with two columns, the starting coordinates for the
-vertices is \code{use.seed} is \code{TRUE}. It is ignored otherwise.}
+vertices is \code{use.seed} is \code{TRUE}. It is ignored otherwise. The default
+\code{NULL} draws uniformly random starting coordinates.}
 
 \item{options}{Options for the layout generator, a named list. See details
-below.}
+below. The default \code{NULL} uses \code{drl_defaults$default}.}
 
 \item{weights}{The weights of the edges. It must be a positive numeric vector,
 \code{NULL} or \code{NA}. If it is \code{NULL} and the input graph has a
diff --git a/man/layout.gem.Rd b/man/layout.gem.Rd
index f71b585e24..9046cfc733 100644
--- a/man/layout.gem.Rd
+++ b/man/layout.gem.Rd
@@ -21,19 +21,19 @@ depending on the \code{dim} argument.
 Default: \code{NULL}.}
 
 \item{maxiter}{The maximum number of iterations to perform. Updating a
-single vertex counts as an iteration.  A reasonable default is 40 * n * n,
+single vertex counts as an iteration. The default \code{NULL} uses 40 * n * n,
 where n is the number of vertices. The original paper suggests 4 * n * n,
 but this usually only works if the other parameters are set up carefully.}
 
-\item{temp.max}{The maximum allowed local temperature. A reasonable default
-is the number of vertices.}
+\item{temp.max}{The maximum allowed local temperature. The default \code{NULL}
+uses the number of vertices.}
 
 \item{temp.min}{The global temperature at which the algorithm terminates
 (even before reaching \code{maxiter} iterations). A reasonable default is
 1/10.}
 
-\item{temp.init}{Initial local temperature of all vertices. A reasonable
-default is the square root of the number of vertices.}
+\item{temp.init}{Initial local temperature of all vertices. The default
+\code{NULL} uses the square root of the number of vertices.}
 }
 \description{
 \ifelse{html}{\href{https://lifecycle.r-lib.org/articles/stages.html#deprecated}{\figure{lifecycle-deprecated.svg}{options: alt='[Deprecated]'}}}{\strong{[Deprecated]}}
diff --git a/man/layout.star.Rd b/man/layout.star.Rd
index 426ec88cbc..7cb3fbf768 100644
--- a/man/layout.star.Rd
+++ b/man/layout.star.Rd
@@ -9,8 +9,8 @@ layout.star(graph, center = V(graph)[1], order = NULL)
 \arguments{
 \item{graph}{The graph to layout.}
 
-\item{center}{The ID of the vertex to put in the center. By default it is
-the first vertex.}
+\item{center}{The ID of the vertex to put in the center. The default
+\code{NULL} uses the first vertex.}
 
 \item{order}{Numeric vector, the order of the vertices along the perimeter.
 The default ordering is given by the vertex IDs.}
diff --git a/man/layout_as_star.Rd b/man/layout_as_star.Rd
index 777bdfcbd0..5e488de3a9 100644
--- a/man/layout_as_star.Rd
+++ b/man/layout_as_star.Rd
@@ -5,7 +5,7 @@
 \alias{as_star}
 \title{Generate coordinates to place the vertices of a graph in a star-shape}
 \usage{
-layout_as_star(graph, ..., center = V(graph)[1], order = NULL)
+layout_as_star(graph, ..., center = NULL, order = NULL)
 
 as_star(...)
 }
@@ -14,8 +14,8 @@ as_star(...)
 
 \item{...}{Arguments to pass to \code{layout_as_star()}.}
 
-\item{center}{The ID of the vertex to put in the center. By default it is
-the first vertex.}
+\item{center}{The ID of the vertex to put in the center. The default
+\code{NULL} uses the first vertex.}
 
 \item{order}{Numeric vector, the order of the vertices along the perimeter.
 The default ordering is given by the vertex IDs.}
diff --git a/man/layout_in_circle.Rd b/man/layout_in_circle.Rd
index e2d51dd08f..4fda9824f9 100644
--- a/man/layout_in_circle.Rd
+++ b/man/layout_in_circle.Rd
@@ -5,7 +5,7 @@
 \alias{in_circle}
 \title{Graph layout with vertices on a circle.}
 \usage{
-layout_in_circle(graph, order = V(graph))
+layout_in_circle(graph, order = NULL)
 
 in_circle(...)
 }
@@ -14,7 +14,8 @@ in_circle(...)
 
 \item{order}{The vertices to place on the circle, in the order of their
 desired placement. Vertices that are not included here will be placed at
-(0,0).}
+(0,0). The default \code{NULL} selects all vertices, in the order of their
+IDs.}
 
 \item{...}{Passed to \code{layout_in_circle()}.}
 }
diff --git a/man/layout_with_dh.Rd b/man/layout_with_dh.Rd
index c9c3a90ec2..107e502166 100644
--- a/man/layout_with_dh.Rd
+++ b/man/layout_with_dh.Rd
@@ -10,13 +10,13 @@ layout_with_dh(
   ...,
   coords = NULL,
   maxiter = 10,
-  fineiter = max(10, log2(vcount(graph))),
+  fineiter = NULL,
   cool.fact = 0.75,
   weight.node.dist = 1,
   weight.border = 0,
-  weight.edge.lengths = edge_density(graph)/10,
-  weight.edge.crossings = 1 - sqrt(edge_density(graph)),
-  weight.node.edge.dist = 0.2 * (1 - edge_density(graph))
+  weight.edge.lengths = NULL,
+  weight.edge.crossings = NULL,
+  weight.node.edge.dist = NULL
 )
 
 with_dh(...)
@@ -32,7 +32,8 @@ coordinates.}
 
 \item{maxiter}{Number of iterations to perform in the first phase.}
 
-\item{fineiter}{Number of iterations in the fine tuning phase.}
+\item{fineiter}{Number of iterations in the fine tuning phase. The
+default \code{NULL} uses \code{max(10, log2(vcount(graph)))}.}
 
 \item{cool.fact}{Cooling factor.}
 
@@ -44,13 +45,15 @@ the energy function. It can be set to zero, if vertices are allowed to sit
 on the border.}
 
 \item{weight.edge.lengths}{Weight for the edge length component of the
-energy function.}
+energy function. The default \code{NULL} uses \code{edge_density(graph) / 10}.}
 
 \item{weight.edge.crossings}{Weight for the edge crossing component of the
-energy function.}
+energy function. The default \code{NULL} uses
+\code{1 - sqrt(edge_density(graph))}.}
 
 \item{weight.node.edge.dist}{Weight for the node-edge distance component of
-the energy function.}
+the energy function. The default \code{NULL} uses
+\code{0.2 * (1 - edge_density(graph))}.}
 }
 \value{
 A matrix with two columns, containing the x and y coordinates
diff --git a/man/layout_with_drl.Rd b/man/layout_with_drl.Rd
index c5e17f7cd0..e7441d81fa 100644
--- a/man/layout_with_drl.Rd
+++ b/man/layout_with_drl.Rd
@@ -15,8 +15,8 @@ layout_with_drl(
   graph,
   ...,
   use.seed = FALSE,
-  seed = matrix(runif(vcount(graph) * 2), ncol = 2),
-  options = drl_defaults$default,
+  seed = NULL,
+  options = NULL,
   weights = NULL,
   dim = c(2, 3)
 )
@@ -32,10 +32,11 @@ with_drl(...)
 \code{seed} argument as a starting point.}
 
 \item{seed}{A matrix with two columns, the starting coordinates for the
-vertices is \code{use.seed} is \code{TRUE}. It is ignored otherwise.}
+vertices is \code{use.seed} is \code{TRUE}. It is ignored otherwise. The default
+\code{NULL} draws uniformly random starting coordinates.}
 
 \item{options}{Options for the layout generator, a named list. See details
-below.}
+below. The default \code{NULL} uses \code{drl_defaults$default}.}
 
 \item{weights}{The weights of the edges. It must be a positive numeric vector,
 \code{NULL} or \code{NA}. If it is \code{NULL} and the input graph has a
diff --git a/man/layout_with_fr.Rd b/man/layout_with_fr.Rd
index 6627fbf1c4..90f32995dc 100644
--- a/man/layout_with_fr.Rd
+++ b/man/layout_with_fr.Rd
@@ -11,7 +11,7 @@ layout_with_fr(
   coords = NULL,
   dim = c(2, 3),
   niter = 500,
-  start.temp = sqrt(vcount(graph)),
+  start.temp = NULL,
   grid = c("auto", "grid", "nogrid"),
   weights = NULL,
   minx = NULL,
@@ -46,7 +46,8 @@ space.}
 
 \item{start.temp}{Real scalar, the start temperature. This is the maximum
 amount of movement alloved along one axis, within one step, for a vertex.
-Currently it is decreased linearly to zero during the iteration.}
+Currently it is decreased linearly to zero during the iteration. The
+default \code{NULL} uses \code{sqrt(vcount(graph))}.}
 
 \item{grid}{Character scalar, whether to use the faster, but less accurate
 grid based implementation of the algorithm. By default (\dQuote{auto}), the
diff --git a/man/layout_with_gem.Rd b/man/layout_with_gem.Rd
index f4b36cb3c9..d0d906cc43 100644
--- a/man/layout_with_gem.Rd
+++ b/man/layout_with_gem.Rd
@@ -9,10 +9,10 @@ layout_with_gem(
   graph,
   ...,
   coords = NULL,
-  maxiter = 40 * vcount(graph)^2,
-  temp.max = max(vcount(graph), 1),
-  temp.min = 1/10,
-  temp.init = sqrt(max(vcount(graph), 1))
+  maxiter = NULL,
+  temp.max = NULL,
+  temp.min = 0.1,
+  temp.init = NULL
 )
 
 with_gem(...)
@@ -27,19 +27,19 @@ depending on the \code{dim} argument.
 Default: \code{NULL}.}
 
 \item{maxiter}{The maximum number of iterations to perform. Updating a
-single vertex counts as an iteration.  A reasonable default is 40 * n * n,
+single vertex counts as an iteration. The default \code{NULL} uses 40 * n * n,
 where n is the number of vertices. The original paper suggests 4 * n * n,
 but this usually only works if the other parameters are set up carefully.}
 
-\item{temp.max}{The maximum allowed local temperature. A reasonable default
-is the number of vertices.}
+\item{temp.max}{The maximum allowed local temperature. The default \code{NULL}
+uses the number of vertices.}
 
 \item{temp.min}{The global temperature at which the algorithm terminates
 (even before reaching \code{maxiter} iterations). A reasonable default is
 1/10.}
 
-\item{temp.init}{Initial local temperature of all vertices. A reasonable
-default is the square root of the number of vertices.}
+\item{temp.init}{Initial local temperature of all vertices. The default
+\code{NULL} uses the square root of the number of vertices.}
 }
 \value{
 A numeric matrix with two columns, and as many rows as the number of
diff --git a/man/layout_with_kk.Rd b/man/layout_with_kk.Rd
index 5b4278c1c9..7d9d6dfda7 100644
--- a/man/layout_with_kk.Rd
+++ b/man/layout_with_kk.Rd
@@ -10,9 +10,9 @@ layout_with_kk(
   ...,
   coords = NULL,
   dim = c(2, 3),
-  maxiter = 50 * vcount(graph),
+  maxiter = NULL,
   epsilon = 0,
-  kkconst = max(vcount(graph), 1),
+  kkconst = NULL,
   weights = NULL,
   minx = NULL,
   maxx = NULL,
@@ -43,7 +43,8 @@ dimensional layouts are places on a plane, three dimensional ones in the 3d
 space.}
 
 \item{maxiter}{The maximum number of iterations to perform. The algorithm
-might terminate earlier, see the \code{epsilon} argument.}
+might terminate earlier, see the \code{epsilon} argument. The default \code{NULL}
+uses \code{50 * vcount(graph)}.}
 
 \item{epsilon}{Numeric scalar, the algorithm terminates, if the maximal
 delta is less than this. (See the reference below for what delta means.) If
@@ -51,7 +52,7 @@ you set this to zero, then the function always performs \code{maxiter}
 iterations.}
 
 \item{kkconst}{Numeric scalar, the Kamada-Kawai vertex attraction constant.
-Typical (and default) value is the number of vertices.}
+The default \code{NULL} uses the number of vertices.}
 
 \item{weights}{Edge weights, larger values will result in longer edges.
 Note that this is the opposite of \code{\link[=layout_with_fr]{layout_with_fr()}}, which produces
diff --git a/man/layout_with_lgl.Rd b/man/layout_with_lgl.Rd
index 7dd652c3f0..ee5fa9bfd3 100644
--- a/man/layout_with_lgl.Rd
+++ b/man/layout_with_lgl.Rd
@@ -9,11 +9,11 @@ layout_with_lgl(
   graph,
   ...,
   maxiter = 150,
-  maxdelta = vcount(graph),
-  area = vcount(graph)^2,
+  maxdelta = NULL,
+  area = NULL,
   coolexp = 1.5,
-  repulserad = area * vcount(graph),
-  cellsize = sqrt(sqrt(area)),
+  repulserad = NULL,
+  cellsize = NULL,
   root = NULL
 )
 
@@ -26,21 +26,21 @@ with_lgl(...)
 
