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Re: [igraph] problem with vertices

From: Tamas Nepusz
Subject: Re: [igraph] problem with vertices
Date: Thu, 27 Aug 2009 13:38:13 +0100
User-agent: Mutt/1.5.17 (2007-11-01)

Hi Figa,

> after deleting a vertex in the graph, the vertex is still there.. how come?
This is because vertices are always numbered from zero to N-1 if you
have N vertices. If you delete vertex I, then the vertices from vertex
I+1 will be renumbered so the range becomes continuous again. In
general, don't add new vertices or delete existing ones when you are
iterating over the vertices in a loop.

> g is an undirected graph;
> 1) I delete all edges whose weight is less than a certain threshold:
There's an easier way to do that:

g.es.select(weight_lt = threshold) selects the edges that have weight
under the given threshold. You can call g.delete_edges() directly on
this object:

g.delete_edges(g.es.select(weight_lt = threshold))

> 2) then I look for the vertices with degree equal to zero:
This can also be done with a similar trick:

g.vs.select(_degree = 0)

This gives you a VertexSeq that contains only the vertices with degree
zero. Then you can delete them straight away:

g.delete_vertices(g.vs.select(_degree = 0))


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