# dfs

##### 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.

- Keywords
- graphs

##### Usage

```
dfs(graph, root, neimode = 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())
```

##### Arguments

- graph
The input graph.

- root
The single root vertex 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.- 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.

##### 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.

See examples below on how to use the callback functions.

##### Value

A named list with the following entries:

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

Character scalar. The `neimode`

argument of the function
call. Note that for undirected graphs this is always ‘all’,
irrespectively of the supplied value.

Numeric vector. The vertex ids, in the order in which they were visited by the search.

Numeric vector, the vertex ids, in the order of the completion of their subtree.

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

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.

##### See Also

`bfs`

for breadth-first search.

##### Examples

```
# NOT RUN {
## 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)
# }
```

*Documentation reproduced from package igraph, version 1.2.3, License: GPL (>= 2)*