
From:  Tamas Nepusz 
Subject:  Re: [igraph] graph.edgelist() behavior 
Date:  Mon, 16 Jun 2008 09:55:40 +0200 
That's the expected behaviour (and I hope you intended to write 6 edges instead of 8, otherwise it is really a bug :)). Consider this as follows: if you have an undirected graph, then adding an edge from vertex A to B "automatically creates" its counterpart from vertex B to A. After that, adding an edge from B to A creates _another_ edge along with its counterpart. You simply get multiple edges between A and B. If you want to get rid of multiple edges, you can use simplify() on your graph after creation  this removes multiple and loop edges.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.
 Tamas
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