igraph (version 1.3.5)

as.directed: Convert between directed and undirected graphs

Description

as.directed converts an undirected graph to directed, as.undirected does the opposite, it converts a directed graph to undirected.

Usage

as.directed(graph, mode = c("mutual", "arbitrary", "random", "acyclic"))
as.undirected(
  graph,
  mode = c("collapse", "each", "mutual"),
  edge.attr.comb = igraph_opt("edge.attr.comb")
)

Value

A new graph object.

Arguments

graph

The graph to convert.

mode

Character constant, defines the conversion algorithm. For as.directed it can be mutual or arbitrary. For as.undirected it can be each, collapse or mutual. See details below.

edge.attr.comb

Specifies what to do with edge attributes, if mode="collapse" or mode="mutual". In these cases many edges might be mapped to a single one in the new graph, and their attributes are combined. Please see attribute.combination for details on this.

Author

Gabor Csardi

Details

Conversion algorithms for as.directed:

"arbitrary"

The number of edges in the graph stays the same, an arbitrarily directed edge is created for each undirected edge, but the direction of the edge is deterministic (i.e. it always points the same way if you call the function multiple times).

"mutual"

Two directed edges are created for each undirected edge, one in each direction.

"random"

The number of edges in the graph stays the same, and a randomly directed edge is created for each undirected edge. You will get different results if you call the function multiple times with the same graph.

"acyclic"

The number of edges in the graph stays the same, and a directed edge is created for each undirected edge such that the resulting graph is guaranteed to be acyclic. This is achieved by ensuring that edges always point from a lower index vertex to a higher index. Note that the graph may include cycles of length 1 if the original graph contained loop edges.

Conversion algorithms for as.undirected:

"each"

The number of edges remains constant, an undirected edge is created for each directed one, this version might create graphs with multiple edges.

"collapse"

One undirected edge will be created for each pair of vertices which are connected with at least one directed edge, no multiple edges will be created.

"mutual"

One undirected edge will be created for each pair of mutual edges. Non-mutual edges are ignored. This mode might create multiple edges if there are more than one mutual edge pairs between the same pair of vertices.

See Also

simplify for removing multiple and/or loop edges from a graph.

Examples

g <- make_ring(10)
as.directed(g, "mutual")
g2 <- make_star(10)
as.undirected(g)

# Combining edge attributes
g3 <- make_ring(10, directed=TRUE, mutual=TRUE)
E(g3)$weight <- seq_len(ecount(g3))
ug3 <- as.undirected(g3)
print(ug3, e=TRUE)
if (FALSE) {
  x11(width=10, height=5)
  layout(rbind(1:2))
  plot( g3, layout=layout_in_circle, edge.label=E(g3)$weight)
  plot(ug3, layout=layout_in_circle, edge.label=E(ug3)$weight)
}

g4 <- graph(c(1,2, 3,2,3,4,3,4, 5,4,5,4,
              6,7, 7,6,7,8,7,8, 8,7,8,9,8,9,
              9,8,9,8,9,9, 10,10,10,10))
E(g4)$weight <- seq_len(ecount(g4))
ug4 <- as.undirected(g4, mode="mutual",
              edge.attr.comb=list(weight=length))
print(ug4, e=TRUE)