# edgeMatrix

##### Compute an Edge Matrix or weight vector for a Graph

For our purposes an *edge matrix* is a matrix with two rows and as
many columns as there are edges. The entries in the first row are the
index of the node the edge is *from*, those in the second row indicate
the node the edge is *to*.

If the graph is “undirected” then the `duplicates`

option
can be used to indicate whether reciprocal edges are wanted. The
default is to leave them out. In this case the notions of *from*
and *to* are not relevant.

- Keywords
- manip

##### Usage

```
edgeMatrix(object, duplicates=FALSE)
eWV(g, eM, sep = ifelse(edgemode(g) == "directed", "->", "--"), useNNames=FALSE)
pathWeights(g, p, eM=NULL)
```

##### Arguments

- object
- An object that inherits from
`graph`

. - g
- An object that inherits from
`graph`

. - duplicates
- Whether or not duplicate edges should be produced for “undirected” graphs.
- eM
- An edge matrix
- sep
- a character string to concatenate node labels in the edge label
- useNNames
- a logical; if TRUE, node names are used in the edge label; if FALSE, node indices are used
- p
- a vector of node names constituting a path in graph
`g`

- ...
- arguments passed to
`edgeMatrix`

.

##### Details

Implementations for `graphNEL`

, `clusterGraph`

and
`distGraph`

are available.

##### Value

`edgeMatrix`

returns a matrix with two rows, *from*and

*to*, and as many columns as there are edges. Entries indicate the index in the node vector that corresponds to the appropriate end of the edge.

`eWV`

uses the edge matrix to create an annotated vector of
edge weights.`pathWeights`

returns an annotated vector of edge weights
for a specified path in a graph.
##### Note

A path through an undirected graph may have several representations as a named vector of edges. Thus in the example, when the weights for path b-a-i are requested, the result is the pair of weights for edges a--b and a--i, as these are the edge labels computed for graph g1.

##### See Also

##### Examples

```
set.seed(123)
g1 <- randomGraph(letters[1:10], 1:4, p=.3)
edgeMatrix(g1)
g2 <- new("clusterGraph", clusters=list(a=c(1,2,3), b=c(4,5,6)))
em2 <- edgeMatrix(g2)
eWV(g1, edgeMatrix(g1))
eWV(g1, edgeMatrix(g1), useNNames=TRUE)
pathWeights(g1, c("b", "a", "i"))
```

*Documentation reproduced from package graph, version 1.44.1, License: Artistic-2.0*