R: Find the multiple or loop edges in a graph

which_multiple {igraph} | R Documentation

## 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

`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.

### Author(s)

Gabor Csardi csardi.gabor@gmail.com

### See Also

`simplify` to eliminate loop and multiple edges.

### Examples

    # 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

[Package igraph version 1.2.4.1 Index]