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## [igraph] Re: plotting planar graphs with igraph?

 From: Gábor Csárdi Subject: [igraph] Re: plotting planar graphs with igraph? Date: Wed, 8 Dec 2010 16:35:16 +0100

Please stay on the list. Thanks.

On Wed, Dec 8, 2010 at 4:25 PM, Christian Jost wrote:
[...]
If you have the 'x' and 'y' coordinates in the corresponding attributes, then do
plot(g, layout=cbind(V(g)\$x, V(g)\$y))

[...]
Why should it reconnect the neighbor vertices? Graphs usually
represent parts of a real system, and the relationships between them.
E.g. people and friendship in case of a social network. If we remove a
person for some particular reason, then why should we reconnect the
network? Her friends do not became friends, just because we removed
her from the network.

I agree that is some special cases it might make sense to reconnect
it, but IMHO in general it does not.

in my case it makes sense (see attached example, since the tunnels are curved I have to add degree-2 nodes simply to model this geometric constraint). But when it comes to identify the bifurcations and the geometric distance between them I thought that I could use the delete.vertices function in order to create the network without degree-2 nodes, assess the topological distance from the origin (as in the Horton Strahler index in river networks) and then come back to the original network in order to compute this geometric distance. Well, I understand now that I have to program this myself, no problem.

Just a final question: when testing/treating for example edge 2
> E(g)[2]
Edge sequence:
e
e [2] 4 -- 0
how do I extract the numbers 4 and 0 ? I tried things like E(g)[2][1] or E(g)[2][[1]], but so far I havn't gotten it working.
Maybe there is another function in igraph to extract the vertice numbers to which an edge is connected?

Best,
G.