For using the igraph C library
igraph_error_t igraph_bfs(const igraph_t *graph, igraph_integer_t root, const igraph_vector_int_t *roots, igraph_neimode_t mode, igraph_bool_t unreachable, const igraph_vector_int_t *restricted, igraph_vector_int_t *order, igraph_vector_int_t *rank, igraph_vector_int_t *father, igraph_vector_int_t *pred, igraph_vector_int_t *succ, igraph_vector_int_t *dist, igraph_bfshandler_t *callback, void *extra);
A simple breadthfirst search, with a lot of different results and the possibility to call a callback whenever a vertex is visited. It is allowed to supply null pointers as the output arguments the user is not interested in, in this case they will be ignored.
If not all vertices can be reached from the supplied root vertex, then additional root vertices will be used, in the order of their vertex IDs.
Consider using igraph_bfs_simple
instead if you set most of the output
arguments provided by this function to a null pointer.
Arguments:

The input graph. 

The id of the root vertex. It is ignored if the 

Pointer to an initialized vector, or a null pointer. If not a null pointer, then it is a vector containing root vertices to start the BFS from. The vertices are considered in the order they appear. If a root vertex was already found while searching from another one, then no search is conducted from it. 

For directed graphs, it defines which edges to follow.


Logical scalar, whether the search should visit the vertices that are unreachable from the given root node(s). If true, then additional searches are performed until all vertices are visited. 

If not a null pointer, then it must be a pointer to a vector containing vertex IDs. The BFS is carried out only on these vertices. 

If not null pointer, then the vertex IDs of the graph are stored here, in the same order as they were visited. 

If not a null pointer, then the rank of each vertex is stored here. 

If not a null pointer, then the id of the father of each vertex is stored here. When a vertex was not visited during the traversal, 2 will be stored as the ID of its father. When a vertex was visited during the traversal and it was one of the roots of the search trees, 1 will be stored as the ID of its father. 

If not a null pointer, then the id of vertex that was visited before the current one is stored here. If there is no such vertex (the current vertex is the root of a search tree), then 1 is stored as the predecessor of the vertex. If the vertex was not visited at all, then 2 is stored for the predecessor of the vertex. 

If not a null pointer, then the id of the vertex that was visited after the current one is stored here. If there is no such vertex (the current one is the last in a search tree), then 1 is stored as the successor of the vertex. If the vertex was not visited at all, then 2 is stored for the successor of the vertex. 

If not a null pointer, then the distance from the root of the current search tree is stored here for each vertex. If a vertex was not reached during the traversal, its distance will be 1 in this vector. 