 \item{maxiter}{The maximum number of iterations to perform (150).}
 
-\item{maxdelta}{The maximum change for a vertex during an iteration (the
-number of vertices).}
+\item{maxdelta}{The maximum change for a vertex during an iteration. The
+default \code{NULL} uses the number of vertices.}
 
-\item{area}{The area of the surface on which the vertices are placed (square
-of the number of vertices).}
+\item{area}{The area of the surface on which the vertices are placed. The
+default \code{NULL} uses the square of the number of vertices.}
 
 \item{coolexp}{The cooling exponent of the simulated annealing (1.5).}
 
-\item{repulserad}{Cancellation radius for the repulsion (the \code{area}
-times the number of vertices).}
+\item{repulserad}{Cancellation radius for the repulsion. The default
+\code{NULL} uses the \code{area} times the number of vertices.}
 
 \item{cellsize}{The size of the cells for the grid. When calculating the
 repulsion forces between vertices only vertices in the same or neighboring
-grid cells are taken into account (the fourth root of the number of
-\code{area}.}
+grid cells are taken into account. The default \code{NULL} uses the square
+root of the square root of the \code{area}.}
 
 \item{root}{The ID of the vertex to place at the middle of the layout. The
 default value is -1 which means that a random vertex is selected.}
diff --git a/man/make_.Rd b/man/make_.Rd
index f9ff4b5cf8..4bb6dfcaba 100644
--- a/man/make_.Rd
+++ b/man/make_.Rd
@@ -31,7 +31,7 @@ to the newly created graphs.
 See the examples and the various constructor modifiers below.
 }
 \section{Related documentation in the C library}{
-\href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_vcount}{\code{vcount()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Operators.html#igraph_simplify}{\code{simplify()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Structural.html#igraph_is_simple}{\code{is_simple()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_edges}{\code{edges()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_get_eids}{\code{get_eids()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_ecount}{\code{ecount()}}
+\href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_vcount}{\code{vcount()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Operators.html#igraph_simplify}{\code{simplify()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Structural.html#igraph_is_simple}{\code{is_simple()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_get_eids}{\code{get_eids()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_edges}{\code{edges()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_ecount}{\code{ecount()}}
 }
 
 \examples{
diff --git a/man/make_graph.Rd b/man/make_graph.Rd
index 37c8cb01f0..ccf05ddeb6 100644
--- a/man/make_graph.Rd
+++ b/man/make_graph.Rd
@@ -12,16 +12,16 @@
 make_graph(
   edges,
   ...,
-  n = max(edges),
+  n = NULL,
   isolates = NULL,
   directed = TRUE,
-  dir = directed,
+  dir = NULL,
   simplify = TRUE
 )
 
-make_directed_graph(edges, n = max(edges))
+make_directed_graph(edges, n = NULL)
 
-make_undirected_graph(edges, n = max(edges))
+make_undirected_graph(edges, n = NULL)
 
 directed_graph(...)
 
@@ -53,7 +53,8 @@ Passed to \code{make_directed_graph()} or \code{make_undirected_graph()}.}
 ignored (with a warning) if \code{edges} are symbolic vertex names. It
 is also ignored if there is a bigger vertex ID in \code{edges}. This
 means that for this function it is safe to supply zero here if the
-vertex with the largest ID is not an isolate.}
+vertex with the largest ID is not an isolate. The default \code{NULL} uses
+the largest vertex ID in \code{edges}.}
 
 \item{isolates}{Character vector, names of isolate vertices,
 for symbolic edge lists. It is ignored for numeric edge lists.}
diff --git a/man/matching.Rd b/man/matching.Rd
index de15b86346..c99eac7679 100644
--- a/man/matching.Rd
+++ b/man/matching.Rd
@@ -10,13 +10,7 @@ is_matching(graph, matching, types = NULL)
 
 is_max_matching(graph, matching, types = NULL)
 
-max_bipartite_match(
-  graph,
-  types = NULL,
-  ...,
-  weights = NULL,
-  eps = .Machine$double.eps
-)
+max_bipartite_match(graph, types = NULL, ..., weights = NULL, eps = NULL)
 }
 \arguments{
 \item{graph}{The input graph. It might be directed, but edge directions will
@@ -40,9 +34,9 @@ much as possible.}
 \item{eps}{A small real number used in equality tests in the weighted
 bipartite matching algorithm. Two real numbers are considered equal in
 the algorithm if their difference is smaller than \code{eps}. This is
-required to avoid the accumulation of numerical errors. By default it is
-set to the smallest \eqn{x}, such that \eqn{1+x \ne 1}{1+x != 1}
-holds. If you are running the algorithm with no weights, this argument
+required to avoid the accumulation of numerical errors. The default
+\code{NULL} stands for the smallest \eqn{x}, such that
+\eqn{1+x \ne 1}{1+x != 1} holds (\code{.Machine$double.eps}). If you are running the algorithm with no weights, this argument
 is ignored.}
 }
 \value{
diff --git a/man/maximum.bipartite.matching.Rd b/man/maximum.bipartite.matching.Rd
index ca6b06fa7a..ee95c7a307 100644
--- a/man/maximum.bipartite.matching.Rd
+++ b/man/maximum.bipartite.matching.Rd
@@ -27,9 +27,9 @@ much as possible.}
 \item{eps}{A small real number used in equality tests in the weighted
 bipartite matching algorithm. Two real numbers are considered equal in
 the algorithm if their difference is smaller than \code{eps}. This is
-required to avoid the accumulation of numerical errors. By default it is
-set to the smallest \eqn{x}, such that \eqn{1+x \ne 1}{1+x != 1}
-holds. If you are running the algorithm with no weights, this argument
+required to avoid the accumulation of numerical errors. The default
+\code{NULL} stands for the smallest \eqn{x}, such that
+\eqn{1+x \ne 1}{1+x != 1} holds (\code{.Machine$double.eps}). If you are running the algorithm with no weights, this argument
 is ignored.}
 }
 \description{
diff --git a/man/merge_coords.Rd b/man/merge_coords.Rd
index 2d3d3e4a89..593289a8cc 100644
--- a/man/merge_coords.Rd
+++ b/man/merge_coords.Rd
@@ -7,7 +7,7 @@
 \usage{
 merge_coords(graphs, layouts, ..., method = "dla")
 
-layout_components(graph, layout = layout_with_kk, ...)
+layout_components(graph, layout = NULL, ...)
 }
 \arguments{
 \item{graphs}{A list of graph objects.}
@@ -22,7 +22,8 @@ function.}
 
 \item{graph}{The input graph.}
 
-\item{layout}{A function object, the layout function to use.}
+\item{layout}{A function object, the layout function to use. The default
+\code{NULL} uses \code{layout_with_kk}.}
 }
 \value{
 A matrix with two columns and as many lines as the total number of
diff --git a/man/neighborhood.size.Rd b/man/neighborhood.size.Rd
index f4d6722866..3264876a6f 100644
--- a/man/neighborhood.size.Rd
+++ b/man/neighborhood.size.Rd
@@ -18,7 +18,8 @@ neighborhood.size(
 \item{order}{Integer giving the order of the neighborhood. Negative values
 indicate an infinite order.}
 
-\item{nodes}{The vertices for which the calculation is performed.}
+\item{nodes}{The vertices for which the calculation is performed.
+The default \code{NULL} selects all vertices.}
 
 \item{mode}{Character constant, it specifies how to use the direction of
 the edges if a directed graph is analyzed. For \sQuote{out} only the
diff --git a/man/normalize.Rd b/man/normalize.Rd
index 76a002ee70..c45f3047ca 100644
--- a/man/normalize.Rd
+++ b/man/normalize.Rd
@@ -7,18 +7,22 @@
 normalize(
   xmin = -1,
   xmax = 1,
-  ymin = xmin,
-  ymax = xmax,
-  zmin = xmin,
-  zmax = xmax
+  ymin = NULL,
+  ymax = NULL,
+  zmin = NULL,
+  zmax = NULL
 )
 }
 \arguments{
 \item{xmin, xmax}{Minimum and maximum for x coordinates.}
 
-\item{ymin, ymax}{Minimum and maximum for y coordinates.}
+\item{ymin, ymax}{Minimum and maximum for y coordinates. When omitted,
+they follow \code{xmin} and \code{xmax}; \code{NULL} disables normalization along
+this axis.}
 
-\item{zmin, zmax}{Minimum and maximum for z coordinates.}
+\item{zmin, zmax}{Minimum and maximum for z coordinates. When omitted,
+they follow \code{xmin} and \code{xmax}; \code{NULL} disables normalization along
+this axis.}
 }
 \description{
 Scale coordinates of a layout.
diff --git a/man/page.rank.Rd b/man/page.rank.Rd
index ad32dc1ed6..0c81f6fdfc 100644
--- a/man/page.rank.Rd
+++ b/man/page.rank.Rd
@@ -27,7 +27,8 @@ for all but small graphs.  \code{"arpack"} uses the ARPACK library, the
 default implementation from igraph version 0.5 until version 0.7. It computes
 PageRank scores by solving an eingevalue problem.}
 
-\item{vids}{The vertices of interest.}
+\item{vids}{The vertices of interest.
+The default \code{NULL} selects all vertices.}
 
 \item{directed}{Logical, if true directed paths will be considered for
 directed graphs. It is ignored for undirected graphs.}
diff --git a/man/page_rank.Rd b/man/page_rank.Rd
index d10c8c1c2d..58937b15df 100644
--- a/man/page_rank.Rd
+++ b/man/page_rank.Rd
@@ -8,7 +8,7 @@ page_rank(
   graph,
   ...,
   algo = c("prpack", "arpack"),
-  vids = V(graph),
+  vids = NULL,
   directed = TRUE,
   damping = 0.85,
   personalized = NULL,
@@ -30,7 +30,8 @@ for all but small graphs.  \code{"arpack"} uses the ARPACK library, the
 default implementation from igraph version 0.5 until version 0.7. It computes
 PageRank scores by solving an eingevalue problem.}
 
-\item{vids}{The vertices of interest.}
+\item{vids}{The vertices of interest.
+The default \code{NULL} selects all vertices.}
 
 \item{directed}{Logical, if true directed paths will be considered for
 directed graphs. It is ignored for undirected graphs.}
diff --git a/man/piecewise.layout.Rd b/man/piecewise.layout.Rd
index 50ca54cc9e..71052f144a 100644
--- a/man/piecewise.layout.Rd
+++ b/man/piecewise.layout.Rd
@@ -9,7 +9,8 @@ piecewise.layout(graph, layout = layout_with_kk, ...)
 \arguments{
 \item{graph}{The input graph.}
 
-\item{layout}{A function object, the layout function to use.}
+\item{layout}{A function object, the layout function to use. The default
+\code{NULL} uses \code{layout_with_kk}.}
 
