igraph: sub-sub-.igraph – R documentation

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sub-sub-.igraph

Query and manipulate a graph as it were an adjacency list

Query and manipulate a graph as it were an adjacency list

## S3 method for class 'igraph'
x[[i, j, from, to, ..., directed = TRUE, edges = FALSE, exact = TRUE]]

`x`	The graph.
`i`	Index, integer, character or logical, see details below.
`j`	Index, integer, character or logical, see details below.
`from`	A numeric or character vector giving vertex ids or names. Together with the `to` argument, it can be used to query/set a sequence of edges. See details below. This argument cannot be present together with any of the `i` and `j` arguments and if it is present, then the `to` argument must be present as well.
`to`	A numeric or character vector giving vertex ids or names. Together with the `from` argument, it can be used to query/set a sequence of edges. See details below. This argument cannot be present together with any of the `i` and `j` arguments and if it is present, then the `from` argument must be present as well.
`...`	Additional arguments are not used currently.
`directed`	Logical scalar, whether to consider edge directions in directed graphs. It is ignored for undirected graphs.
`edges`	Logical scalar, whether to return edge ids.
`exact`	Ignored.

The double bracket operator indexes the (imaginary) adjacency list of the graph. This can used for the following operations:

Querying the adjacent vertices for one or more vertices:
```
  graph[[1:3,]]
graph[[,1:3]]
```
The first form gives the successors, the second the predecessors or the 1:3 vertices. (For undirected graphs they are equivalent.)
Querying the incident edges for one or more vertices, if the edges argument is set to TRUE:
```
  graph[[1:3, , edges=TRUE]]
graph[[, 1:3, edges=TRUE]]
```
Querying the edge ids between two sets or vertices, if both indices are used. E.g.
```
  graph[[v, w, edges=TRUE]]
```
gives the edge ids of all the edges that exist from vertices v to vertices w.

The alternative argument names from and to can be used instead of the usual i and j, to make the code more readable:

graph[[from = 1:3]]
graph[[from = v, to = w, edges = TRUE]]

‘[[’ operators allows logical indices and negative indices as well, with the usual R semantics.

Vertex names are also supported, so instead of a numeric vertex id a vertex can also be given to ‘[’ and ‘[[’.