R igraph manual pages

Depth-first search

Description

Depth-first search is an algorithm to traverse a graph. It starts from a root vertex and tries to go quickly as far from as possible.

Usage

dfs(
  graph,
  root,
  mode = c("out", "in", "all", "total"),
  unreachable = TRUE,
  order = TRUE,
  order.out = FALSE,
  father = FALSE,
  dist = FALSE,
  in.callback = NULL,
  out.callback = NULL,
  extra = NULL,
  rho = parent.frame(),
  neimode
)

Arguments

graph	The input graph.
root	The single root vertex 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.
order	Logical scalar, whether to return the DFS ordering of the vertices.
order.out	Logical scalar, whether to return the ordering based on leaving the subtree of the vertex.
father	Logical scalar, whether to return the father of the vertices.
dist	Logical scalar, whether to return the distance from the root of the search tree.
in.callback	If not NULL, then it must be callback function. This is called whenever a vertex is visited. See details below.
out.callback	If not NULL, then it must be callback function. This is called whenever the subtree of a vertex is completed by the algorithm. 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.

Details

The callback functions 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 and ‘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 functions.

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.
order.out	Numeric vector, the vertex ids, in the order of the completion of their subtree.
father	Numeric vector. The father of each vertex, i.e. the vertex it was discovered from.
dist	Numeric vector, for each vertex its distance from the root of the search tree.

Note that order, order.out, father, 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

bfs for breadth-first search.

Examples

## A graph with two separate trees
dfs(make_tree(10) %du% make_tree(10), root=1, "out",
          TRUE, TRUE, TRUE, TRUE)

## How to use a callback
f.in <- function(graph, data, extra) {
  cat("in:", paste(collapse=", ", data), "\n")
  FALSE
}
f.out <- function(graph, data, extra) {
  cat("out:", paste(collapse=", ", data), "\n")
  FALSE
}
tmp <- dfs(make_tree(10), root=1, "out",
                 in.callback=f.in, out.callback=f.out)

## Terminate after the first component, using a callback
f.out <- function(graph, data, extra) {
 data['vid'] == 1
}
tmp <- dfs(make_tree(10) %du% make_tree(10), root=1,
                 out.callback=f.out)

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