# which_multiple

##### Find the multiple or loop edges in a graph

A loop edge is an edge from a vertex to itself. An edge is a multiple edge if it has exactly the same head and tail vertices as another edge. A graph without multiple and loop edges is called a simple graph.

- Keywords
- graphs

##### Usage

`which_multiple(graph, eids = E(graph))`

##### Arguments

- graph
The input graph.

- eids
The edges to which the query is restricted. By default this is all edges in the graph.

##### Details

`which_loop`

decides whether the edges of the graph are loop edges.

`any_multiple`

decides whether the graph has any multiple edges.

`which_multiple`

decides whether the edges of the graph are multiple
edges.

`count_multiple`

counts the multiplicity of each edge of a graph.

Note that the semantics for `which_multiple`

and `count_multiple`

is
different. `which_multiple`

gives `TRUE`

for all occurences of a
multiple edge except for one. Ie. if there are three `i-j`

edges in the
graph then `which_multiple`

returns `TRUE`

for only two of them while
`count_multiple`

returns ‘3’ for all three.

See the examples for getting rid of multiple edges while keeping their original multiplicity as an edge attribute.

##### Value

`any_multiple`

returns a logical scalar. `which_loop`

and
`which_multiple`

return a logical vector. `count_multiple`

returns a
numeric vector.

##### See Also

`simplify`

to eliminate loop and multiple edges.

##### Examples

```
# NOT RUN {
# Loops
g <- graph( c(1,1,2,2,3,3,4,5) )
which_loop(g)
# Multiple edges
g <- barabasi.game(10, m=3, algorithm="bag")
any_multiple(g)
which_multiple(g)
count_multiple(g)
which_multiple(simplify(g))
all(count_multiple(simplify(g)) == 1)
# Direction of the edge is important
which_multiple(graph( c(1,2, 2,1) ))
which_multiple(graph( c(1,2, 2,1), dir=FALSE ))
# Remove multiple edges but keep multiplicity
g <- barabasi.game(10, m=3, algorithm="bag")
E(g)$weight <- count_multiple(g)
g <- simplify(g)
any(which_multiple(g))
E(g)$weight
# }
```

*Documentation reproduced from package igraph, version 1.0.1, License: GPL (>= 2)*