 \item{...}{Additional arguments to pass to the \code{layout} layout
 function.}
diff --git a/man/plot.igraph.Rd b/man/plot.igraph.Rd
index 71fc17e8e4..a2eb1a656b 100644
--- a/man/plot.igraph.Rd
+++ b/man/plot.igraph.Rd
@@ -12,9 +12,9 @@
   xlim = NULL,
   ylim = NULL,
   mark.groups = list(),
-  mark.shape = 1/2,
-  mark.col = rainbow(length(mark.groups), alpha = 0.3),
-  mark.border = rainbow(length(mark.groups), alpha = 1),
+  mark.shape = 0.5,
+  mark.col = NULL,
+  mark.border = NULL,
   mark.expand = 15,
   mark.lwd = 1,
   loop.size = 1,
@@ -48,11 +48,12 @@ used for the different vertex groups.}
 
 \item{mark.col}{A scalar or vector giving the colors of marking the
 polygons, in any format accepted by \code{\link[graphics:xspline]{graphics::xspline()}}; e.g.
-numeric color IDs, symbolic color names, or colors in RGB.}
+numeric color IDs, symbolic color names, or colors in RGB. The default
+\code{NULL} uses semi-transparent rainbow colors.}
 
 \item{mark.border}{A scalar or vector giving the colors of the borders of
 the vertex group marking polygons. If it is \code{NA}, then no border is
-drawn.}
+drawn. The default \code{NULL} uses rainbow colors.}
 
 \item{mark.expand}{A numeric scalar or vector, the size of the border around
 the marked vertex groups. It is in the same units as the vertex sizes. If a
diff --git a/man/plotHierarchy.Rd b/man/plotHierarchy.Rd
index 9ed2994130..81fb79b068 100644
--- a/man/plotHierarchy.Rd
+++ b/man/plotHierarchy.Rd
@@ -12,8 +12,8 @@ plotHierarchy(
 }
 \arguments{
 \item{layout}{The layout of a plot, it is simply passed on to
-\code{plot.igraph()}, see the possible formats there. By default the
-Reingold-Tilford layout generator is used.}
+\code{plot.igraph()}, see the possible formats there. The default \code{NULL} uses
+the Reingold-Tilford layout generator.}
 
 \item{...}{Additional arguments. \code{plot_hierarchy()} and \code{\link[=plot]{plot()}} pass
 them to \code{plot.igraph()}.  \code{\link[=print]{print()}} and \code{\link[=summary]{summary()}} ignore them.}
diff --git a/man/plot_dendrogram.communities.Rd b/man/plot_dendrogram.communities.Rd
index 33a205c286..0b8bc7f9ff 100644
--- a/man/plot_dendrogram.communities.Rd
+++ b/man/plot_dendrogram.communities.Rd
@@ -5,11 +5,11 @@
 \alias{plot_dendrogram.communities}
 \title{Community structure dendrogram plots}
 \usage{
-plot_dendrogram(x, mode = igraph_opt("dend.plot.type"), ...)
+plot_dendrogram(x, mode = NULL, ...)
 
 \method{plot_dendrogram}{communities}(
   x,
-  mode = igraph_opt("dend.plot.type"),
+  mode = NULL,
   ...,
   use.modularity = FALSE,
   palette = categorical_pal(8)
@@ -19,7 +19,8 @@ plot_dendrogram(x, mode = igraph_opt("dend.plot.type"), ...)
 \item{x}{An object containing the community structure of a graph. See
 \code{\link[=communities]{communities()}} for details.}
 
-\item{mode}{Which dendrogram plotting function to use. See details below.}
+\item{mode}{Which dendrogram plotting function to use. See details below.
+The default \code{NULL} uses the \code{dend.plot.type} igraph option.}
 
 \item{\dots}{Additional arguments to supply to the dendrogram plotting
 function.}
diff --git a/man/plot_dendrogram.igraphHRG.Rd b/man/plot_dendrogram.igraphHRG.Rd
index fd00e13ada..caf346887d 100644
--- a/man/plot_dendrogram.igraphHRG.Rd
+++ b/man/plot_dendrogram.igraphHRG.Rd
@@ -4,13 +4,14 @@
 \alias{plot_dendrogram.igraphHRG}
 \title{HRG dendrogram plot}
 \usage{
-\method{plot_dendrogram}{igraphHRG}(x, mode = igraph_opt("dend.plot.type"), ...)
+\method{plot_dendrogram}{igraphHRG}(x, mode = NULL, ...)
 }
 \arguments{
 \item{x}{An \code{igraphHRG}, a hierarchical random graph, as returned by
 the \code{\link[=fit_hrg]{fit_hrg()}} function.}
 
-\item{mode}{Which dendrogram plotting function to use. See details below.}
+\item{mode}{Which dendrogram plotting function to use. See details below.
+The default \code{NULL} uses the \code{dend.plot.type} igraph option.}
 
 \item{\dots}{Additional arguments to supply to the dendrogram plotting
 function.}
diff --git a/man/power_centrality.Rd b/man/power_centrality.Rd
index 2549e592ea..a471df4bd1 100644
--- a/man/power_centrality.Rd
+++ b/man/power_centrality.Rd
@@ -6,7 +6,7 @@
 \usage{
 power_centrality(
   graph,
-  nodes = V(graph),
+  nodes = NULL,
   ...,
   loops = FALSE,
   exponent = 1,
@@ -20,7 +20,7 @@ power_centrality(
 \item{graph}{the input graph.}
 
 \item{nodes}{vertex sequence indicating which vertices are to be included in
-the calculation.  By default, all vertices are included.}
+the calculation. The default \code{NULL} selects all vertices.}
 
 \item{...}{These dots are for future extensions and must be empty.}
 
diff --git a/man/preference.game.Rd b/man/preference.game.Rd
index 736009aab8..d55dea8377 100644
--- a/man/preference.game.Rd
+++ b/man/preference.game.Rd
@@ -21,7 +21,8 @@ preference.game(
 
 \item{type.dist}{The distribution of the vertex types, a numeric vector of
 length \sQuote{types} containing non-negative numbers. The vector will be
-normed to obtain probabilities.}
+normed to obtain probabilities. The default \code{NULL} gives a uniform
+distribution.}
 
 \item{fixed.sizes}{Fix the number of vertices with a given vertex type
 label. The \code{type.dist} argument gives the group sizes (i.e. number of
@@ -29,7 +30,8 @@ vertices with the different labels) in this case.}
 
 \item{pref.matrix}{A square matrix giving the preferences of the vertex
 types. The matrix has \sQuote{types} rows and columns. When generating
-an undirected graph, it must be symmetric.}
+an undirected graph, it must be symmetric. The default \code{NULL} sets all
+preferences to one.}
 
 \item{directed}{Logical, whether to create a directed graph.}
 
diff --git a/man/print.igraph.Rd b/man/print.igraph.Rd
index 2b34d4b130..0aa9e07f11 100644
--- a/man/print.igraph.Rd
+++ b/man/print.igraph.Rd
@@ -9,13 +9,13 @@
 \usage{
 \method{print}{igraph}(
   x,
-  full = igraph_opt("print.full"),
-  graph.attributes = igraph_opt("print.graph.attributes"),
-  vertex.attributes = igraph_opt("print.vertex.attributes"),
-  edge.attributes = igraph_opt("print.edge.attributes"),
+  full = NULL,
+  graph.attributes = NULL,
+  vertex.attributes = NULL,
+  edge.attributes = NULL,
   names = TRUE,
-  max.lines = igraph_opt("auto.print.lines"),
-  id = igraph_opt("print.id"),
+  max.lines = NULL,
+  id = NULL,
   ...
 )
 
@@ -25,22 +25,27 @@
 \item{x}{The graph to print.}
 
 \item{full}{Logical, whether to print the graph structure itself as
-well.}
+well. The default \code{NULL} uses the \code{print.full} igraph option.}
 
-\item{graph.attributes}{Logical, whether to print graph attributes.}
+\item{graph.attributes}{Logical, whether to print graph attributes. The
+default \code{NULL} uses the \code{print.graph.attributes} igraph option.}
 
 \item{vertex.attributes}{Logical, whether to print vertex
-attributes.}
+attributes. The default \code{NULL} uses the \code{print.vertex.attributes} igraph
+option.}
 
-\item{edge.attributes}{Logical, whether to print edge attributes.}
+\item{edge.attributes}{Logical, whether to print edge attributes. The
+default \code{NULL} uses the \code{print.edge.attributes} igraph option.}
 
 \item{names}{Logical, whether to print symbolic vertex names (i.e.
 the \code{name} vertex attribute) or vertex IDs.}
 
 \item{max.lines}{The maximum number of lines to use. The rest of the
-output will be truncated.}
+output will be truncated. If not given, the \code{auto.print.lines} igraph
+option applies; \code{NULL} prints all lines.}
 
-\item{id}{Whether to print the graph ID.}
+\item{id}{Whether to print the graph ID. The default \code{NULL} uses the
+\code{print.id} igraph option.}
 
 \item{\dots}{Additional agruments.}
 
diff --git a/man/print.igraph.es.Rd b/man/print.igraph.es.Rd
index b5d2dbe95f..5d3a9505ca 100644
--- a/man/print.igraph.es.Rd
+++ b/man/print.igraph.es.Rd
@@ -12,7 +12,8 @@
 \item{full}{Whether to show the full sequence, or truncate the output
 to the screen size.}
 
-\item{id}{Whether to print the graph ID.}
+\item{id}{Whether to print the graph ID. The default \code{NULL} uses the
+\code{print.id} igraph option.}
 
 \item{...}{Currently ignored.}
 }
diff --git a/man/print.igraph.vs.Rd b/man/print.igraph.vs.Rd
index 15342a5c9a..92504a5dcd 100644
--- a/man/print.igraph.vs.Rd
+++ b/man/print.igraph.vs.Rd
@@ -12,7 +12,8 @@
 \item{full}{Whether to show the full sequence, or truncate the output
 to the screen size.}
 
-\item{id}{Whether to print the graph ID.}
+\item{id}{Whether to print the graph ID. The default \code{NULL} uses the
+\code{print.id} igraph option.}
 
 \item{...}{These arguments are currently ignored.}
 }
diff --git a/man/reverse_edges.Rd b/man/reverse_edges.Rd
index 7ca28fec47..f4d0cd085f 100644
--- a/man/reverse_edges.Rd
+++ b/man/reverse_edges.Rd
@@ -5,14 +5,15 @@
 \alias{t.igraph}
 \title{Reverse edges in a graph}
 \usage{
-reverse_edges(graph, eids = E(graph))
+reverse_edges(graph, eids = NULL)
 
 \method{t}{igraph}(x)
 }
 \arguments{
 \item{graph}{The input graph.}
 
-\item{eids}{The edge IDs of the edges to reverse.}
+\item{eids}{The edge IDs of the edges to reverse. The default \code{NULL}
+reverses all edges.}
 
 \item{x}{The input graph.}
 }
diff --git a/man/sample_.Rd b/man/sample_.Rd
index a7c4390887..13ea902286 100644
--- a/man/sample_.Rd
+++ b/man/sample_.Rd
@@ -31,7 +31,7 @@ to the newly created graphs.
 See the examples and the various constructor modifiers below.
 }
 \section{Related documentation in the C library}{
-\href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_vcount}{\code{vcount()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Operators.html#igraph_simplify}{\code{simplify()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Structural.html#igraph_is_simple}{\code{is_simple()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_edges}{\code{edges()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_get_eids}{\code{get_eids()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_ecount}{\code{ecount()}}
+\href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_vcount}{\code{vcount()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Operators.html#igraph_simplify}{\code{simplify()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Structural.html#igraph_is_simple}{\code{is_simple()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_get_eids}{\code{get_eids()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_edges}{\code{edges()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_ecount}{\code{ecount()}}
 }
 
