Breadth-first search is an algorithm to traverse a graph. We start from a root vertex and spread along every edge “simultaneously”.

## Usage

```
bfs(
graph,
root,
mode = c("out", "in", "all", "total"),
unreachable = TRUE,
restricted = NULL,
order = TRUE,
rank = FALSE,
father = FALSE,
pred = FALSE,
succ = FALSE,
dist = FALSE,
callback = NULL,
extra = NULL,
rho = parent.frame(),
neimode
)
```

## Arguments

- graph
The input graph.

- root
Numeric vector, usually of length one. The root vertex, or root vertices to start the search from.

- mode
For directed graphs specifies the type of edges to follow. ‘out’ follows outgoing, ‘in’ incoming edges. ‘all’ ignores edge directions completely. ‘total’ is a synonym for ‘all’. This argument is ignored for undirected graphs.

- unreachable
Logical scalar, whether the search should visit the vertices that are unreachable from the given root vertex (or vertices). If

`TRUE`

, then additional searches are performed until all vertices are visited.- restricted
`NULL`

(=no restriction), or a vector of vertices (ids or symbolic names). In the latter case, the search is restricted to the given vertices.- order
Logical scalar, whether to return the ordering of the vertices.

- rank
Logical scalar, whether to return the rank of the vertices.

- father
Logical scalar, whether to return the father of the vertices.

- pred
Logical scalar, whether to return the predecessors of the vertices.

- succ
Logical scalar, whether to return the successors of the vertices.

- dist
Logical scalar, whether to return the distance from the root of the search tree.

- callback
If not

`NULL`

, then it must be callback function. This is called whenever a vertex is visited. See details below.- extra
Additional argument to supply to the callback function.

- rho
The environment in which the callback function is evaluated.

- neimode
This argument is deprecated from igraph 1.3.0; use

`mode`

instead.

## Value

A named list with the following entries:

- root
Numeric scalar. The root vertex that was used as the starting point of the search.

- neimode
Character scalar. The

`mode`

argument of the function call. Note that for undirected graphs this is always ‘all’, irrespectively of the supplied value.- order
Numeric vector. The vertex ids, in the order in which they were visited by the search.

- rank
Numeric vector. The rank for each vertex, zero for unreachable vertices.

- father
Numeric vector. The father of each vertex, i.e. the vertex it was discovered from.

- pred
Numeric vector. The previously visited vertex for each vertex, or 0 if there was no such vertex.

- succ
Numeric vector. The next vertex that was visited after the current one, or 0 if there was no such vertex.

- dist
Numeric vector, for each vertex its distance from the root of the search tree. Unreachable vertices have a negative distance as of igraph 1.6.0, this used to be

`NaN`

.

Note that `order`

, `rank`

, `father`

, `pred`

, `succ`

and `dist`

might be `NULL`

if their corresponding argument is
`FALSE`

, i.e. if their calculation is not requested.

## Details

The callback function must have the following arguments:

- graph
The input graph is passed to the callback function here.

- data
A named numeric vector, with the following entries: ‘vid’, the vertex that was just visited, ‘pred’, its predecessor (zero if this is the first vertex), ‘succ’, its successor (zero if this is the last vertex), ‘rank’, the rank of the current vertex, ‘dist’, its distance from the root of the search tree.

- extra
The extra argument.

The callback must return `FALSE`

to continue the search or `TRUE`

to terminate it. See examples below on how to
use the callback function.

## See also

`dfs()`

for depth-first search.