If not null, then it should be a pointer to a
function of type 

Extra argument to pass to the callback function. 
Returns:
Error code. 
Time complexity: O(V+E), linear in the number of vertices and edges.
Example 15.1. File examples/simple/igraph_bfs.c
/* * mode: C * */ /* IGraph library. Copyright (C) 20092021 The igraph development team This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 021101301 USA */ #include <igraph.h> igraph_bool_t bfs_callback(const igraph_t *graph, igraph_integer_t vid, igraph_integer_t pred, igraph_integer_t succ, igraph_integer_t rank, igraph_integer_t dist, void *extra) { IGRAPH_UNUSED(graph); IGRAPH_UNUSED(pred); IGRAPH_UNUSED(succ); IGRAPH_UNUSED(rank); IGRAPH_UNUSED(dist); printf(" %" IGRAPH_PRId "", vid); return 0; } int main() { igraph_t graph, ring; igraph_vector_int_t order, rank, father, pred, succ, dist; /* Create a disjoint union of two rings */ igraph_ring(&ring, 10, /*directed=*/ 0, /*mutual=*/ 0, /*circular=*/ 1); igraph_disjoint_union(&graph, &ring, &ring); igraph_destroy(&ring); /* Initialize the vectors where the result will be stored. Any of these * can be omitted and replaced with a null pointer when calling * igraph_bfs() */ igraph_vector_int_init(&order, 0); igraph_vector_int_init(&rank, 0); igraph_vector_int_init(&father, 0); igraph_vector_int_init(&pred, 0); igraph_vector_int_init(&succ, 0); igraph_vector_int_init(&dist, 0); /* Now call the BFS function */ igraph_bfs(&graph, /*root=*/0, /*roots=*/ 0, /*neimode=*/ IGRAPH_OUT, /*unreachable=*/ 1, /*restricted=*/ 0, &order, &rank, &father, &pred, &succ, &dist, /*callback=*/ 0, /*extra=*/ 0); /* Print the results */ igraph_vector_int_print(&order); igraph_vector_int_print(&rank); igraph_vector_int_print(&father); igraph_vector_int_print(&pred); igraph_vector_int_print(&succ); igraph_vector_int_print(&dist); /* Cleam up after ourselves */ igraph_vector_int_destroy(&order); igraph_vector_int_destroy(&rank); igraph_vector_int_destroy(&father); igraph_vector_int_destroy(&pred); igraph_vector_int_destroy(&succ); igraph_vector_int_destroy(&dist); igraph_destroy(&graph); return 0; }
Example 15.2. File examples/simple/igraph_bfs_callback.c
/* * mode: C * */ /* IGraph library. Copyright (C) 20092021 The igraph development team This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 021101301 USA */ #include <igraph.h> igraph_bool_t bfs_callback(const igraph_t *graph, igraph_integer_t vid, igraph_integer_t pred, igraph_integer_t succ, igraph_integer_t rank, igraph_integer_t dist, void *extra) { IGRAPH_UNUSED(graph); IGRAPH_UNUSED(pred); IGRAPH_UNUSED(succ); IGRAPH_UNUSED(rank); IGRAPH_UNUSED(dist); printf(" %" IGRAPH_PRId "", vid); return 0; } int main() { igraph_t graph, ring; /* Create a disjoint union of two rings */ igraph_ring(&ring, 10, /*directed=*/ 0, /*mutual=*/ 0, /*circular=*/ 1); igraph_disjoint_union(&graph, &ring, &ring); igraph_destroy(&ring); /* Now call the BFS function */ printf("("); igraph_bfs(&graph, /*root=*/0, /*roots=*/ 0, /*neimode=*/ IGRAPH_OUT, /*unreachable=*/ 1, /*restricted=*/ 0, /*order=*/ 0, /*rank=*/ 0, /*father=*/ 0, /*pred=*/ 0, /*succ=*/ 0, /*dist=*/ 0, /*callback=*/ bfs_callback, /*extra=*/ 0); printf(" )\n"); /* Cleam up after ourselves */ igraph_destroy(&graph); return 0; }
igraph_error_t igraph_bfs_simple(igraph_t *graph, igraph_integer_t vid, igraph_neimode_t mode, igraph_vector_int_t *vids, igraph_vector_int_t *layers, igraph_vector_int_t *parents);
An alternative breadthfirst search implementation to cater for the
simpler usecases when only a single breadthfirst search has to be conducted
from a source node and most of the output arguments from igraph_bfs
are not needed. It is allowed to supply null pointers as
the output arguments the user is not interested in, in this case they will
be ignored.
Arguments:

The input graph. 

The id of the root vertex. 

For directed graphs, it defines which edges to follow.


If not a null pointer, then an initialized vector must be passed here. The IDs of the vertices visited during the traversal will be stored here, in the same order as they were visited. 

If not a null pointer, then an initialized vector must be
passed here. The ith element of the vector will contain the index
into 

If not a null pointer, then an initialized vector must be passed here. The vector will be resized so its length is equal to the number of nodes, and it will contain the index of the parent node for each visited node. The values in the vector are undefined for vertices that were not visited. 
Returns:
Error code. 
Time complexity: O(V+E), linear in the number of vertices and edges.
Example 15.3. File examples/simple/igraph_bfs_simple.c
/* * mode: C * */ /* IGraph library. Copyright (C) 20092021 The igraph development team This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 021101301 USA */ #include <igraph.h> int main() { igraph_t g; igraph_vector_int_t vids, layers, parents; igraph_ring(&g, 10, IGRAPH_UNDIRECTED, 0, 0); igraph_vector_int_init(&vids, 0); igraph_vector_int_init(&layers, 0); igraph_vector_int_init(&parents, 0); igraph_bfs_simple(&g, 0, IGRAPH_ALL, &vids, &layers, &parents); igraph_vector_int_print(&vids); igraph_vector_int_print(&layers); igraph_vector_int_print(&parents); igraph_destroy(&g); igraph_vector_int_destroy(&vids); igraph_vector_int_destroy(&layers); igraph_vector_int_destroy(&parents); return 0; }
typedef igraph_error_t igraph_bfshandler_t(const igraph_t *graph, igraph_integer_t vid, igraph_integer_t pred, igraph_integer_t succ, igraph_integer_t rank, igraph_integer_t dist, void *extra);
igraph_bfs()
is able to call a callback function, whenever a
new vertex is found, while doing the breadthfirst search. This
callback function must be of type igraph_bfshandler_t
. It has
the following arguments:
Arguments:

The graph that that algorithm is working on. Of course this must not be modified. 

The id of the vertex just found by the breadthfirst search. 

The id of the previous vertex visited. It is 1 if there is no previous vertex, because the current vertex is the root is a search tree. 

