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## [igraph] Modularity (Q) based on the Louvain split, unexpected values

 From: serafim loukas Subject: [igraph] Modularity (Q) based on the Louvain split, unexpected values Date: Sun, 29 Mar 2020 12:56:16 +0000

```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?

Makis

———————————

My code and data:

PYTHON

```
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)
print(Q)

-0.001847596203445795
```

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

```
clear all
load('A.mat')
[M,Q]=community_louvain(A);

Q =

0.1466
```

```

A.mat
Description: A.mat

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