[igraph] Edge selection

[Matthew Walker]

Hi,

I've been having a small problem with edge selection.  I think there 
might be an issue with the use of the %->% operator when there are no 
edges that match the given specification.  I think the following code 
shows this most clearly:


> library(igraph)
> g <- graph( c(0,1, 1,1, 1,2, 2,3, 3,1) )
> g
Vertices: 4
Edges: 5
Directed: TRUE
Edges:
        
[0] 0 -> 1
[1] 1 -> 1
[2] 1 -> 2
[3] 2 -> 3
[4] 3 -> 1
> E(g) [ 0:1 %->% 2:3 ]
Edge sequence:
   e      
e [2] 1 -> 2
> E(g) [ 0:1 %->% 3 ]
Edge sequence:
  e    
e []  ->
Warning message:
In max(el) : no non-missing arguments to max; returning -Inf


However, the "empty edge selector" seems to be fairly functional 
nonetheless:

> E(g)$value <- 1:5
> E(g) [ 0:1 %->% 2 ]$value
[1] 3
> E(g) [ 0:1 %->% 3 ]$value
integer(0)


For both cases, I would have instead expected an "NA" (or similar) 
response.  Something along the lines of:

> v <- 1:5
> v[6]
[1] NA


What do you think of this?  Or am I missing something?


Cheers,

Matthew