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

## Usage

```
# S3 method for igraph
modularity(x, membership, weights = NULL, resolution = 1, directed = TRUE, ...)
modularity_matrix(
graph,
membership,
weights = NULL,
resolution = 1,
directed = TRUE
)
```

## Arguments

- x, graph
The input graph.

- membership
Numeric vector, one value for each vertex, the membership vector of the community structure.

- weights
If not

`NULL`

then a numeric vector giving edge weights.- resolution
The resolution parameter. Must be greater than or equal to 0. Set it to 1 to use the classical definition of modularity.

- directed
Whether to use the directed or undirected version of modularity. Ignored for undirected graphs.

- ...
Additional arguments, none currently.

## Value

For `modularity()`

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

For `modularity_matrix()`

a numeric square matrix, its order is the number of
vertices in the graph.

## 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 $$Q=\frac{1}{2m} \sum_{i,j} (A_{ij}-\gamma\frac{k_i k_j}{2m})\delta(c_i,c_j),$$ here \(m\) is the number of edges, \(A_{ij}\) is the element of the \(A\) adjacency matrix in row \(i\) and column \(j\), \(k_i\) is the degree of \(i\), \(k_j\) is the degree of \(j\), \(c_i\) is the type (or component) of \(i\), \(c_j\) 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. For directed graphs, it is defined as $$Q = \frac{1}{m} \sum_{i,j} (A_{ij}-\gamma \frac{k_i^{out} k_j^{in}}{m})\delta(c_i,c_j).$$

The resolution parameter \(\gamma\) allows weighting the random null model, which might be useful when finding partitions with a high modularity. Maximizing modularity with higher values of the resolution parameter typically results in more, smaller clusters when finding partitions with a high modularity. Lower values typically results in fewer, larger clusters. The original definition of modularity is retrieved when setting \(\gamma\) to 1.

If edge weights are given, then these are considered as the element of the \(A\) adjacency matrix, and \(k_i\) 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
\(M_{ij}\) is given as \(A_{ij}-d_i
d_j/(2m)\), where \(A_{ij}\) 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).

## References

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

## See also

`cluster_walktrap()`

,
`cluster_edge_betweenness()`

,
`cluster_fast_greedy()`

, `cluster_spinglass()`

,
`cluster_louvain()`

and `cluster_leiden()`

for
various community detection methods.

Community detection
`as_membership()`

,
`cluster_edge_betweenness()`

,
`cluster_fast_greedy()`

,
`cluster_fluid_communities()`

,
`cluster_infomap()`

,
`cluster_label_prop()`

,
`cluster_leading_eigen()`

,
`cluster_leiden()`

,
`cluster_louvain()`

,
`cluster_optimal()`

,
`cluster_spinglass()`

,
`cluster_walktrap()`

,
`compare()`

,
`groups()`

,
`make_clusters()`

,
`membership()`

,
`plot_dendrogram()`

,
`split_join_distance()`

,
`voronoi_cells()`

## Author

Gabor Csardi csardi.gabor@gmail.com

## 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)
#> [1] 0.5757576
modularity(g, membership(wtc))
#> [1] 0.5757576
```