Depth-first search

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.

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 = deprecated()
)

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.

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.

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.

See also

bfs() for breadth-first search.

Other structural.properties: bfs(), component_distribution(), connect(), constraint(), coreness(), degree(), 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

## A graph with two separate trees
dfs(make_tree(10) %du% make_tree(10),
  root = 1, "out",
  TRUE, TRUE, TRUE, TRUE
)
#> $root
#> [1] 1
#> 
#> $mode
#> [1] "out"
#> 
#> $order
#> + 20/20 vertices, from a51e1c1:
#>  [1]  1  2  4  8  9  5 10  3  6  7 11 12 14 18 19 15 20 13 16 17
#> 
#> $order.out
#> + 20/20 vertices, from a51e1c1:
#>  [1]  8  9  4 10  5  2  6  7  3  1 18 19 14 20 15 12 16 17 13 11
#> 
#> $father
#> + 20/20 vertices, from a51e1c1:
#>  [1] NA  1  1  2  2  3  3  4  4  5 NA 11 11 12 12 13 13 14 14 15
#> 
#> $dist
#> NULL
#> 
#> $neimode
#> [1] "out"
#> 

## 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
)
#> in: 1, 0 
#> in: 2, 1 
#> in: 4, 2 
#> in: 8, 3 
#> out: 8, 2 
#> in: 9, 3 
#> out: 9, 2 
#> out: 4, 1 
#> in: 5, 2 
#> in: 10, 3 
#> out: 10, 2 
#> out: 5, 1 
#> out: 2, 0 
#> in: 3, 1 
#> in: 6, 2 
#> out: 6, 1 
#> in: 7, 2 
#> out: 7, 1 
#> out: 3, 0 
#> out: 1, -1 

## 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
)