 \examples{
diff --git a/man/sample_correlated_gnp.Rd b/man/sample_correlated_gnp.Rd
index 0c119fe8f5..9c7e5c11f7 100644
--- a/man/sample_correlated_gnp.Rd
+++ b/man/sample_correlated_gnp.Rd
@@ -5,13 +5,7 @@
 \title{Generate a new random graph from a given graph by randomly
 adding/removing edges}
 \usage{
-sample_correlated_gnp(
-  old.graph,
-  corr,
-  ...,
-  p = edge_density(old.graph),
-  permutation = NULL
-)
+sample_correlated_gnp(old.graph, corr, ..., p = NULL, permutation = NULL)
 }
 \arguments{
 \item{old.graph}{The original graph.}
@@ -23,8 +17,8 @@ graph (the adjacency matrix being used as a vector).}
 \item{...}{These dots are for future extensions and must be empty.}
 
 \item{p}{A numeric scalar, the probability of an edge between two
-vertices, it must in the open (0,1) interval. The default is the empirical
-edge density of the graph. If you are resampling an Erdős-Rényi graph and
+vertices, it must in the open (0,1) interval. The default \code{NULL} uses the
+empirical edge density of the graph. If you are resampling an Erdős-Rényi graph and
 you know the original edge probability of the Erdős-Rényi model, you should
 supply that explicitly.}
 
diff --git a/man/sample_last_cit.Rd b/man/sample_last_cit.Rd
index 03db1e4376..6f26215231 100644
--- a/man/sample_last_cit.Rd
+++ b/man/sample_last_cit.Rd
@@ -13,26 +13,19 @@ sample_last_cit(
   n,
   edges = 1,
   ...,
-  agebins = n/7100,
-  pref = (1:(agebins + 1))^-3,
+  agebins = NULL,
+  pref = NULL,
   directed = TRUE
 )
 
-last_cit(
-  n,
-  edges = 1,
-  ...,
-  agebins = n/7100,
-  pref = (1:(agebins + 1))^-3,
-  directed = TRUE
-)
+last_cit(n, edges = 1, ..., agebins = NULL, pref = NULL, directed = TRUE)
 
 sample_cit_types(
   n,
   edges = 1,
-  types = rep(0, n),
+  types = NULL,
   ...,
-  pref = rep(1, length(types)),
+  pref = NULL,
   directed = TRUE,
   attr = TRUE
 )
@@ -40,9 +33,9 @@ sample_cit_types(
 cit_types(
   n,
   edges = 1,
-  types = rep(0, n),
+  types = NULL,
   ...,
-  pref = rep(1, length(types)),
+  pref = NULL,
   directed = TRUE,
   attr = TRUE
 )
@@ -50,9 +43,9 @@ cit_types(
 sample_cit_cit_types(
   n,
   edges = 1,
-  types = rep(0, n),
+  types = NULL,
   ...,
-  pref = matrix(1, nrow = length(types), ncol = length(types)),
+  pref = NULL,
   directed = TRUE,
   attr = TRUE
 )
@@ -60,9 +53,9 @@ sample_cit_cit_types(
 cit_cit_types(
   n,
   edges = 1,
-  types = rep(0, n),
+  types = NULL,
   ...,
-  pref = matrix(1, nrow = length(types), ncol = length(types)),
+  pref = NULL,
   directed = TRUE,
   attr = TRUE
 )
@@ -74,16 +67,19 @@ cit_cit_types(
 
 \item{...}{These dots are for future extensions and must be empty.}
 
-\item{agebins}{Number of aging bins.}
+\item{agebins}{Number of aging bins. The default \code{NULL} uses \code{n / 7100}.}
 
 \item{pref}{Vector (\code{sample_last_cit()} and \code{sample_cit_types()} or
 matrix (\code{sample_cit_cit_types()}) giving the (unnormalized) citation
-probabilities for the different vertex types.}
+probabilities for the different vertex types. The default \code{NULL} uses
+\code{(1:(agebins + 1))^-3} for \code{sample_last_cit()} and all-one probabilities
+for the other two.}
 
 \item{directed}{Logical, whether to generate directed networks.}
 
 \item{types}{Vector of length \sQuote{\code{n}}, the types of the vertices.
-Types are numbered from zero.}
+Types are numbered from zero. The default \code{NULL} gives all vertices
+type zero.}
 
 \item{attr}{Logical, whether to add the vertex types to the generated
 graph as a vertex attribute called \sQuote{\code{type}}.}
diff --git a/man/sample_motifs.Rd b/man/sample_motifs.Rd
index 266ca0665f..f2d2dedcba 100644
--- a/man/sample_motifs.Rd
+++ b/man/sample_motifs.Rd
@@ -8,7 +8,7 @@ sample_motifs(
   graph,
   size = 3,
   ...,
-  cut.prob = rep(0, size),
+  cut.prob = NULL,
   sample.size = NULL,
   sample = NULL
 )
diff --git a/man/sample_pref.Rd b/man/sample_pref.Rd
index bb452c0351..35b69cbd0d 100644
--- a/man/sample_pref.Rd
+++ b/man/sample_pref.Rd
@@ -11,9 +11,9 @@ sample_pref(
   nodes,
   types,
   ...,
-  type.dist = rep(1, types),
+  type.dist = NULL,
   fixed.sizes = FALSE,
-  pref.matrix = matrix(1, types, types),
+  pref.matrix = NULL,
   directed = FALSE,
   loops = FALSE
 )
@@ -22,9 +22,9 @@ pref(
   nodes,
   types,
   ...,
-  type.dist = rep(1, types),
+  type.dist = NULL,
   fixed.sizes = FALSE,
-  pref.matrix = matrix(1, types, types),
+  pref.matrix = NULL,
   directed = FALSE,
   loops = FALSE
 )
@@ -33,8 +33,8 @@ sample_asym_pref(
   nodes,
   types,
   ...,
-  type.dist.matrix = matrix(1, types, types),
-  pref.matrix = matrix(1, types, types),
+  type.dist.matrix = NULL,
+  pref.matrix = NULL,
   loops = FALSE
 )
 
@@ -42,8 +42,8 @@ asym_pref(
   nodes,
   types,
   ...,
-  type.dist.matrix = matrix(1, types, types),
-  pref.matrix = matrix(1, types, types),
+  type.dist.matrix = NULL,
+  pref.matrix = NULL,
   loops = FALSE
 )
 }
@@ -56,7 +56,8 @@ asym_pref(
 
 \item{type.dist}{The distribution of the vertex types, a numeric vector of
 length \sQuote{types} containing non-negative numbers. The vector will be
-normed to obtain probabilities.}
+normed to obtain probabilities. The default \code{NULL} gives a uniform
+distribution.}
 
 \item{fixed.sizes}{Fix the number of vertices with a given vertex type
 label. The \code{type.dist} argument gives the group sizes (i.e. number of
@@ -64,14 +65,15 @@ vertices with the different labels) in this case.}
 
 \item{pref.matrix}{A square matrix giving the preferences of the vertex
 types. The matrix has \sQuote{types} rows and columns. When generating
-an undirected graph, it must be symmetric.}
+an undirected graph, it must be symmetric. The default \code{NULL} sets all
+preferences to one.}
 
 \item{directed}{Logical, whether to create a directed graph.}
 
 \item{loops}{Logical, whether self-loops are allowed in the graph.}
 
 \item{type.dist.matrix}{The joint distribution of the in- and out-vertex
-types.}
+types. The default \code{NULL} gives a uniform distribution.}
 }
 \value{
 An igraph graph.
diff --git a/man/sample_traits_callaway.Rd b/man/sample_traits_callaway.Rd
index b472c39185..29f831bc9b 100644
--- a/man/sample_traits_callaway.Rd
+++ b/man/sample_traits_callaway.Rd
@@ -12,8 +12,8 @@ sample_traits_callaway(
   types,
   ...,
   edge.per.step = 1,
-  type.dist = rep(1, types),
-  pref.matrix = matrix(1, types, types),
+  type.dist = NULL,
+  pref.matrix = NULL,
   directed = FALSE
 )
 
@@ -22,8 +22,8 @@ traits_callaway(
   types,
   ...,
   edge.per.step = 1,
-  type.dist = rep(1, types),
-  pref.matrix = matrix(1, types, types),
+  type.dist = NULL,
+  pref.matrix = NULL,
   directed = FALSE
 )
 
@@ -32,8 +32,8 @@ sample_traits(
   types,
   k = 1,
   ...,
-  type.dist = rep(1, types),
-  pref.matrix = matrix(1, types, types),
+  type.dist = NULL,
+  pref.matrix = NULL,
   directed = FALSE
 )
 
@@ -42,8 +42,8 @@ traits(
   types,
   k = 1,
   ...,
-  type.dist = rep(1, types),
-  pref.matrix = matrix(1, types, types),
+  type.dist = NULL,
+  pref.matrix = NULL,
   directed = FALSE
 )
 }
@@ -57,10 +57,11 @@ traits(
 \item{edge.per.step}{The number of edges to add to the graph per time step.}
 
 \item{type.dist}{The distribution of the vertex types. This is assumed to be
-stationary in time.}
+stationary in time. The default \code{NULL} gives a uniform distribution.}
 
 \item{pref.matrix}{A matrix giving the preferences of the given vertex
-types. These should be probabilities, i.e. numbers between zero and one.}
+types. These should be probabilities, i.e. numbers between zero and one.
+The default \code{NULL} sets all preferences to one.}
 
 \item{directed}{Logical, whether to generate directed graphs.}
 
diff --git a/man/set.edge.attribute.Rd b/man/set.edge.attribute.Rd
index 71dd0b2101..d88601f71b 100644
--- a/man/set.edge.attribute.Rd
+++ b/man/set.edge.attribute.Rd
@@ -12,7 +12,7 @@ set.edge.attribute(graph, name, index = E(graph), value)
 \item{name}{The name of the attribute to set.}
 
 \item{index}{An optional edge sequence to set the attributes of
-a subset of edges.}
+a subset of edges. The default \code{NULL} selects all edges.}
 
 \item{value}{The new value of the attribute for all (or \code{index})
 edges.
diff --git a/man/set.vertex.attribute.Rd b/man/set.vertex.attribute.Rd
index 98702f4fec..f24c2f3077 100644
--- a/man/set.vertex.attribute.Rd
+++ b/man/set.vertex.attribute.Rd
@@ -12,7 +12,7 @@ set.vertex.attribute(graph, name, index = V(graph), value)
 \item{name}{The name of the attribute to set.}
 
 \item{index}{An optional vertex sequence to set the attributes
-of a subset of vertices.}
+of a subset of vertices. The default \code{NULL} selects all vertices.}
 
 \item{value}{The new value of the attribute for all (or \code{index})
 vertices.
diff --git a/man/set_edge_attr.Rd b/man/set_edge_attr.Rd
index 8b50ec8fbe..ab5dd24576 100644
--- a/man/set_edge_attr.Rd
+++ b/man/set_edge_attr.Rd
@@ -4,7 +4,7 @@
 \alias{set_edge_attr}
 \title{Set edge attributes}
 \usage{
-set_edge_attr(graph, name, index = E(graph), value)
+set_edge_attr(graph, name, index = NULL, value)
 }
 \arguments{
 \item{graph}{The graph}
@@ -12,7 +12,7 @@ set_edge_attr(graph, name, index = E(graph), value)
 \item{name}{The name of the attribute to set.}
 
 \item{index}{An optional edge sequence to set the attributes of
-a subset of edges.}
+a subset of edges. The default \code{NULL} selects all edges.}
 
 \item{value}{The new value of the attribute for all (or \code{index})
 edges.
diff --git a/man/set_vertex_attr.Rd b/man/set_vertex_attr.Rd
index 025bb58193..5d315ad020 100644
--- a/man/set_vertex_attr.Rd
+++ b/man/set_vertex_attr.Rd
@@ -4,7 +4,7 @@
 \alias{set_vertex_attr}
 \title{Set vertex attributes}
 \usage{
-set_vertex_attr(graph, name, index = V(graph), value)
+set_vertex_attr(graph, name, index = NULL, value)
 }
 \arguments{
 \item{graph}{The graph.}
@@ -12,7 +12,7 @@ set_vertex_attr(graph, name, index = V(graph), value)
 \item{name}{The name of the attribute to set.}
 
