igraph nonreciprocal edges after converting to undirected graph using mutual

Question

I'm working on a directed graph in igraph for R. I'm trying to convert it to an undirected graph where just reciprocal edges of the former persist. Should be easy but I'm getting strange results.

first I did it like that

    library(igraph)
    load("dmNet.Rdata")
    #http://www.unet.univie.ac.at/~a0406222/dmNet.Rdata

    recNet <- as.undirected(net, mode = "mutual",edge.attr.comb="sum")

when I check E(recNet)$weight there are a lot of edges with a weight of 1, which should not be possible since the sum of two reciprocal edges has to be at least 2. Then I did it like that

    recNet <- as.undirected(net, mode = "mutual",edge.attr.comb="c")

now I can see that there are actually some edges containing just one value. My new graph recNet seems to contain nonreciprocal edges of net. What am I doing wrong, or am I missunderstandig the "mutual option"?

Gabor Csardi Gabor Csardi · Accepted Answer · 2012-10-22T14:18:53

This happens because some edges are self-loops in your graph. All the edges that end up with a weight of 1 are self-loops:

all(which(E(recNet)$weight == 1) %in% which(is.loop(recNet)))
# [1] TRUE

Apparently, a self-loop is considered as a mutual edge in a directed graph. If you want to consider self-loops as non-mutual, then you can just remove them from the graph. Be careful, though, because some vertices have multiple self-loops, and you might not want to remove these.

igraph nonreciprocal edges after converting to undirected graph using mutual

1 Answers