R igraph convert parallel edges to weight attribute

Question

I'm working with igraph for R. My graph is based on an edgelist which includes parallel edges (more than one edge with the same source and target). I would like to convert these parallel edges to an edge attribute weight. Is there an eay way to do this?

If there is no easy way. how can I identify these parallel edges?

    duplicated(E(net))

does not return a single duplicate. I suppose its looking for duplicated edge ids.

score 29 · Accepted Answer · answered Oct 21 '12 at 21:05

29

You can also use E(graph)$weight <- 1 followed by simplify(graph, edge.attr.comb=list(weight="sum")) to assign a weight of 1 to each edge and then collapsing multiple edges into single ones while summing the weights.

answered Oct 21 '12 at 21:05

Tamás

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This solution is actually much better, because it uses linear space and time, whereas the adjacency matrix solution uses quadratic (in terms of the number of vertices) space and time. – Gabor Csardi Oct 22 '12 at 00:38
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Btw. this also eliminates loop edges, so if you don't want that, use the `remove.loops=FALSE` argument to `simplify()`. – Gabor Csardi Oct 22 '12 at 00:39

score 5 · Answer 2 · answered Oct 21 '12 at 14:32

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It seems exporting an unweighted graph with parallel edges to an adjacency matrix in igraph creates a weights list with the number of edges as weight, which can then be read again:

library("igraph")
E <- matrix(c(1,1,1,2,2,2),3,2)
G <- graph.edgelist(E)

G2 <- graph.adjacency(get.adjacency(G),weighted=TRUE)

answered Oct 21 '12 at 14:32

Sacha Epskamp

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This is a really helpful solution for very large graphs. – timothyjgraham Sep 12 '17 at 05:22

score 1 · Answer 3 · answered Dec 12 '17 at 09:20

In case you want to get the number of parallel edges of a graph without adding a weight attribute to your graph, you could use the following function:

duplicated <- function(graph){
  g_local <- graph
  E(g_local)$weight <- 1
  g_simp <- simplify(g_local, edge.attr.comb=list(weight="sum"))
  w <- E(g_simp)$weight
  return(sum(w-1))
}

R igraph convert parallel edges to weight attribute

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