layout_as_tree does not check for empty graph.

[clpippel]

Describe the bug layout_as_tree does not check for empty graph.

To reproduce make_empty_graph() |> plot(layout=layout_as_tree)

produces warning:

Warning message:
In max(res[, 2]) : no non-missing arguments to max; returning -Inf

If you are confident that the issue is not in the R interface but in the C core of igraph, please add it to the main igraph repo instead.

If you are unsure, feel free to add your issue here - we will transfer it to the main igraph repo if the root cause is in the C core of igraph.

Version information [1] igraph_1.3.4

[clpippel]

The functions below also produce warnings:

g <- make_empty_graph()

g |> plot(layout=component_wise)
g |> plot(layout=layout_as_bipartite)
g |> plot(layout=layout_as_star)
g |> plot(layout=layout_with_gem)
g |> plot(layout=layout_with_kk)
g |> plot(layout=layout_with_lgl)
g |> plot(layout=layout_with_mds)
g |> plot(layout=layout_with_sugiyama)
g |> plot(layout=layout_)
g |> plot(layout=merge_coords)
g |> plot(layout=norm_coords)
g |> plot(layout=normalize)

[ntamas]

I've updated your comment above to remove redundant (duplicate) entries. As for the rest:

• component_wise() and normalize() are layout modifiers, not layouts, so they are not meant to be used this way.
• merge_coords(), norm_coords() and layout_() are not layout functions either.
• layout_as_bipartite() prints an error saying that the graph is not bipartite, which is correct (it needs a type vertex attribute, even if the graph has no vertices).
• layout_as_star() prints an error, which is in some sense correct as you need a center vertex for a star layout and there are no vertices, so we cannot produce a star layout. Nevertheless, we should probably just special-case it and return an empty matrix.
• layout_as_tree(), layout_with_gem(), layout_with_kk() and layout_with_sugiyama() are fixed in 8bdb4627
• layout_with_lgl() and layout_with_mds() have to be fixed in the C core; fixed in the C core now in https://github.com/igraph/igraph/commit/17d3cbeafa16758964c1d0732a0ac0daaf081347 , https://github.com/igraph/igraph/commit/a4ff2b3a0aacf8d752262072d383a9d9f7f5bfdf and https://github.com/igraph/igraph/commit/fb778eef25def547bb053023b72d0e56b4c1aaed

[ntamas]

3bd98f43 fixes layout_as_star(), updates the C core to fix layout_with_lgl() and layout_with_mds(), and adds test to validate that these functions print no warnings for a null graph.