Breadth-first search

Source: R/structural.properties.R

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

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 = deprecated()
)

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

[Deprecated] 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.

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 2c8005d:
#>  [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 2c8005d:
#>  [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 2c8005d:
#>  [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 2c8005d:
#>  [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 d56aa95:
#>  [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"
#>