# girth

##### Girth of a graph

The girth of a graph is the length of the shortest circle in it.

- Keywords
- graphs

##### Usage

`girth(graph, circle = TRUE)`

##### Arguments

- graph
The input graph. It may be directed, but the algorithm searches for undirected circles anyway.

- circle
Logical scalar, whether to return the shortest circle itself.

##### Details

The current implementation works for undirected graphs only, directed graphs are treated as undirected graphs. Loop edges and multiple edges are ignored. If the graph is a forest (ie. acyclic), then zero is returned.

This implementation is based on Alon Itai and Michael Rodeh: Finding a
minimum circuit in a graph *Proceedings of the ninth annual ACM
symposium on Theory of computing*, 1-10, 1977. The first implementation of
this function was done by Keith Briggs, thanks Keith.

##### Value

A named list with two components:

Integer constant, the girth of the graph, or 0 if the graph is acyclic.

Numeric vector with the vertex ids in the shortest circle.

##### References

Alon Itai and Michael Rodeh: Finding a minimum circuit in a
graph *Proceedings of the ninth annual ACM symposium on Theory of
computing*, 1-10, 1977

##### Examples

```
# NOT RUN {
# No circle in a tree
g <- make_tree(1000, 3)
girth(g)
# The worst case running time is for a ring
g <- make_ring(100)
girth(g)
# What about a random graph?
g <- sample_gnp(1000, 1/1000)
girth(g)
# }
```

*Documentation reproduced from package igraph, version 1.2.3, License: GPL (>= 2)*