igraph edge between two vertices

Question

I'm new to R and igraph and I was wondering if anybody can help me with the following.

I want to find the edge weight between two vertices in a graph. My graph structure is defined by the normal ego (node1), alter (node2) and the weight of the edge between them.

I know that I can get the weight for each of the edges in the list of edges that originate from node number 5 using E(igraph_friendship) [ from(5) ]$weight And that I can find the weight for each of the edges in the list of edges that end onto node number 10 using E(igraph_friendship) [ to(10) ]$weight

But what if I simply want to find the weight of the edge that simple connects just node 5 and node 10?

Alternatively, if I can get the identifier of the edge that connects node 5 and 10 in the list of all edges, E(igraph_friendship), that would work too.

Thanks a lot for your help, I've been looking around a lot for it and I really appreciate your help!

Answer 1

Gabor's use of the adjacency matrix helped. However, it took me a while to figure out how to get my edge lists with weights into an adjacency matrix. I tried to do it the usual way using graph.data.frame but then would get a weird error when I tried translating the igraph object to and adjacency matrix (error: Error in .M.kind(x) : not yet implemented for matrix w/ typeof character). This post helped do the trick: https://sites.google.com/site/daishizuka/toolkits/sna/weighted-edgelists.

However, what I found out from the R help email list help to work best was this simple operator directly on the igraph object: E(g)[5 %--% 10]$weight. See http://igraph.sourceforge.net/doc/R/iterators.html for details

Answer 2

This is actually quite easy in igraph 0.6 and above, because you can treat the graph as if it was an adjacency matrix (or weighed adjacency matrix in your case):

library(igraph)
g <- graph.ring(10)
g[1,2]
# [1] 1
E(g)$weight <- runif(ecount(g))
g[1,2]
# [1] 0.8115639

If you want to do this for the whole matrix, then you can simply do

g[]

Or, if you don't want sparse matrices, then

g[sparse=FALSE]

Please see ?"[.igraph" for more.