Re: [igraph] Obtaining communities and modularity index for an undirected graph

[Giuseppe G.]

Thanks again Tamás.

I think I have some working code based on your recommendation now. One 
more thing. It seems like, in certain cases, the number of merges is not 
always Nodes-1 but smaller. Eg. I have a network with 101 nodes and 95 
merges. I obtain these numbers by doing

nodes = igraph_vcount(&graph);
fprintf(stdout,"Vertices: %li\n", (long int) nodes);
number_of_merges = igraph_matrix_nrow(&merges);
fprintf(stdout,"number of merges: %li\n", (long int) number_of_merges);

Is this expected behaviour? I.e. is N-1 just an upper boundary for the 
number of potential merges? Thanks!

G

On 21/05/12 13:51, Tamás Nepusz wrote:
> > All I've used so far is only "ready-made" implementations of the Girvan
> > Neumann algorithm, and I don't remember having had to set the number of
> > steps. All I remember getting was one community assignment presented to
> > me as THE GN clustering for that undirected network.
>
> The original publication of the Girvan-Newman algorithm (see 
> http://www.pnas.org/content/99/12/7821, preprint: 
> http://www.santafe.edu/media/workingpapers/01-12-077.pdf) does not describe how 
> to select the number of communities (i.e. how to decide where to cut the 
> dendrogram). As far as I know, most of the existing implementations use the 
> modularity score or some other internal quality criterion to decide the number 
> of communities. For instance, the SNAP library uses the maximum modularity 
> criterion.
>
> Cheers,
> Tamas
>
>
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