# R igraph manual pages

Use this if you are using igraph from R

## Modularity of a community structure of a graph

### Description

This function calculates how modular is a given division of a graph into subgraphs.

### Usage

```## S3 method for class 'igraph'
modularity(x, membership, weights = NULL, ...)

modularity_matrix(graph, membership, weights = NULL)
```

### Arguments

 `x, graph` The input graph. `membership` Numeric vector, for each vertex it gives its community. The communities are numbered from one. `weights` If not `NULL` then a numeric vector giving edge weights. `...` Additional arguments, none currently.

### Details

`modularity` calculates the modularity of a graph with respect to the given `membership` vector.

The modularity of a graph with respect to some division (or vertex types) measures how good the division is, or how separated are the different vertex types from each other. It defined as

Q=1/(2m) * sum( (Aij-ki*kj/(2m) ) delta(ci,cj),i,j),

here m is the number of edges, Aij is the element of the A adjacency matrix in row i and column j, ki is the degree of i, kj is the degree of j, ci is the type (or component) of i, cj that of j, the sum goes over all i and j pairs of vertices, and delta(x,y) is 1 if x=y and 0 otherwise.

If edge weights are given, then these are considered as the element of the A adjacency matrix, and ki is the sum of weights of adjacent edges for vertex i.

`modularity_matrix` calculates the modularity matrix. This is a dense matrix, and it is defined as the difference of the adjacency matrix and the configuration model null model matrix. In other words element M[i,j] is given as A[i,j]-d[i]d[j]/(2m), where A[i,j] is the (possibly weighted) adjacency matrix, d[i] is the degree of vertex i, and m is the number of edges (or the total weights in the graph, if it is weighed).

### Value

For `modularity` a numeric scalar, the modularity score of the given configuration.

For `modularity_matrix` a numeic square matrix, its order is the number of vertices in the graph.

### Author(s)

Gabor Csardi csardi.gabor@gmail.com

### References

Clauset, A.; Newman, M. E. J. & Moore, C. Finding community structure in very large networks, Phyisical Review E 2004, 70, 066111

`cluster_walktrap`, `cluster_edge_betweenness`, `cluster_fast_greedy`, `cluster_spinglass` for various community detection methods.

### Examples

```
g <- make_full_graph(5) %du% make_full_graph(5) %du% make_full_graph(5)
g <- add_edges(g, c(1,6, 1,11, 6, 11))
wtc <- cluster_walktrap(g)
modularity(wtc)
modularity(g, membership(wtc))

```

[Package igraph version 1.2.6 Index]