The id of the next vertex that will be visited. It is 1 if there is no next vertex, because the current vertex is the last one in a search tree. 

The rank of the current vertex, it starts with zero. 

The distance (number of hops) of the current vertex from the root of the current search tree. 

The extra argument that was passed to 
Returns:

See also:
igraph_error_t igraph_dfs(const igraph_t *graph, igraph_integer_t root, igraph_neimode_t mode, igraph_bool_t unreachable, igraph_vector_int_t *order, igraph_vector_int_t *order_out, igraph_vector_int_t *father, igraph_vector_int_t *dist, igraph_dfshandler_t *in_callback, igraph_dfshandler_t *out_callback, void *extra);
A simple depthfirst search, with the possibility to call a callback whenever a vertex is discovered and/or whenever a subtree is finished. It is allowed to supply null pointers as the output arguments the user is not interested in, in this case they will be ignored.
If not all vertices can be reached from the supplied root vertex, then additional root vertices will be used, in the order of their vertex IDs.
Arguments:

The input graph. 

The id of the root vertex. 

For directed graphs, it defines which edges to follow.


Logical scalar, whether the search should visit the vertices that are unreachable from the given root node(s). If true, then additional searches are performed until all vertices are visited. 

If not null pointer, then the vertex IDs of the graph are stored here, in the same order as they were discovered. The tail of the vector will be padded with 1 to ensure that the length of the vector is the same as the number of vertices, even if some vertices were not visited during the traversal. 

If not a null pointer, then the vertex IDs of the graphs are stored here, in the order of the completion of their subtree. The tail of the vector will be padded with 1 to ensure that the length of the vector is the same as the number of vertices, even if some vertices were not visited during the traversal. 

If not a null pointer, then the id of the father of each vertex is stored here. 1 will be stored for the root of the search tree; 2 will be stored for vertices that were not visited. 

If not a null pointer, then the distance from the root of the current search tree is stored here. 1 will be stored for vertices that were not visited. 

If not null, then it should be a pointer to a
function of type 

If not null, then it should be a pointer to a
function of type 

Extra argument to pass to the callback function(s). 
Returns:
Error code. 
Time complexity: O(V+E), linear in the number of vertices and edges.
typedef igraph_error_t igraph_dfshandler_t(const igraph_t *graph, igraph_integer_t vid, igraph_integer_t dist, void *extra);
igraph_dfs()
is able to call a callback function, whenever a
new vertex is discovered, and/or whenever a subtree is
completed. These callbacks must be of type igraph_dfshandler_t
. They have the following arguments:
Arguments:

The graph that that algorithm is working on. Of course this must not be modified. 

The id of the vertex just found by the depthfirst search. 

The distance (number of hops) of the current vertex from the root of the current search tree. 

The extra argument that was passed to 
Returns:

See also:
igraph_error_t igraph_random_walk(const igraph_t *graph, igraph_vector_int_t *walk, igraph_integer_t start, igraph_neimode_t mode, igraph_integer_t steps, igraph_random_walk_stuck_t stuck);
Performs a random walk with a given length on a graph, from the given
start vertex. Edge directions are (potentially) considered, depending on
the mode
argument.
Arguments:

The input graph, it can be directed or undirected. Multiple edges are respected, so are loop edges. 

An allocated vector, the result is stored here. It will be resized as needed. 

The start vertex for the walk. 

The number of steps to take. If the random walk gets
stuck, then the 

How to walk along the edges in directed graphs.


What to do if the random walk gets stuck.

Returns:
Error code. 
Time complexity: O(l + d), where l
is the length of the
walk, and d
is the total degree of the visited nodes.
igraph_error_t igraph_random_edge_walk(const igraph_t *graph, const igraph_vector_t *weights, igraph_vector_int_t *edgewalk, igraph_integer_t start, igraph_neimode_t mode, igraph_integer_t steps, igraph_random_walk_stuck_t stuck);
Performs a random walk with a given length on a graph, from the given
start vertex. Edge directions are (potentially) considered, depending on
the mode
argument.
Arguments:

The input graph, it can be directed or undirected. Multiple edges are respected, so are loop edges. 

A vector of nonnegative edge weights. It is assumed that at least one strictly positive weight is found among the outgoing edges of each vertex. Additionally, no edge weight may be NaN. If either case does not hold, an error is returned. If it is a NULL pointer, all edges are considered to have equal weight. 

An initialized vector; the indices of traversed edges are stored here. It will be resized as needed. 

The start vertex for the walk. 

The number of steps to take. If the random walk gets
stuck, then the 

How to walk along the edges in directed graphs.


What to do if the random walk gets stuck.

Returns:
Error code. 
← Chapter 14. Graph cycles  Chapter 16. Cliques and independent vertex sets → 