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Re: [igraph] Extracting sub-graph

From: Tamás Nepusz
Subject: Re: [igraph] Extracting sub-graph
Date: Wed, 12 Oct 2011 15:09:36 +0200

> decompose() seems to work for that purpose, but not on directed graphs.
>    aa = i.decompose()
> igraph._igraph.InternalError: Error at components.c:463: only
> 'IGRAPH_WEAK' is implemented, Invalid value
> Note: unable to select mode='WEAK' with decompose(), is this a bug?
Well, sort of. The decompose() method (or more precisely, the underlying 
igraph_decompose function) has only been implemented to extract the weakly 
connected components. However, I made two mistakes:

1) I have inadvertedly set the default value of the mode=... argument to 
STRONG, which works for undirected graphs (since this argument is simply 
ignored) but not for directed ones.

2) I have forgot to handle cases when the mode argument is specified as string. 
So, this will work:

from igraph import Graph, WEAK

aa = i.decompose(mode=WEAK)

but this will not:

aa = i.decompose(mode="weak")

I'll fix this in the development version.


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