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

[Giuseppe G.]

Hi,

I would like to partition an undirected graph into communities using the 
Girvan-Newman edge betweenness method and then obtain a modularity index 
for the partitioned graph. I seem to have found exactly what I need in 
the following two methods:

igraph_community_edge_betweenness
igraph_modularity

I have read the reference guide and if I understand correctly I need to 
call a third method between the two, igraph_community_to_membership, to 
map the dendrogram matrix populated by igraph_community_edge_betweenness 
into a vector of cluster assignments for the nodes. I'm not sure if 
that's correct however. So far I have

//................
igraph_vector_t result;
igraph_vector_t eb;
igraph_matrix_t merges;
igraph_vector_t bridges;
igraph_vector_t membership;
igraph_real_t modularity_index;
igraph_integer_t nodes;
igraph_integer_t steps;
igraph_vector_init(&result, 2);
igraph_vector_init(&eb, 2);
igraph_vector_init(&bridges, 2);
igraph_matrix_init(&merges, 2, 2);

igraph_community_edge_betweenness(&graph,&result,&eb,&merges,&bridges,IGRAPH_UNDIRECTED);

long int rows = igraph_matrix_nrow(&merges);//leaves of the dendrogram

igraph_community_to_membership(&merges,rows,steps,&membership,NULL);

igraph_modularity(&graph, &membership, &modularity_index, NULL);

//................


However I'm not sure about two arguments of 
igraph_community_to_membership, rows, and steps. Firstly is my way of 
getting the rows parameter from the matrix correct? Secondly, how to 
obtain the steps argument? Any help would be greatly appreciated.

Best
Giuseppe

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