graph.empty(n=NULL) results in undefined vcount(g)

[clpippel]

L.S.

I happen to run into the following (simplified) R-code:

# allocate graph.empty with undefined vcount
library(igraph)
g <- graph.empty(n=NULL, directed=TRUE)
print(vcount(g))  # <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<

sessionInfo()

The result is a graph with an uninitialized number of vertices, instead of as expected zero.

(screenshot included)

0.

[gaborcsardi]

I am not sure why you'd expect that NULL is the same as 0, it is not. You want an empty graph, that's n = 0.

[clpippel]

I am not sure why you'd expect that NULL is the same as 0, it is not. You want an empty graph, that's n = 0.

I expect NULL is NULL. In my original program NULL was actual a variable erroneously set to NULL (not NA).

g <- graph.empty(n=npls, directed=TRUE)

What I did expect (or hoped for) was at least an error message for example:
At type_indexededgelist.c:117 : cannot create empty graph, Invalid value n=NULL

I did not expect a graph with 72228592 vertices.

With no warning it took me some time to sort out what went wrong and why the next statement took so much time:

V(g)$name <- paste(seq_len(vcount(g)), sep="")

Note that:
g <- make_tree(NULL)
treats NULL as zero

[clpippel]

g <- make_graph(NULL)

gives errors and warnings:

Error in (function (edges, n = max(edges), directed = TRUE)  : 
  At type_indexededgelist.c:117 : cannot create empty graph with negative number of vertices, Invalid value
In addition: Warning message:
In max(edges) : no non-missing arguments to max; returning -Inf

and creates a graph with zero vertices and edges.

Note that a proper empty graph is created with no warning when:

g <- make_graph(NULL, n=NULL)
or
g <- make_graph(edges=c(), n=NULL)

However,
g <- graph.empty(NULL)
gives when sufficient memory is available

IGRAPH 1554744 D--- 333586160 0 -- 
+ edges from 1554744:

[ntamas]

I have recently patched make_graph() to accept NULL as an argument without warnings and produce an empty graph (see this commit).

g <- graph.empty(NULL) also works now after applying this commit).

These will be part of the upcoming patch release.