 \item{index}{An optional vertex sequence to set the attributes
-of a subset of vertices.}
+of a subset of vertices. The default \code{NULL} selects all vertices.}
 
 \item{value}{The new value of the attribute for all (or \code{index})
 vertices.
diff --git a/man/set_vertex_attrs.Rd b/man/set_vertex_attrs.Rd
index 61b81c8f25..ca504251f0 100644
--- a/man/set_vertex_attrs.Rd
+++ b/man/set_vertex_attrs.Rd
@@ -4,7 +4,7 @@
 \alias{set_vertex_attrs}
 \title{Set multiple vertex attributes}
 \usage{
-set_vertex_attrs(graph, ..., index = V(graph))
+set_vertex_attrs(graph, ..., index = NULL)
 }
 \arguments{
 \item{graph}{The graph.}
@@ -12,7 +12,7 @@ set_vertex_attrs(graph, ..., index = V(graph))
 \item{...}{<\code{\link[rlang:dyn-dots]{dynamic-dots}}> Named arguments, where the names are the attributes}
 
 \item{index}{An optional vertex sequence to set the attributes
-of a subset of vertices.}
+of a subset of vertices. The default \code{NULL} selects all vertices.}
 }
 \value{
 The graph, with the vertex attributes added or set.
diff --git a/man/shapes.Rd b/man/shapes.Rd
index a06011fffe..6223055e10 100644
--- a/man/shapes.Rd
+++ b/man/shapes.Rd
@@ -14,13 +14,7 @@ shape_noclip(coords, el, params, end = c("both", "from", "to"))
 
 shape_noplot(coords, v = NULL, params)
 
-add_shape(
-  shape,
-  ...,
-  clip = shape_noclip,
-  plot = shape_noplot,
-  parameters = list()
-)
+add_shape(shape, ..., clip = NULL, plot = NULL, parameters = list())
 }
 \arguments{
 \item{shape}{Character scalar, name of a vertex shape. If it is
@@ -32,9 +26,11 @@ functions below.}
 
 \item{...}{These dots are for future extensions and must be empty.}
 
-\item{clip}{An R function object, the clipping function.}
+\item{clip}{An R function object, the clipping function. The default
+\code{NULL} uses \code{shape_noclip}.}
 
-\item{plot}{An R function object, the plotting function.}
+\item{plot}{An R function object, the plotting function. The default
+\code{NULL} uses \code{shape_noplot}.}
 
 \item{parameters}{Named list, additional plot/vertex/edge
 parameters. The element named define the new parameters, and the
diff --git a/man/shortest.paths.Rd b/man/shortest.paths.Rd
index 0fc77c7806..041f8de75b 100644
--- a/man/shortest.paths.Rd
+++ b/man/shortest.paths.Rd
@@ -17,10 +17,10 @@ shortest.paths(
 \item{graph}{The graph to work on.}
 
 \item{v}{Numeric vector, the vertices from which the shortest paths will be
-calculated.}
+calculated. The default \code{NULL} selects all vertices.}
 
 \item{to}{Numeric vector, the vertices to which the shortest paths will be
-calculated. By default it includes all vertices. Note that for
+calculated. The default \code{NULL} includes all vertices. Note that for
 \code{distances()} every vertex must be included here at most once. (This
 is not required for \code{shortest_paths()}.}
 
diff --git a/man/similarity.Rd b/man/similarity.Rd
index 2b254e0d5a..505e887ec6 100644
--- a/man/similarity.Rd
+++ b/man/similarity.Rd
@@ -6,7 +6,7 @@
 \usage{
 similarity(
   graph,
-  vids = V(graph),
+  vids = NULL,
   ...,
   mode = c("all", "out", "in", "total"),
   loops = FALSE,
@@ -16,7 +16,8 @@ similarity(
 \arguments{
 \item{graph}{The input graph.}
 
-\item{vids}{The vertex IDs for which the similarity is calculated.}
+\item{vids}{The vertex IDs for which the similarity is calculated. The
+default \code{NULL} selects all vertices.}
 
 \item{...}{These dots are for future extensions and must be empty.}
 
diff --git a/man/similarity.dice.Rd b/man/similarity.dice.Rd
index d9616b8d86..eef1059ad7 100644
--- a/man/similarity.dice.Rd
+++ b/man/similarity.dice.Rd
@@ -14,7 +14,8 @@ similarity.dice(
 \arguments{
 \item{graph}{The input graph.}
 
-\item{vids}{The vertex IDs for which the similarity is calculated.}
+\item{vids}{The vertex IDs for which the similarity is calculated. The
+default \code{NULL} selects all vertices.}
 
 \item{mode}{The type of neighboring vertices to use for the calculation,
 possible values: \sQuote{\code{out}}, \sQuote{\verb{in}},
diff --git a/man/similarity.invlogweighted.Rd b/man/similarity.invlogweighted.Rd
index 8f8dbdd2b7..8fbc142be0 100644
--- a/man/similarity.invlogweighted.Rd
+++ b/man/similarity.invlogweighted.Rd
@@ -13,7 +13,8 @@ similarity.invlogweighted(
 \arguments{
 \item{graph}{The input graph.}
 
-\item{vids}{The vertex IDs for which the similarity is calculated.}
+\item{vids}{The vertex IDs for which the similarity is calculated. The
+default \code{NULL} selects all vertices.}
 
 \item{mode}{The type of neighboring vertices to use for the calculation,
 possible values: \sQuote{\code{out}}, \sQuote{\verb{in}},
diff --git a/man/similarity.jaccard.Rd b/man/similarity.jaccard.Rd
index 433fca4c62..f28b381f80 100644
--- a/man/similarity.jaccard.Rd
+++ b/man/similarity.jaccard.Rd
@@ -14,7 +14,8 @@ similarity.jaccard(
 \arguments{
 \item{graph}{The input graph.}
 
-\item{vids}{The vertex IDs for which the similarity is calculated.}
+\item{vids}{The vertex IDs for which the similarity is calculated. The
+default \code{NULL} selects all vertices.}
 
 \item{mode}{The type of neighboring vertices to use for the calculation,
 possible values: \sQuote{\code{out}}, \sQuote{\verb{in}},
diff --git a/man/simplify.Rd b/man/simplify.Rd
index bf1643497a..91a279f97b 100644
--- a/man/simplify.Rd
+++ b/man/simplify.Rd
@@ -10,7 +10,7 @@ simplify(
   graph,
   remove.multiple = TRUE,
   remove.loops = TRUE,
-  edge.attr.comb = igraph_opt("edge.attr.comb")
+  edge.attr.comb = NULL
 )
 
 is_simple(graph)
@@ -28,7 +28,8 @@ removed.}
 \item{edge.attr.comb}{Specifies what to do with edge attributes, if
 \code{remove.multiple=TRUE}. In this case many edges might be mapped to a
 single one in the new graph, and their attributes are combined. Please see
-\code{\link[=attribute.combination]{attribute.combination()}} for details on this.}
+\code{\link[=attribute.combination]{attribute.combination()}} for details on this. The default \code{NULL} uses
+the \code{edge.attr.comb} igraph option.}
 }
 \value{
 a graph object with the loop and/or multiple edges removed; the
diff --git a/man/stochastic_matrix.Rd b/man/stochastic_matrix.Rd
index df24220d9d..3c1ad6b6e1 100644
--- a/man/stochastic_matrix.Rd
+++ b/man/stochastic_matrix.Rd
@@ -4,12 +4,7 @@
 \alias{stochastic_matrix}
 \title{Stochastic matrix of a graph}
 \usage{
-stochastic_matrix(
-  graph,
-  ...,
-  column.wise = FALSE,
-  sparse = igraph_opt("sparsematrices")
-)
+stochastic_matrix(graph, ..., column.wise = FALSE, sparse = NULL)
 }
 \arguments{
 \item{graph}{The input graph. Must be of class \code{igraph}.}
@@ -20,7 +15,8 @@ stochastic_matrix(
 sum up to one; otherwise it is the columns.}
 
 \item{sparse}{Logical, whether to return a sparse matrix. The
-\code{Matrix} package is needed for sparse matrices.}
+\code{Matrix} package is needed for sparse matrices. The default \code{NULL} uses
+the \code{sparsematrices} igraph option.}
 }
 \value{
 A regular matrix or a matrix of class \code{Matrix} if a
diff --git a/man/strength.Rd b/man/strength.Rd
index 3dfd73932e..cf0e0765c5 100644
--- a/man/strength.Rd
+++ b/man/strength.Rd
@@ -6,7 +6,7 @@
 \usage{
 strength(
   graph,
-  vids = V(graph),
+  vids = NULL,
   ...,
   mode = c("all", "out", "in", "total"),
   loops = TRUE,
@@ -16,7 +16,8 @@ strength(
 \arguments{
 \item{graph}{The input graph.}
 
-\item{vids}{The vertices for which the strength will be calculated.}
+\item{vids}{The vertices for which the strength will be calculated.
+The default \code{NULL} selects all vertices.}
 
 \item{...}{These dots are for future extensions and must be empty.}
 
diff --git a/man/union.igraph.Rd b/man/union.igraph.Rd
index 95aacbd76c..f75202bc1d 100644
--- a/man/union.igraph.Rd
+++ b/man/union.igraph.Rd
@@ -62,7 +62,7 @@ An error is generated if some input graphs are directed and others are
 undirected.
 }
 \section{Related documentation in the C library}{
-\href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_vcount}{\code{vcount()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Isomorphism.html#igraph_permute_vertices}{\code{permute_vertices()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_edges}{\code{edges()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_get_eids}{\code{get_eids()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_ecount}{\code{ecount()}}
+\href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_vcount}{\code{vcount()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Isomorphism.html#igraph_permute_vertices}{\code{permute_vertices()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_get_eids}{\code{get_eids()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_edges}{\code{edges()}}, \href{https://igraph.org/c/html/0.10.17/igraph-Basic.html#igraph_ecount}{\code{ecount()}}
 }
 
 \examples{
diff --git a/man/vertex_attr-set.Rd b/man/vertex_attr-set.Rd
index 56324443d7..817c5abfe5 100644
--- a/man/vertex_attr-set.Rd
+++ b/man/vertex_attr-set.Rd
@@ -5,7 +5,7 @@
 \alias{vertex.attributes<-}
 \title{Set one or more vertex attributes}
 \usage{
-vertex_attr(graph, name, index = V(graph)) <- value
+vertex_attr(graph, name, index = NULL) <- value
 }
 \arguments{
 \item{graph}{The graph.}
@@ -15,7 +15,7 @@ then \code{value} must be a named list, and its entries are
 set as vertex attributes.}
 
 \item{index}{An optional vertex sequence to set the attributes
-of a subset of vertices.}
+of a subset of vertices. The default \code{NULL} selects all vertices.}
 
 \item{value}{The new value of the attribute(s) for all
 (or \code{index}) vertices.}
diff --git a/man/vertex_attr.Rd b/man/vertex_attr.Rd
index ff6476e401..5a864092fe 100644
--- a/man/vertex_attr.Rd
+++ b/man/vertex_attr.Rd
@@ -5,7 +5,7 @@
 \alias{vertex.attributes}
 \title{Query vertex attributes of a graph}
 \usage{
-vertex_attr(graph, name, index = V(graph))
+vertex_attr(graph, name, index = NULL)
 }
 \arguments{
 \item{graph}{The graph.}
@@ -14,7 +14,7 @@ vertex_attr(graph, name, index = V(graph))
 all vertex attributes are returned in a list.}
 
