R: Find triangles in graphs

count_triangles {igraph} | R Documentation

## Find triangles in graphs

### Description

Count how many triangles a vertex is part of, in a graph, or just list the triangles of a graph.

### Usage

    count_triangles(graph, vids = V(graph))

### Arguments

`graph` | The input graph. It might be directed, but edge directions are ignored.
`vids` | The vertices to query, all of them by default. This might be a vector of numeric ids, or a character vector of symbolic vertex names for named graphs.

### Details

`triangles` lists all triangles of a graph. For efficiency, all triangles are returned in a single vector. The first three vertices belong to the first triangle, etc.

`count_triangles` counts how many triangles a vertex is part of.

### Value

For `triangles` a numeric vector of vertex ids, the first three vertices belong to the first triangle found, etc.

For `count_triangles` a numeric vector, the number of triangles for all vertices queried.

### Author(s)

Gabor Csardi csardi.gabor@gmail.com

### See Also

`transitivity`

### Examples

    ## A small graph
    kite <- make_graph("Krackhardt_Kite")
    plot(kite)
    matrix(triangles(kite), nrow=3)

    ## Adjacenct triangles
    atri <- count_triangles(kite)
    plot(kite, vertex.label=atri)

    ## Always true
    sum(count_triangles(kite)) == length(triangles(kite))

    ## Should match, local transitivity is the
    ## number of adjacent triangles divided by the number
    ## of adjacency triples
    transitivity(kite, type="local")
    count_triangles(kite) / (degree(kite) * (degree(kite)-1)/2)

[Package igraph version 1.3.2 Index]