Other structural.properties:
`component_distribution()`

,
`connect()`

,
`constraint()`

,
`coreness()`

,
`degree()`

,
`dfs()`

,
`distance_table()`

,
`edge_density()`

,
`feedback_arc_set()`

,
`girth()`

,
`is_acyclic()`

,
`is_dag()`

,
`is_matching()`

,
`k_shortest_paths()`

,
`knn()`

,
`reciprocity()`

,
`subcomponent()`

,
`subgraph()`

,
`topo_sort()`

,
`transitivity()`

,
`unfold_tree()`

,
`which_multiple()`

,
`which_mutual()`

## Author

Gabor Csardi csardi.gabor@gmail.com

## Examples

```
## Two rings
bfs(make_ring(10) %du% make_ring(10),
root = 1, "out",
order = TRUE, rank = TRUE, father = TRUE, pred = TRUE,
succ = TRUE, dist = TRUE
)
#> $root
#> [1] 1
#>
#> $mode
#> [1] "out"
#>
#> $order
#> + 20/20 vertices, from f75503d:
#> [1] 1 2 10 3 9 4 8 5 7 6 11 12 20 13 19 14 18 15 17 16
#>
#> $rank
#> [1] 1 2 4 6 8 10 9 7 5 3 11 12 14 16 18 20 19 17 15 13
#>
#> $father
#> + 20/20 vertices, from f75503d:
#> [1] NA 1 2 3 4 5 8 9 10 1 NA 11 12 13 14 15 18 19 20 11
#>
#> $pred
#> + 20/20 vertices, from f75503d:
#> [1] NA 1 10 9 8 7 5 4 3 2 NA 11 20 19 18 17 15 14 13 12
#>
#> $succ
#> + 20/20 vertices, from f75503d:
#> [1] 2 10 9 8 7 NA 6 5 4 3 12 20 19 18 17 NA 16 15 14 13
#>
#> $dist
#> [1] 0 1 2 3 4 5 4 3 2 1 0 1 2 3 4 5 4 3 2 1
#>
#> $neimode
#> [1] "out"
#>
## How to use a callback
f <- function(graph, data, extra) {
print(data)
FALSE
}
tmp <- bfs(make_ring(10) %du% make_ring(10),
root = 1, "out",
callback = f
)
#> vid pred succ rank dist
#> 1 0 2 1 0
#> vid pred succ rank dist
#> 2 1 10 2 1
#> vid pred succ rank dist
#> 10 2 3 3 1
#> vid pred succ rank dist
#> 3 10 9 4 2
#> vid pred succ rank dist
#> 9 3 4 5 2
#> vid pred succ rank dist
#> 4 9 8 6 3
#> vid pred succ rank dist
#> 8 4 5 7 3
#> vid pred succ rank dist
#> 5 8 7 8 4
#> vid pred succ rank dist
#> 7 5 6 9 4
#> vid pred succ rank dist
#> 6 7 0 10 5
#> vid pred succ rank dist
#> 11 0 12 11 0
#> vid pred succ rank dist
#> 12 11 20 12 1
#> vid pred succ rank dist
#> 20 12 13 13 1
#> vid pred succ rank dist
#> 13 20 19 14 2
#> vid pred succ rank dist
#> 19 13 14 15 2
#> vid pred succ rank dist
#> 14 19 18 16 3
#> vid pred succ rank dist
#> 18 14 15 17 3
#> vid pred succ rank dist
#> 15 18 17 18 4
#> vid pred succ rank dist
#> 17 15 16 19 4
#> vid pred succ rank dist
#> 16 17 0 20 5
## How to use a callback to stop the search
## We stop after visiting all vertices in the initial component
f <- function(graph, data, extra) {
data["succ"] == -1
}
bfs(make_ring(10) %du% make_ring(10), root = 1, callback = f)
#> $root
#> [1] 1
#>
#> $mode
#> [1] "out"
#>
#> $order
#> + 20/20 vertices, from 2d51a5a:
#> [1] 1 2 10 3 9 4 8 5 7 6 11 12 20 13 19 14 18 15 17 16
#>
#> $rank
#> NULL
#>
#> $father
#> NULL
#>
#> $pred
#> NULL
#>
#> $succ
#> NULL
#>
#> $dist
#> NULL
#>
#> $neimode
#> [1] "out"
#>
```