igraph (version 1.3.5)

count_triangles: 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))

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.

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.

Author

Gabor Csardi

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.

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)