R: Breadth-first search

bfs {igraph} | R Documentation

## Breadth-first search

### Description

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,
      neimode = 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()
    )

### Arguments

`graph` | The input graph.
`root` | Numeric vector, usually of length one. The root vertex, or root vertices to start the search from.
`neimode` | 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.

### 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, ‘succ’, its successor, ‘rank’, the rank of the current vertex, ‘dist’, its distance from the root of the search tree.

extra

  The extra argument.

See examples below on how to use the callback function.

### 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 `neimode` 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.
`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.

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.

### Author(s)

Gabor Csardi csardi.gabor@gmail.com

### See Also

`dfs` for depth-first search.

### 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)

    ## 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)

    ## 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)

[Package igraph version 1.2.6 Index]