`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")
)
```

## 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.

## 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.

Other conversion:
`as.matrix.igraph()`

,
`as_adj_list()`

,
`as_adjacency_matrix()`

,
`as_biadjacency_matrix()`

,
`as_data_frame()`

,
`as_edgelist()`

,
`as_graphnel()`

,
`as_long_data_frame()`

,
`graph_from_adj_list()`

,
`graph_from_graphnel()`

## Author

Gabor Csardi csardi.gabor@gmail.com

## Examples

```
g <- make_ring(10)
as.directed(g, "mutual")
#> IGRAPH ba78111 D--- 10 20 -- Ring graph
#> + attr: name (g/c), mutual (g/l), circular (g/l)
#> + edges from ba78111:
#> [1] 1-> 2 2-> 3 3-> 4 4-> 5 5-> 6 6-> 7 7-> 8 8-> 9 9->10 1->10
#> [11] 2-> 1 3-> 2 4-> 3 5-> 4 6-> 5 7-> 6 8-> 7 9-> 8 10-> 9 10-> 1
g2 <- make_star(10)
as.undirected(g)
#> IGRAPH cb0824a U--- 10 10 -- Ring graph
#> + attr: name (g/c), mutual (g/l), circular (g/l)
#> + edges from cb0824a:
#> [1] 1-- 2 2-- 3 3-- 4 4-- 5 5-- 6 6-- 7 7-- 8 8-- 9 9--10 1--10
# Combining edge attributes
g3 <- make_ring(10, directed = TRUE, mutual = TRUE)
E(g3)$weight <- seq_len(ecount(g3))
ug3 <- as.undirected(g3)
print(ug3, e = TRUE)
#> IGRAPH cb8dfd2 U-W- 10 10 -- Ring graph
#> + attr: name (g/c), mutual (g/l), circular (g/l), weight (e/n)
#> + edges from cb8dfd2:
#> [1] 1-- 2 2-- 3 3-- 4 4-- 5 5-- 6 6-- 7 7-- 8 8-- 9 1--10 9--10
if (FALSE) { # rlang::is_interactive()
x11(width = 10, height = 5)
layout(rbind(1:2))
plot(g3, layout = layout_in_circle, edge.label = E(g3)$weight)
plot(ug3, layout = layout_in_circle, edge.label = E(ug3)$weight)
}
g4 <- make_graph(c(
1, 2, 3, 2, 3, 4, 3, 4, 5, 4, 5, 4,
6, 7, 7, 6, 7, 8, 7, 8, 8, 7, 8, 9, 8, 9,
9, 8, 9, 8, 9, 9, 10, 10, 10, 10
))
E(g4)$weight <- seq_len(ecount(g4))
ug4 <- as.undirected(g4,
mode = "mutual",
edge.attr.comb = list(weight = length)
)
print(ug4, e = TRUE)
#> IGRAPH b03beb7 U-W- 10 7 --
#> + attr: weight (e/n)
#> + edges from b03beb7:
#> [1] 6-- 7 7-- 8 8-- 9 8-- 9 9-- 9 10--10 10--10
```