Issue with vertices in boost::subgraph

Question

The issue

The code in the bottom of the post prints out:

Vertices in g  = [ 0 1 2 3 4 ]
Vertices in g' = [ 0 1 ]

I was expected the output to be:

Vertices in g  = [ 0 1 2 3 4 ]
Vertices in g' = [ 3 4 ]

Is this a bug in boost::subgraph or in my understanding of the library?

The code in question

#include <sstream>
#include <iostream>

#include <boost/graph/subgraph.hpp>
#include <boost/graph/adjacency_list.hpp>

using namespace std;
using namespace boost;

// Underlying graph representation and implementation
typedef adjacency_list_traits<vecS, vecS, directedS> Traits;

// Graph representation
typedef subgraph< adjacency_list<vecS, vecS, directedS,
    property<vertex_color_t, int>, property<edge_index_t, int> > > Graph;

// Iterating over vertices and edges
typedef graph_traits<Graph>::vertex_iterator vertex_iter;
typedef graph_traits<Graph>::edge_iterator edge_iter;

int main(void)
{
    Graph g;
    add_edge(0,1, g);
    add_edge(1,2, g);
    add_edge(3,4, g);

    Graph sub = g.create_subgraph();
    add_vertex(3, sub);
    add_vertex(4, sub);

    pair<vertex_iter, vertex_iter> vip;

    cout << "Vertices in g  = [ ";
    vip = vertices(g);
    for(vertex_iter vi = vip.first; vi != vip.second; ++vi) {
        cout << *vi << " ";
    }
    cout << "]" << endl;

    cout << "Vertices in g' = [ ";
    vip = vertices(sub);
    for(vertex_iter vi = vip.first; vi != vip.second; ++vi) {
        cout << *vi << " ";
    }
    cout << "]" << endl;

    return 0;
}

Answer 1

I was too lazy to read the documentation. boost::subgraph distinguished between "local" and "global" descriptors.

The add_vertex function uses global descriptors when adding vertices to the subgraph. The vertices() function returns local descriptors.

What I needed to do, was to use the method local_to_global() on the subgraph to "resolve" the local descriptor into the global descriptor that I was expecting.

The output:

Vertices in g  = [ 0 1 2 3 4 ]
Vertices (local) in g' = [ 0 1 ]
Vertices (global) in g' = [ 3 4 ]

Comes from the code:

#include <sstream>
#include <iostream>

#include <boost/graph/subgraph.hpp>
#include <boost/graph/adjacency_list.hpp>

using namespace std;
using namespace boost;

// Underlying graph representation and implementation
typedef adjacency_list_traits<vecS, vecS, directedS> Traits;

// Graph representation
typedef subgraph< adjacency_list<vecS, vecS, directedS,
    property<vertex_color_t, int>, property<edge_index_t, int> > > Graph;

// Iterating over vertices and edges
typedef graph_traits<Graph>::vertex_iterator vertex_iter;
typedef graph_traits<Graph>::edge_iterator edge_iter;

int main(void)
{
    Graph g;
    add_edge(0,1, g);
    add_edge(1,2, g);
    add_edge(3,4, g);

    Graph sub = g.create_subgraph();
    add_vertex(3, sub);
    add_vertex(4, sub);

    pair<vertex_iter, vertex_iter> vip;

    cout << "Vertices in g  = [ ";
    vip = vertices(g);
    for(vertex_iter vi = vip.first; vi != vip.second; ++vi) {
        cout << *vi << " ";
    }
    cout << "]" << endl;

    cout << "Vertices (local) in g' = [ ";
    vip = vertices(sub);
    for(vertex_iter vi = vip.first; vi != vip.second; ++vi) {
        cout << *vi << " ";
    }
    cout << "]" << endl;

    cout << "Vertices (global) in g' = [ ";
    vip = vertices(sub);
    for(vertex_iter vi = vip.first; vi != vip.second; ++vi) {
        cout << sub.local_to_global(*vi) << " ";
    }
    cout << "]" << endl;

    return 0;
}