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Re: [igraph] Modularity (Q) based on the Louvain split, unexpected value

From: serafim loukas
Subject: Re: [igraph] Modularity (Q) based on the Louvain split, unexpected values
Date: Sun, 29 Mar 2020 13:07:55 +0000

Just to add that I just found the python version of BCT (the Matlab toolbox that I used to benchmark igraph).

Using this: https://github.com/aestrivex/bctpy/blob/f9526a693a9af57051762442d8490dcdf2ebf4e3/bct/algorithms/modularity.py#L71, again I get approx. 0.1466 that matches the Matlab based results but is far from the python output (Q).


On 29 Mar 2020, at 14:56, serafim loukas <address@hidden> wrote:

Hi igraph community,

I have a graph and I want to estimate the modularity (Q) based on the Louvain split of the graph.

In python I used igraph and to compare, I also estimated Q in Matlab.
Based on igraph, the Q is negative (weird) whereas the Q based on the Matlab estimation is possitive.

I would expect differences in the values but not by so far (+ shows modularity, - shows anti-modularity).
Any idea why this happens?



My code and data:


import numpy as np
import scipy.io
from igraph import *

A = scipy.io.loadmat('A.mat')['A']

graph = Graph.Weighted_Adjacency(A.tolist(), mode=ADJ_UNDIRECTED, attr="weight", loops=False)
Louvain = graph.community_multilevel(weights=graph.es['weight'], return_levels=False)
Q = graph.modularity(Louvain)


MATLAB (https://sites.google.com/site/bctnet/measures/list)
using community_louvain.m: Louvain community detection algorithm

clear all

Q =


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