Multiple bugs in setbyname.R (patch attached)

[Alex Lubbock]

I've had a few issues with the set operation functions in setbyname.R, namely graph.union.by.name, graph.intersection.by.name and graph.difference.by.name. Here's a list:

 - difference, union and intersection by name don't work on directed graphs, despite the documentation saying so
 - safer.merge fails when one data frame has 0 rows and indicate.origin is TRUE
 - get.vertices.as.data.frame returns a 0 column data frame on empty graphs, rather than one empty column as expected
 - related to the above two points, set operations fail if either graph has an empty node or edge list
 - set operations fail if either graph doesn't have a vertex name attribute (sometimes failing silently) - instead, a meaningful error should probably be generated

I've attached a patch which fixes these issues (but could do with further testing). Hope it's helpful.

[Gábor Csárdi (gabor.csardi)]

Thanks, this looks great, I'll add it soon. I am actually in the process of rewriting these functions to make them more general and faster. BUt until that is finished, your patches will be very useful. Thanks again!

[Gábor Csárdi (gabor.csardi)]

The development of igraph has moved to github, so please do not comment on this bug here. You are of course welcome to comment on github, here:
https://github.com/igraph/igraph/issues/136