graph levelization using BGL - boost-graph

Question

Can you please post example code to levelize a directed graph using BGL?
Definition of levelization: Vertex has a property "int level". During BFS traversal of the graph, when a vertex is being "examined", look at its predecessor vertices' levels, take max of these, increment, and assign this to the "level" of this vertex.

Answer 1

If you mean the BFS depth then this is already built in to boost BFS and can be gotten easily.
Just use a vector to store the depths and a depth BFS visitor like this example I made:
#include <iostream>
#include <vector>
#include <boost/graph/adjacency_list.hpp>
#include <boost/graph/graph_traits.hpp>
#include <boost/graph/breadth_first_search.hpp>
using namespace std;
using namespace boost;
typedef adjacency_list < vecS, vecS, directedS,
property< vertex_index_t, size_t> ,
property< edge_index_t, size_t > > Graph;
typedef graph_traits<Graph>::vertex_descriptor Vertex;
typedef graph_traits<Graph>::edge_descriptor Edge;
int main(int argc, char* argv[]){
Graph G;
vector<Vertex> verts;
for(size_t i = 0; i < 9; ++i){
Vertex v = add_vertex(G);
verts.push_back(v);
}
/*
0 0
/ \
1 1 4
/ \
2 2 5
/ \
3 3 6
\
4 7
\
5 8
*/
add_edge(verts.at(0),verts.at(1),G);
add_edge(verts.at(1),verts.at(2),G);
add_edge(verts.at(2),verts.at(3),G);
add_edge(verts.at(0),verts.at(4),G);
add_edge(verts.at(4),verts.at(5),G);
add_edge(verts.at(5),verts.at(6),G);
add_edge(verts.at(6),verts.at(7),G);
add_edge(verts.at(7),verts.at(8),G);
cout << "vertices " << num_vertices(G) << endl;
cout << "edges " << num_edges(G) << endl;
//store depths
vector<size_t> d(num_vertices(G));
//get an index map, from Graph definition property< vertex_index_t, size_t>
typedef boost::property_map< Graph, boost::vertex_index_t>::type VertexIndexMap;
VertexIndexMap v_index = get(boost::vertex_index, G);
// Create the external property map, this map wraps the storage vector d
boost::iterator_property_map< std::vector< size_t >::iterator, VertexIndexMap >
d_map(d.begin(), v_index);
//Start at 0
boost::breadth_first_search(G, verts.at(0),
boost::visitor(boost::make_bfs_visitor(
boost::record_distances(d_map, boost::on_tree_edge())
)));
cout << "Starting at 0" << endl;
for(size_t i = 0; i < 9; ++i){
//depth (level) of BFS
cout << "vertex " << i << "\t" << d.at(i) << endl;
}
vector<size_t> d2(num_vertices(G));
cout << "Starting at 4" << endl;
// Create the external property map, this map wraps the storage vector d
boost::iterator_property_map< std::vector< size_t >::iterator, VertexIndexMap >
d2_map(d2.begin(), v_index);
//start at 4
boost::breadth_first_search(G, verts.at(4),
boost::visitor(boost::make_bfs_visitor(
boost::record_distances(d2_map, boost::on_tree_edge())
)));
for(size_t i = 0; i < 9; ++i){
//depth (level) of BFS
cout << "vertex " << i << "\t" << d2.at(i) << endl;
}
}
Output should look like this:
vertices 9
edges 8
Starting at 0
vertex 0 0
vertex 1 1
vertex 2 2
vertex 3 3
vertex 4 1
vertex 5 2
vertex 6 3
vertex 7 4
vertex 8 5
Starting at 4
vertex 0 0
vertex 1 0
vertex 2 0
vertex 3 0
vertex 4 0
vertex 5 1
vertex 6 2
vertex 7 3
vertex 8 4
When you start at 4 the other vertices are not reachable (directed) so the vector contains default values (0 in this case). This should work for undirected too.