# R igraph manual pages

Use this if you are using igraph from R

## Find the multiple or loop edges in a graph

### Description

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.

### 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

`any_loop` decides whether the graph has any loop edges.

`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 occurrences 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_loop` and `any_multiple` return a logical scalar. `which_loop` and `which_multiple` return a logical vector. `count_multiple` returns a numeric vector.

### Author(s)

Gabor Csardi csardi.gabor@gmail.com

`simplify` to eliminate loop and multiple edges.

### Examples

```
# Loops
g <- graph( c(1,1,2,2,3,3,4,5) )
any_loop(g)
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, edge.attr.comb=list(weight = "min"))
any(which_multiple(g))
E(g)\$weight

```

[Package igraph version 1.3.0 Index]