graph_from_adj_list: Create graphs from adjacency lists

Description

An adjacency list is a list of numeric vectors, containing the neighbor vertices for each vertex. This function creates an igraph graph object from such a list.

Usage

graph_from_adj_list(
  adjlist,
  mode = c("out", "in", "all", "total"),
  duplicate = TRUE
)

Arguments

adjlist

The adjacency list. It should be consistent, i.e. the maximum throughout all vectors in the list must be less than the number of vectors (=the number of vertices in the graph). Note that the list is expected to be 0-indexed.

mode

Character scalar, it specifies whether the graph to create is undirected (‘all’ or ‘total’) or directed; and in the latter case, whether it contains the outgoing (‘out’) or the incoming (‘in’) neighbors of the vertices.

duplicate

Logical scalar. For undirected graphs it gives whether edges are included in the list twice. E.g. if it is TRUE then for an undirected {A,B} edge graph_from_adj_list expects A included in the neighbors of B and B to be included in the neighbors of A.

This argument is ignored if mode is out or in.

Value

An igraph graph object.

Details

Adjacency lists are handy if you intend to do many (small) modifications to a graph. In this case adjacency lists are more efficient than igraph graphs.

The idea is that you convert your graph to an adjacency list by as_adj_list, do your modifications to the graphs and finally create again an igraph graph by calling graph_from_adj_list.

Examples

Run this code

# NOT RUN {
## Directed
g <- make_ring(10, dir=TRUE)
al <- as_adj_list(g, mode="out")
g2 <- graph_from_adj_list(al)
graph.isomorphic(g, g2)

## Undirected
g <- make_ring(10)
al <- as_adj_list(g)
g2 <- graph_from_adj_list(al, mode="all")
graph.isomorphic(g, g2)
ecount(g2)
g3 <- graph_from_adj_list(al, mode="all", duplicate=FALSE)
ecount(g3)
which_multiple(g3)
# }