 \item{index}{An optional vertex sequence to query the attribute only
-for these vertices.}
+for these vertices. The default \code{NULL} selects all vertices.}
 }
 \value{
 The value of the vertex attribute, or the list of
diff --git a/man/which_multiple.Rd b/man/which_multiple.Rd
index 52f8c68739..a705616f7d 100644
--- a/man/which_multiple.Rd
+++ b/man/which_multiple.Rd
@@ -9,13 +9,13 @@
 \alias{count_loops}
 \title{Find the multiple or loop edges in a graph}
 \usage{
-which_multiple(graph, eids = E(graph))
+which_multiple(graph, eids = NULL)
 
 any_multiple(graph)
 
-count_multiple(graph, eids = E(graph))
+count_multiple(graph, eids = NULL)
 
-which_loop(graph, eids = E(graph))
+which_loop(graph, eids = NULL)
 
 any_loop(graph)
 
@@ -24,8 +24,8 @@ count_loops(graph)
 \arguments{
 \item{graph}{The input graph.}
 
-\item{eids}{The edges to which the query is restricted. By default this is
-all edges in the graph.}
+\item{eids}{The edges to which the query is restricted. The default
+\code{NULL} selects all edges.}
 }
 \value{
 \code{any_loop()} and \code{any_multiple()} return a Logical.
diff --git a/man/which_mutual.Rd b/man/which_mutual.Rd
index a576fe94c7..46346529c7 100644
--- a/man/which_mutual.Rd
+++ b/man/which_mutual.Rd
@@ -4,13 +4,13 @@
 \alias{which_mutual}
 \title{Find mutual edges in a directed graph}
 \usage{
-which_mutual(graph, eids = E(graph), ..., loops = TRUE)
+which_mutual(graph, eids = NULL, ..., loops = TRUE)
 }
 \arguments{
 \item{graph}{The input graph.}
 
-\item{eids}{Edge sequence, the edges that will be probed. By default is
-includes all edges in the order of their IDs.}
+\item{eids}{Edge sequence, the edges that will be probed. The default
+\code{NULL} includes all edges in the order of their IDs.}
 
 \item{...}{These dots are for future extensions and must be empty.}
 
diff --git a/tests/testthat/test-constant-defaults.R b/tests/testthat/test-constant-defaults.R
new file mode 100644
index 0000000000..f8770f4eb4
--- /dev/null
+++ b/tests/testthat/test-constant-defaults.R
@@ -0,0 +1,110 @@
+# Migrated signatures carry only constant defaults; complex defaults are
+# declared as `NULL` and resolved in the body after all arguments are
+# available. These tests pin the user-visible semantics of that pattern.
+
+test_that("NULL selector defaults stand for the full vertex/edge set", {
+  g <- make_ring(5)
+  E(g)$weight <- seq_len(5)
+
+  # Passing NULL explicitly is now the same as not passing the argument.
+  # (Before the constant-defaults cleanup an explicit NULL was coerced to an
+  # empty selection -- an accident of as_igraph_vs(), never documented.)
+  expect_identical(degree(g, v = NULL), degree(g))
+  expect_identical(distances(g, v = NULL, to = NULL), distances(g))
+  expect_identical(diversity(g, vids = NULL), diversity(g))
+  expect_identical(which_mutual(g, eids = NULL), which_mutual(g))
+  expect_identical(closeness(g, vids = NULL), closeness(g))
+})
+
+test_that("NULL non-selector defaults resolve in the body", {
+  g <- make_ring(4)
+
+  # sparse = NULL falls back to the sparsematrices option.
+  local_igraph_options(sparsematrices = TRUE)
+  expect_s4_class(as_adjacency_matrix(g), "dgCMatrix")
+  local_igraph_options(sparsematrices = FALSE)
+  expect_true(is.matrix(as_adjacency_matrix(g)))
+
+  # eps = NULL falls back to the machine epsilon.
+  bip <- make_bipartite_graph(c(0, 1, 0, 1), c(1, 2, 3, 4))
+  expect_identical(max_bipartite_match(bip)$matching_size, 2)
+})
+
+test_that("NULL selector defaults of round-2 functions select the full set", {
+  g <- make_ring(5)
+
+  expect_identical(max_degree(g, v = NULL), max_degree(g))
+  expect_identical(which_loop(g, eids = NULL), which_loop(g))
+  expect_identical(which_multiple(g, eids = NULL), which_multiple(g))
+  expect_identical(count_multiple(g, eids = NULL), count_multiple(g))
+  expect_identical(count_triangles(g, vids = NULL), count_triangles(g))
+  expect_identical(cocitation(g, v = NULL), cocitation(g))
+  expect_identical(similarity(g, vids = NULL), similarity(g))
+  expect_identical_graphs(reverse_edges(g, eids = NULL), reverse_edges(g))
+
+  V(g)$name <- letters[1:5]
+  expect_identical(vertex_attr(g, "name", index = NULL), vertex_attr(g, "name"))
+  expect_identical(vertex.attributes(g, index = NULL), vertex.attributes(g))
+  g2 <- set_vertex_attr(g, "x", index = NULL, value = 1)
+  expect_identical(vertex_attr(g2, "x"), rep(1, 5))
+})
+
+test_that("NULL non-selector defaults of round-2 functions resolve in the body", {
+  g <- make_ring(4)
+
+  # option-backed defaults fall back to the igraph option
+  expect_identical_graphs(
+    simplify(g + edge(1, 2), edge.attr.comb = NULL),
+    simplify(g + edge(1, 2))
+  )
+  expect_identical_graphs(
+    as_undirected(as_directed(g), edge.attr.comb = NULL),
+    as_undirected(as_directed(g))
+  )
+
+  # cross-referencing defaults resolve after all arguments are available
+  igraph_local_seed(42)
+  g1 <- sample_pref(20, types = 2, pref.matrix = NULL, type.dist = NULL)
+  igraph_local_seed(42)
+  g2 <- sample_pref(20, types = 2)
+  expect_identical_graphs(g1, g2)
+
+  igraph_local_seed(1)
+  l1 <- layout_with_kk(g, kkconst = NULL, maxiter = NULL)
+  igraph_local_seed(1)
+  l2 <- layout_with_kk(g)
+  expect_identical(l1, l2)
+})
+
+test_that("empty-sequence defaults are spelled as typed empty vectors", {
+  # `c()` evaluates to NULL, so a `c()` default would collide with the
+  # resolve-in-body sentinel. Typed empties stay constant and disjoint from
+  # NULL: an explicit empty selection keeps meaning "nothing selected",
+  # while NULL now always means "use the default".
+  g <- make_graph(c(1, 2, 2, 2, 2, 3), directed = TRUE)
+  expect_equal(max_degree(g, v = integer()), 0)
+  expect_identical(which_loop(g, eids = integer()), logical(0))
+  expect_gt(max_degree(g, v = NULL), 0)
+
+  # layout_as_tree(): the typed empty stays the documented default
+  tree <- make_tree(5)
+  expect_identical(
+    layout_as_tree(tree, root = numeric()),
+    layout_as_tree(tree)
+  )
+})
+
+test_that("positional recovery of a selector with a NULL default works", {
+  # Regression: with `vids = V(graph)` as the default, re-evaluating the
+  # default during recovery produced a fresh igraph.vs whose weakref `env`
+  # attribute never compares identical(), so the legacy positional call
+  # `diversity(g, weights, vids)` died with a spurious
+  # "supplied more than once" error. A constant NULL default is stable, so
+  # the recovery path works again.
+  g <- make_ring(5)
+  E(g)$weight <- seq_len(5)
+  lifecycle::expect_deprecated(
+    res <- diversity(g, NULL, V(g)[1:3])
+  )
+  expect_identical(res, diversity(g, weights = NULL, vids = V(g)[1:3]))
+})
diff --git a/tests/testthat/test-generate-migrations.R b/tests/testthat/test-generate-migrations.R
index 3798e3970e..e9ee4ffaac 100644
--- a/tests/testthat/test-generate-migrations.R
+++ b/tests/testthat/test-generate-migrations.R
@@ -140,19 +140,88 @@ test_that("render_call_arg() wraps a single long item without a stray comma", {
 })
 
 test_that("default expressions keep air's spacing around binary `/`", {
+  # The constant-defaults rule keeps arithmetic like `n / 7100` out of real
+  # registries, but the renderer must stay air-clean for any deparsed
+  # expression, so exercise the helpers directly.
+  gen <- local_generator()
+  fmls <- formals(function(agebins = n / 7100, base = "http://a/b") {})
+  expect_identical(gen$default_expr(fmls, "agebins"), "n / 7100")
+  # slashes inside string literals stay untouched
+  expect_identical(gen$default_expr(fmls, "base"), "\"http://a/b\"")
+})
+
+test_that("is_constant_default() classifies expressions", {
+  gen <- local_generator()
+  const <- alist(
+    NULL,
+    TRUE,
+    1,
+    -1,
+    "out",
+    NA,
+    NA_character_,
+    Inf,
+    c("a", "b"),
+    c(1, -2.5),
+    list(),
+    (2),
+    deprecated(),
+    lifecycle::deprecated(),
+    # typed empty vectors: the canonical spelling of an empty sequence
+    logical(),
+    integer(),
+    numeric(),
+    double(),
+    complex(),
+    character(),
+    raw()
+  )
+  for (e in const) {
+    expect_true(gen$is_constant_default(e), label = deparse(e))
+  }
+  nonconst <- alist(
+    igraph_opt("sparsematrices"),
+    V(graph),
+    bins * 2,
+    new.env(),
+    stats::runif(1),
+    rep(0, 3),
+    sqrt(.Machine$double.eps),
+    .Machine$double.eps,
+    x,
+    T,
+    c(1, n),
+    # not empty constructors: sized or converting calls stay non-constant
+    numeric(2),
+    integer(n),
+    vector("numeric")
+  )
+  for (e in nonconst) {
+    expect_false(gen$is_constant_default(e), label = deparse(e))
+  }
+})
+
+test_that("non-constant defaults are rejected, with no escape hatch", {
   gen <- local_generator()
   entry <- list(
-    old = function(graph, agebins, base) {},
-    new = function(
-      graph,
-      ...,
-      agebins = n / 7100,
-      base = "http://a/b"
-    ) {},
+    old = function(graph, vids) {},
+    new = function(graph, ..., vids = V(graph)) {},
     when = "3.0.0"
   )
-  norm <- gen$normalise_migration("fn_slash", entry)
-  expect_identical(norm$defaults$agebins, "n / 7100")
-  # slashes inside string literals stay untouched
-  expect_identical(norm$defaults$base, "\"http://a/b\"")
+  expect_error(
+    gen$normalise_migration("fn_nc", entry),
+    "non-constant default"
+  )
+
+  # the constant replacement passes
+  entry$new <- function(graph, ..., vids = NULL) {}
+  expect_silent(norm <- gen$normalise_migration("fn_nc", entry))
+  expect_identical(norm$tail, "vids")
+
+  # the retired grandfather field is called out explicitly
+  entry$nonconst_defaults <- "vids"
+  expect_error(
+    gen$normalise_migration("fn_nc", entry),
+    "not supported"
+  )
 })
diff --git a/tests/testthat/test-structural-properties.R b/tests/testthat/test-structural-properties.R
index b7768c9e90..6c4e575202 100644
--- a/tests/testthat/test-structural-properties.R
+++ b/tests/testthat/test-structural-properties.R
@@ -69,7 +69,8 @@ test_that("max_degree() works", {
   expect_equal(max_degree(g, loops = FALSE), 2)
   expect_equal(max_degree(g, mode = "out", loops = FALSE), 1)
   expect_equal(max_degree(g, mode = "in", loops = FALSE), 1)
-  expect_equal(max_degree(g, v = c()), 0)
+  expect_equal(max_degree(g, v = integer()), 0)
+  expect_equal(max_degree(g, v = NULL), max_degree(g))
   expect_equal(max_degree(make_empty_graph()), 0)
 })
 
diff --git a/tools/generate-migrations.R b/tools/generate-migrations.R
index c28bbfe615..96709c2a07 100644
--- a/tools/generate-migrations.R
+++ b/tools/generate-migrations.R
@@ -130,9 +130,111 @@ space_binary_slash <- function(text) {
   gsub("[ ]+/[ ]+", " / ", paste(chars, collapse = ""))
 }
 
