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[igraph] graph.edgelist() behavior

From: Melody Lam
Subject: [igraph] graph.edgelist() behavior
Date: Sun, 15 Jun 2008 21:18:15 -0500

Dear List:

I've been working with igraph for a few weeks for a research project
and I noticed this strange behavior with graph.edgelist() -

Say I have a simple edge list like this (named E):

1 2
2 3
3 4
4 5

graph.edgelist(E, directed = FALSE) gives out 4 edges, which is
correct. The adjacency matrix for this edge list only has zeros and

but if I had this edge list:

1 2
2 1
3 4
4 3
4 5
5 4

and give the command graph.edgelist(E, directed = FALSE), I instead
get 8 edges. The adjacency matrix for this graph instead has zeros and
twos, which means that 1 2 and 2 1 were counted separately and they're
NOT the same.

So I'm wondering why in igraph the edge connecting vertex 1 to 2 is
not the same as the edge from 2 to 1 if the graph is undirected.

Melody Lam

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