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## Re: [igraph] Inter and intra vertices' attribute value using subgraph

**From**: |
Tamás Nepusz |

**Subject**: |
Re: [igraph] Inter and intra vertices' attribute value using subgraph |

**Date**: |
Fri, 26 Aug 2011 16:51:15 +0200 |

Dear Ali,
>* If I understand Sugraphs correctly, it returns vertices with attributes and *
>* edges which satisfy some rule based on either the index or the regular *
>* expression of any kind. I don't seem to have the desired results when I run *
>* the following code over my graph.*
>* *
>* celticgroup <- which(V(g)$group == "CELTIC")*
>* subg <- subgraph(g, celticgroup)*
I think you are bitten by the classical "indices-off-by-one" bug here. The R
interface of igraph indexes the vertices from zero up to 0.5.5, but R uses a
1-based indexing. Therefore, you have to subtract 1 from "celticgroup" before
passing it to subgraph().
>* As I am interested in inter attribute level links as well, I would like to *
>* know how to do that - all edges which have "CELTIC" in either the source or *
>* the target vertex. I do not know how to get this calculated.*
>* Could anyone please share some insight here?*
Try something like:
celticgroup <- which(V(g)$group == "CELTIC") - 1
E(g)[celticgroup %--% V(g)]
The trick here is the %--% operator; see the following page for an explanation:
http://igraph.sourceforge.net/doc/R/iterators.html
--
T.