+# ---- constant-default rule --------------------------------------------------
+#
+# Defaults in migrated (`new`) signatures must be *constant expressions*:
+# literals, NULL/TRUE/FALSE/NA*/Inf/NaN, `c()`/`list()` of constants, a typed
+# empty vector (`integer()`, `numeric()`, ...), a unary sign, or the
+# `deprecated()` sentinel. Anything else -- option lookups, `V(graph)`,
+# cross-references to other arguments, RNG draws, `rep()`, ... -- is evaluated
+# lazily at an unpredictable time, invites forcing hazards in the recovery
+# machinery, and hides the real default from the signature. There is no escape
+# hatch: express a complex default as `NULL` and resolve it in the body after
+# all arguments are available.
+
+is_constant_default <- function(expr) {
+  if (is.null(expr)) {
+    return(TRUE)
+  }
+  if (is.atomic(expr)) {
+    return(TRUE)
+  }
+  if (is.symbol(expr)) {
+    # TRUE/FALSE/NULL/NA/Inf/NaN parse as literals, not symbols; any symbol
+    # here references an argument or a global -- not constant. (T/F included.)
+    return(FALSE)
+  }
+  if (is.call(expr)) {
+    head <- expr[[1]]
+    if (is.call(head)) {
+      # namespaced sentinel: lifecycle::deprecated()
+      return(
+        length(expr) == 1L &&
+          identical(head, quote(lifecycle::deprecated))
+      )
+    }
+    if (!is.symbol(head)) {
+      return(FALSE)
+    }
+    fn <- as.character(head)
+    if (fn %in% c("c", "list", "(")) {
+      args <- as.list(expr)[-1]
+      return(all(vapply(args, is_constant_default, logical(1))))
+    }
+    empty_ctors <- c(
+      "logical",
+      "integer",
+      "numeric",
+      "double",
+      "complex",
+      "character",
+      "raw"
+    )
+    if (fn %in% empty_ctors && length(expr) == 1L) {
+      # A typed empty vector is a literal: the canonical spelling of an
+      # empty-sequence default (`c()` would be NULL in disguise).
+      return(TRUE)
+    }
+    if (fn %in% c("+", "-") && length(expr) == 2L) {
+      return(is_constant_default(expr[[2]]))
+    }
+    if (fn == "deprecated" && length(expr) == 1L) {
+      return(TRUE)
+    }
+    return(FALSE)
+  }
+  FALSE
+}
+
+check_constant_defaults <- function(fn, new_fmls) {
+  flagged <- character(0)
+  for (nm in names(new_fmls)) {
+    if (nm == "...") {
+      next
+    }
+    if (!nzchar(default_expr(new_fmls, nm))) {
+      next
+    } # no default
+    if (!is_constant_default(new_fmls[[nm]])) {
+      flagged <- c(flagged, nm)
+    }
+  }
+  if (length(flagged)) {
+    stop(
+      "Migration `",
+      fn,
+      "`: non-constant default(s) for ",
+      paste0("`", flagged, "`", collapse = ", "),
+      ".\nDeclare the formal as `NULL` and resolve it in the body after all\n",
+      "arguments are available (see tools/migrations/README.md).",
+      call. = FALSE
+    )
+  }
+  invisible(TRUE)
+}
+
 # Turn one registry entry (with `old`/`new` as function objects) into the flat
 # structure the renderer consumes.
 normalise_migration <- function(fn, entry) {
+  if (!is.null(entry$nonconst_defaults)) {
+    stop(
+      "Migration `",
+      fn,
+      "`: `nonconst_defaults` is not supported; defaults must be constant\n",
+      "expressions (declare the formal as `NULL` and resolve it in the body).",
+      call. = FALSE
+    )
+  }
   for (field in c("old", "new")) {
     if (!is.function(entry[[field]])) {
       stop(
@@ -150,6 +252,7 @@ normalise_migration <- function(fn, entry) {
 
   old_fmls <- formals(entry$old)
   new_fmls <- formals(entry$new)
+  check_constant_defaults(fn, new_fmls)
   entry$old <- names(old_fmls)
   entry$new <- names(new_fmls)
 
diff --git a/tools/migrations/README.md b/tools/migrations/README.md
index cba13e02c6..2c42465154 100644
--- a/tools/migrations/README.md
+++ b/tools/migrations/README.md
@@ -17,6 +17,39 @@ Regenerate the spliced blocks after editing any registry file:
 Rscript tools/generate-migrations.R
 ```
 
+## Constant defaults
+
+Defaults in migrated (`new`) signatures must be **constant expressions**:
+literals, `NULL`/`TRUE`/`FALSE`/`NA*`/`Inf`/`NaN`,
+`c()`/`list()` of constants,
+a typed empty vector (`integer()`, `numeric()`, ...),
+a unary sign,
+or the `deprecated()` sentinel.
+An empty-sequence default must be spelled as a typed empty vector,
+never as `c()`:
+`c()` evaluates to `NULL`,
+and `NULL` is reserved as the resolve-in-body sentinel.
+Anything else -- option lookups, `V(graph)`,
+references to other arguments, RNG draws --
+is evaluated lazily at an unpredictable time,
+invites forcing hazards in the recovery machinery,
+and hides the real default from the signature.
+The generator stops with an error on any non-constant default;
+there is no escape hatch.
+Express a complex default as `NULL`
+and resolve it in the body
+after all arguments are available:
+
+```r
+some_fun <- function(graph, ..., vids = NULL) {
+  # (generated ARG_HANDLE block)
+  if (is.null(vids)) {
+    vids <- V(graph)
+  }
+  ...
+}
+```
+
 ## Entry schema
 
 Every registry file defines a `migrations` list named by function.
diff --git a/tools/migrations/centrality.R b/tools/migrations/centrality.R
index 45a7912fd1..4e6c02ac37 100644
--- a/tools/migrations/centrality.R
+++ b/tools/migrations/centrality.R
@@ -7,7 +7,7 @@ migrations <- list(
     old = function(graph, nodes, alpha, loops, exo, weights, tol, sparse) {},
     new = function(
       graph,
-      nodes = V(graph),
+      nodes = NULL,
       ...,
       alpha = 1,
       loops = FALSE,
@@ -23,7 +23,7 @@ migrations <- list(
     old = function(graph, v, directed, weights, normalized, cutoff) {},
     new = function(
       graph,
-      v = V(graph),
+      v = NULL,
       ...,
       directed = TRUE,
       weights = NULL,
@@ -37,7 +37,7 @@ migrations <- list(
     old = function(graph, vids, mode, weights, normalized, cutoff) {},
     new = function(
       graph,
-      vids = V(graph),
+      vids = NULL,
       ...,
       mode = c("out", "in", "all", "total"),
       weights = NULL,
@@ -53,7 +53,7 @@ migrations <- list(
       graph,
       ...,
       weights = NULL,
-      vids = V(graph)
+      vids = NULL
     ) {},
     when = "3.0.0"
   ),
@@ -62,7 +62,7 @@ migrations <- list(
     old = function(graph, e, directed, weights, cutoff) {},
     new = function(
       graph,
-      e = E(graph),
+      e = NULL,
       ...,
       directed = TRUE,
       weights = NULL,
@@ -75,7 +75,7 @@ migrations <- list(
     old = function(graph, vids, mode, weights, normalized, cutoff) {},
     new = function(
       graph,
-      vids = V(graph),
+      vids = NULL,
       ...,
       mode = c("out", "in", "all", "total"),
       weights = NULL,
@@ -100,7 +100,7 @@ migrations <- list(
       graph,
       ...,
       algo = c("prpack", "arpack"),
-      vids = V(graph),
+      vids = NULL,
       directed = TRUE,
       damping = 0.85,
       personalized = NULL,
@@ -123,7 +123,7 @@ migrations <- list(
     ) {},
     new = function(
       graph,
-      nodes = V(graph),
+      nodes = NULL,
       ...,
       loops = FALSE,
       exponent = 1,
@@ -139,7 +139,7 @@ migrations <- list(
     old = function(graph, vids, mode, loops, weights) {},
     new = function(
       graph,
-      vids = V(graph),
+      vids = NULL,
       ...,
       mode = c("all", "out", "in", "total"),
       loops = TRUE,
diff --git a/tools/migrations/conversion.R b/tools/migrations/conversion.R
index f70729ddca..05e9104015 100644
--- a/tools/migrations/conversion.R
+++ b/tools/migrations/conversion.R
@@ -16,7 +16,7 @@ migrations <- list(
       ...,
       weights = NULL,
       names = TRUE,
-      sparse = igraph_opt("sparsematrices"),
+      sparse = NULL,
       edges = deprecated(),
       attr = deprecated()
     ) {},
diff --git a/tools/migrations/games.R b/tools/migrations/games.R
index fdbc774737..89d95e6fc1 100644
--- a/tools/migrations/games.R
+++ b/tools/migrations/games.R
@@ -9,8 +9,8 @@ migrations <- list(
       nodes,
       types,
       ...,
-      type.dist.matrix = matrix(1, types, types),
-      pref.matrix = matrix(1, types, types),
+      type.dist.matrix = NULL,
+      pref.matrix = NULL,
       loops = FALSE
     ) {},
     when = "3.0.0"
@@ -21,9 +21,9 @@ migrations <- list(
     new = function(
       n,
       edges = 1,
-      types = rep(0, n),
+      types = NULL,
       ...,
-      pref = matrix(1, nrow = length(types), ncol = length(types)),
+      pref = NULL,
       directed = TRUE,
       attr = TRUE
     ) {},
@@ -35,9 +35,9 @@ migrations <- list(
     new = function(
       n,
       edges = 1,
-      types = rep(0, n),
+      types = NULL,
       ...,
-      pref = rep(1, length(types)),
+      pref = NULL,
       directed = TRUE,
       attr = TRUE
     ) {},
@@ -96,8 +96,8 @@ migrations <- list(
       n,
       edges = 1,
       ...,
-      agebins = n / 7100,
-      pref = (1:(agebins + 1))^-3,
+      agebins = NULL,
+      pref = NULL,
       directed = TRUE
     ) {},
     when = "3.0.0"
@@ -183,9 +183,9 @@ migrations <- list(
       nodes,
       types,
       ...,
-      type.dist = rep(1, types),
+      type.dist = NULL,
       fixed.sizes = FALSE,
-      pref.matrix = matrix(1, types, types),
+      pref.matrix = NULL,
       directed = FALSE,
       loops = FALSE
     ) {},
@@ -198,8 +198,8 @@ migrations <- list(
       nodes,
       types,
       ...,
-      type.dist.matrix = matrix(1, types, types),
-      pref.matrix = matrix(1, types, types),
+      type.dist.matrix = NULL,
+      pref.matrix = NULL,
       loops = FALSE
     ) {},
     when = "3.0.0"
@@ -210,9 +210,9 @@ migrations <- list(
     new = function(
       n,
       edges = 1,
-      types = rep(0, n),
+      types = NULL,
       ...,
-      pref = matrix(1, nrow = length(types), ncol = length(types)),
+      pref = NULL,
       directed = TRUE,
       attr = TRUE
     ) {},
@@ -224,9 +224,9 @@ migrations <- list(
     new = function(
       n,
       edges = 1,
-      types = rep(0, n),
+      types = NULL,
       ...,
-      pref = rep(1, length(types)),
+      pref = NULL,
       directed = TRUE,
       attr = TRUE
     ) {},
@@ -239,7 +239,7 @@ migrations <- list(
       old.graph,
       corr,
       ...,
-      p = edge_density(old.graph),
+      p = NULL,
       permutation = NULL
     ) {},
     when = "3.0.0"
@@ -375,8 +375,8 @@ migrations <- list(
       n,
       edges = 1,
       ...,
-      agebins = n / 7100,
-      pref = (1:(agebins + 1))^-3,
+      agebins = NULL,
+      pref = NULL,
       directed = TRUE
     ) {},
     when = "3.0.0"
@@ -466,9 +466,9 @@ migrations <- list(
       nodes,
       types,
       ...,
-      type.dist = rep(1, types),
+      type.dist = NULL,
       fixed.sizes = FALSE,
-      pref.matrix = matrix(1, types, types),
+      pref.matrix = NULL,
       directed = FALSE,
       loops = FALSE
     ) {},
@@ -509,8 +509,8 @@ migrations <- list(
       types,
       k = 1,
       ...,
-      type.dist = rep(1, types),
-      pref.matrix = matrix(1, types, types),
+      type.dist = NULL,
+      pref.matrix = NULL,
       directed = FALSE
     ) {},
     when = "3.0.0"
@@ -530,8 +530,8 @@ migrations <- list(
       types,
       ...,
       edge.per.step = 1,
-      type.dist = rep(1, types),
-      pref.matrix = matrix(1, types, types),
+      type.dist = NULL,
+      pref.matrix = NULL,
       directed = FALSE
     ) {},
     when = "3.0.0"
@@ -571,8 +571,8 @@ migrations <- list(
       types,
       k = 1,
       ...,
-      type.dist = rep(1, types),
-      pref.matrix = matrix(1, types, types),
+      type.dist = NULL,
+      pref.matrix = NULL,
       directed = FALSE
     ) {},
     when = "3.0.0"
@@ -592,8 +592,8 @@ migrations <- list(
       types,
       ...,
       edge.per.step = 1,
-      type.dist = rep(1, types),
-      pref.matrix = matrix(1, types, types),
+      type.dist = NULL,
+      pref.matrix = NULL,
       directed = FALSE
     ) {},
     when = "3.0.0"
diff --git a/tools/migrations/hrg-embedding.R b/tools/migrations/hrg-embedding.R
index 59ec37972c..6377eaceb7 100644
--- a/tools/migrations/hrg-embedding.R
+++ b/tools/migrations/hrg-embedding.R
@@ -12,8 +12,8 @@ migrations <- list(
       weights = NULL,
       which = c("lm", "la", "sa"),
       scaled = TRUE,
-      cvec = strength(graph, weights = weights) / (vcount(graph) - 1),
-      options = arpack_defaults()
+      cvec = NULL,
+      options = NULL
     ) {},
     when = "3.0.0"
   ),
@@ -28,7 +28,7 @@ migrations <- list(
       which = c("lm", "la", "sa"),
       type = c("default", "D-A", "DAD", "I-DAD", "OAP"),
       scaled = TRUE,
-      options = arpack_defaults()
+      options = NULL
     ) {},
     when = "3.0.0"
   ),
diff --git a/tools/migrations/layout.R b/tools/migrations/layout.R
index 52e0df783c..b9ac4be525 100644
--- a/tools/migrations/layout.R
+++ b/tools/migrations/layout.R
@@ -21,7 +21,7 @@ migrations <- list(
     new = function(
       graph,
       ...,
-      center = V(graph)[1],
+      center = NULL,
       order = NULL
     ) {},
     when = "3.0.0"
@@ -81,13 +81,13 @@ migrations <- list(
       ...,
       coords = NULL,
       maxiter = 10,
-      fineiter = max(10, log2(vcount(graph))),
+      fineiter = NULL,
       cool.fact = 0.75,
       weight.node.dist = 1.0,
       weight.border = 0.0,
-      weight.edge.lengths = edge_density(graph) / 10,
-      weight.edge.crossings = 1.0 - sqrt(edge_density(graph)),
-      weight.node.edge.dist = 0.2 * (1 - edge_density(graph))
+      weight.edge.lengths = NULL,
+      weight.edge.crossings = NULL,
+      weight.node.edge.dist = NULL
     ) {},
     when = "3.0.0"
   ),
@@ -98,8 +98,8 @@ migrations <- list(
       graph,
       ...,
       use.seed = FALSE,
-      seed = matrix(runif(vcount(graph) * 2), ncol = 2),
-      options = drl_defaults$default,
+      seed = NULL,
+      options = NULL,
       weights = NULL,
       dim = c(2, 3)
     ) {},
@@ -133,7 +133,7 @@ migrations <- list(
       coords = NULL,
       dim = c(2, 3),
       niter = 500,
-      start.temp = sqrt(vcount(graph)),
+      start.temp = NULL,
       grid = c("auto", "grid", "nogrid"),
       weights = NULL,
       minx = NULL,
@@ -157,10 +157,10 @@ migrations <- list(
       graph,
       ...,
       coords = NULL,
-      maxiter = 40 * vcount(graph)^2,
-      temp.max = max(vcount(graph), 1),
-      temp.min = 1 / 10,
-      temp.init = sqrt(max(vcount(graph), 1))
+      maxiter = NULL,
+      temp.max = NULL,
+      temp.min = 0.1,
+      temp.init = NULL
     ) {},
     when = "3.0.0"
   ),
@@ -216,9 +216,9 @@ migrations <- list(
       ...,
       coords = NULL,
       dim = c(2, 3),
-      maxiter = 50 * vcount(graph),
+      maxiter = NULL,
       epsilon = 0.0,
-      kkconst = max(vcount(graph), 1),
+      kkconst = NULL,
       weights = NULL,
       minx = NULL,
       maxx = NULL,
@@ -250,11 +250,11 @@ migrations <- list(
       graph,
       ...,
       maxiter = 150,
-      maxdelta = vcount(graph),
-      area = vcount(graph)^2,
+      maxdelta = NULL,
+      area = NULL,
       coolexp = 1.5,
-      repulserad = area * vcount(graph),
-      cellsize = sqrt(sqrt(area)),
+      repulserad = NULL,
+      cellsize = NULL,
       root = NULL
     ) {},
     when = "3.0.0"
diff --git a/tools/migrations/misc.R b/tools/migrations/misc.R
index 32bd22ca25..471d08a71a 100644
--- a/tools/migrations/misc.R
+++ b/tools/migrations/misc.R
@@ -68,7 +68,7 @@ migrations <- list(
       graph,
       ...,
       column.wise = FALSE,
-      sparse = igraph_opt("sparsematrices")
+      sparse = NULL
     ) {},
     when = "3.0.0"
   )
diff --git a/tools/migrations/motifs-graphlets.R b/tools/migrations/motifs-graphlets.R
index 21f26a2012..7bf83db155 100644
--- a/tools/migrations/motifs-graphlets.R
+++ b/tools/migrations/motifs-graphlets.R
@@ -21,7 +21,7 @@ migrations <- list(
       weights = NULL,
       cliques,
       niter = 1000,
-      Mu = rep(1, length(cliques))
+      Mu = NULL
     ) {},
     when = "3.0.0"
   ),
@@ -66,7 +66,7 @@ migrations <- list(
       graph,
       size = 3,
       ...,
-      cut.prob = rep(0, size),
+      cut.prob = NULL,
       sample.size = NULL,
       sample = NULL
     ) {},
diff --git a/tools/migrations/operators.R b/tools/migrations/operators.R
index 219b04a518..fb6ac480c2 100644
--- a/tools/migrations/operators.R
+++ b/tools/migrations/operators.R
@@ -27,7 +27,7 @@ migrations <- list(
       g2,
       ...,
       byname = "auto",
-      graph.attr.comb = igraph_opt("graph.attr.comb"),
+      graph.attr.comb = NULL,
       vertex.attr.comb = "rename",
       edge.attr.comb = "rename"
     ) {},
diff --git a/tools/migrations/plotting.R b/tools/migrations/plotting.R
index 876a80666f..22036c7f6a 100644
--- a/tools/migrations/plotting.R
+++ b/tools/migrations/plotting.R
@@ -28,8 +28,8 @@ migrations <- list(
     new = function(
       shape,
       ...,
-      clip = shape_noclip,
-      plot = shape_noplot,
+      clip = NULL,
+      plot = NULL,
       parameters = list()
     ) {},
     when = "3.0.0"
diff --git a/tools/migrations/similarity-efficiency.R b/tools/migrations/similarity-efficiency.R
index c6f1ad256a..1860778604 100644
--- a/tools/migrations/similarity-efficiency.R
+++ b/tools/migrations/similarity-efficiency.R
@@ -52,7 +52,7 @@ migrations <- list(
     old = function(graph, vids, weights, directed, mode) {},
     new = function(
       graph,
-      vids = V(graph),
+      vids = NULL,
       ...,
       weights = NULL,
       directed = TRUE,
@@ -65,7 +65,7 @@ migrations <- list(
     old = function(graph, vids, mode, loops, method) {},
     new = function(
       graph,
-      vids = V(graph),
+      vids = NULL,
       ...,
       mode = c(
         "all",
diff --git a/tools/migrations/structural-properties.R b/tools/migrations/structural-properties.R
index 66294f7737..b7b781964f 100644
--- a/tools/migrations/structural-properties.R
+++ b/tools/migrations/structural-properties.R
@@ -41,7 +41,7 @@ migrations <- list(
     new = function(
       graph,
       from,
-      to = V(graph),
+      to = NULL,
       ...,
       mode = c("out", "in", "all", "total"),
       cutoff = -1
@@ -64,7 +64,7 @@ migrations <- list(
     new = function(
       graph,
       from,
-      to = V(graph),
+      to = NULL,
       ...,
       mode = c("out", "all", "in"),
       weights = NULL
@@ -86,7 +86,7 @@ migrations <- list(
     old = function(graph, nodes, weights) {},
     new = function(
       graph,
-      nodes = V(graph),
+      nodes = NULL,
       ...,
       weights = NULL
     ) {},
@@ -127,7 +127,7 @@ migrations <- list(
     old = function(graph, v, mode, loops, normalized) {},
     new = function(
       graph,
-      v = V(graph),
+      v = NULL,
       ...,
       mode = c("all", "out", "in", "total"),
       loops = TRUE,
@@ -152,8 +152,8 @@ migrations <- list(
     old = function(graph, v, to, mode, weights, algorithm) {},
     new = function(
       graph,
-      v = V(graph),
-      to = V(graph),
+      v = NULL,
+      to = NULL,
       ...,
       mode = c("all", "out", "in"),
       weights = NULL,
@@ -184,7 +184,7 @@ migrations <- list(
     new = function(
       graph,
       order = 1,
-      nodes = V(graph),
+      nodes = NULL,
       ...,
       mode = c("all", "out", "in"),
       mindist = 0
@@ -197,7 +197,7 @@ migrations <- list(
     new = function(
       graph,
       order = 1,
-      nodes = V(graph),
+      nodes = NULL,
       ...,
       mode = c("all", "out", "in"),
       mindist = 0
@@ -286,7 +286,7 @@ migrations <- list(
     old = function(graph, vids, mode, neighbor.degree.mode, weights) {},
     new = function(
       graph,
-      vids = V(graph),
+      vids = NULL,
       ...,
       mode = c("all", "out", "in", "total"),
       neighbor.degree.mode = c("all", "out", "in", "total"),
@@ -300,7 +300,7 @@ migrations <- list(
     new = function(
       graph,
       order = 1,
-      nodes = V(graph),
+      nodes = NULL,
       ...,
       mode = c("all", "out", "in"),
       mindist = 0
@@ -315,7 +315,7 @@ migrations <- list(
       types = NULL,
       ...,
       weights = NULL,
-      eps = .Machine$double.eps
+      eps = NULL
     ) {},
     when = "3.0.0"
   ),
@@ -369,7 +369,7 @@ migrations <- list(
     new = function(
       graph,
       from,
-      to = V(graph),
+      to = NULL,
       ...,
       mode = c("out", "all", "in"),
       weights = NULL,
@@ -452,7 +452,7 @@ migrations <- list(
     old = function(graph, eids, loops) {},
     new = function(
       graph,
-      eids = E(graph),
+      eids = NULL,
       ...,
       loops = TRUE
     ) {},