Question

This question is connected with: Using boost connected components with cartesian points

I’ve made some changes in example to use cartesian points. Here is my current code:

using namespace boost;
typedef adjacency_list <vecS, vecS, undirectedS, cv::Point> Graph;
typedef graph_traits<Graph>::vertex_descriptor Vertex;
typedef graph_traits<Graph>::vertices_size_type VertexIndex;

int main()
{
    const int VERTEX_COUNT = 10;
    Graph graph(VERTEX_COUNT);

    std::vector<VertexIndex> rank(num_vertices(graph));
    std::vector<Vertex> parent(num_vertices(graph));

    typedef VertexIndex* Rank;
    typedef Vertex* Parent;

    disjoint_sets<Rank,Parent> ds(&rank[0], &parent[0]);

    initialize_incremental_components(graph,ds);
    incremental_components(graph,ds);

    graph_traits<Graph>::edge_descriptor edge;
    bool flag;

    std::vector<Vertex> verticesVector;
    //vector of points which creates graph
    std::vector<cv::Point> points;

    points.push_back(cv::Point(0,0));
    points.push_back(cv::Point(1,1));
    points.push_back(cv::Point(2,2));
    points.push_back(cv::Point(3,1));
    points.push_back(cv::Point(4,2));
    points.push_back(cv::Point(0,2));
    points.push_back(cv::Point(2,3));
    points.push_back(cv::Point(3,3));

    int i = 0;
    for(std::vector<cv::Point>::iterator it = points.begin(); 
            it !=points.end();it++, i++)  
    {
        verticesVector.push_back(add_vertex(graph));
        graph[i].x = (*it).x;
        graph[i].y = (*it).y;
    }

    typedef component_index<VertexIndex> Components;
    Components components(parent.begin(), parent.end());

    BOOST_FOREACH(VertexIndex current_index, components)
    {
        std::cout<<"component "<<current_index<<" contains: ";
        BOOST_FOREACH(VertexIndex child_index, components[current_index])
        {
            std::cout<<child_index<<" "<<" x = "<<graph[current_index].x<<";"<<graph[current_index].y<<"||";
        }
        std::cout<<std::endl;
    }

    boost::tie(edge,flag) = add_edge(verticesVector[0],verticesVector[1],graph);
    ds.union_set(verticesVector[0],verticesVector[1]);
    boost::tie(edge,flag) = add_edge(verticesVector[1],verticesVector[2],graph);
    ds.union_set(verticesVector[1],verticesVector[2]);
    boost::tie(edge,flag) = add_edge(verticesVector[2],verticesVector[3],graph);
    ds.union_set(verticesVector[2],verticesVector[3]);
    boost::tie(edge,flag) = add_edge(verticesVector[3],verticesVector[4],graph);
    ds.union_set(verticesVector[3],verticesVector[4]);
    boost::tie(edge,flag) = add_edge(verticesVector[1],verticesVector[5],graph);
    ds.union_set(verticesVector[1],verticesVector[5]);
    boost::tie(edge,flag) = add_edge(verticesVector[5],verticesVector[6],graph);
    ds.union_set(verticesVector[5],verticesVector[6]);
    boost::tie(edge,flag) = add_edge(verticesVector[6],verticesVector[7],graph);
    ds.union_set(verticesVector[6],verticesVector[7]);

    Components components2(parent.begin(), parent.end());


    /*BOOST_FOREACH(Vertex current_vertex, vertices(graph)) 
    {
        std::cout<< "representative["<<current_vertex<<"] = "
        << ds.find_set(current_vertex)<<std::endl;
    }

    std::cout<<std::endl;*/

    for(std::vector<Vertex>::iterator it = verticesVector.begin(); it !=  verticesVector.end(); it++)
    {
        std::cout<<"representative = "<<ds.find_set((*it))<<std::endl;
    }

    BOOST_FOREACH(VertexIndex current_index, components2)
    {
        std::cout<<"component "<<current_index<<" contains: ";
        BOOST_FOREACH(VertexIndex child_index, components[current_index])
        {
            std::cout<<child_index<<" "<<" x,y = "<<graph[current_index].x<<";"<<graph[current_index].y<<"||";
        }
        std::cout<<std::endl;
    }

    //write_graphviz(std::cout, graph);

    getchar();
    return 0;
}

I’m getting exception when trying to union_set(): Access violation reading location…
I can not figure out the reason. I’ve read all topics on boost lib, tried to search in docs.

Answer 1

I’ve figured it out thanks to this tutorial: Algorithm for selecting all edges and vertices connected to one vertex

The problems was:

• I initialized vector with some vertices first (copy paste from example and modifying wasn’t best idea)
• As tutorial says “Notice that graph[ a VertexID ] gives a Vertex, but graph[ an EdgeID ] gives an Edge “
• Disjoint set should be initialized after adding some vertices

Working code:

using namespace boost;

typedef adjacency_list <vecS, vecS, undirectedS, cv::Point> Graph;
typedef graph_traits<Graph>::vertex_descriptor Vertex;
typedef graph_traits<Graph>::vertices_size_type VertexIndex;

int main()
{
    Graph graph;

    graph_traits<Graph>::edge_descriptor edge;
    bool flag;

    std::vector<Vertex> verticesVector;
    Vertex verticesTable[8];
    std::vector<cv::Point> points;

    points.push_back(cv::Point(0,0));
    points.push_back(cv::Point(1,1));
    points.push_back(cv::Point(2,2));
    points.push_back(cv::Point(3,1));
    points.push_back(cv::Point(4,2));
    points.push_back(cv::Point(0,2));
    points.push_back(cv::Point(2,3));
    points.push_back(cv::Point(3,3));

    int i = 0;
    for(std::vector<cv::Point>::iterator it = points.begin(); it != points.end(); it++, i++)
    {
        verticesTable[i] = add_vertex(graph);
        graph[verticesTable[i]].x = (*it).x;
        graph[verticesTable[i]].y = (*it).y;
    }

    std::vector<VertexIndex> rank(num_vertices(graph));
    std::vector<Vertex> parent(num_vertices(graph));

    typedef VertexIndex* Rank;
    typedef Vertex* Parent;

    disjoint_sets<Rank,Parent> ds(&rank[0], &parent[0]);

    initialize_incremental_components(graph,ds);
    incremental_components(graph,ds);

    typedef component_index<VertexIndex> Components;
    Components components(parent.begin(), parent.end());

    Graph::vertex_iterator vertexIt, vertexEnd;
    boost::tie(vertexIt,vertexEnd) = vertices(graph);
    for(;vertexIt != vertexEnd; ++vertexIt)
    {
        Vertex a = *vertexIt;
        cv::Point &b = graph[a];
        std::cout<<"Point: "<<b.x<<";"<<b.y<<std::endl;
    }

    boost::tie(edge,flag) = add_edge(verticesTable[0],verticesTable[1],graph);
    ds.union_set(verticesTable[0],verticesTable[1]);
    boost::tie(edge,flag) = add_edge(verticesTable[1],verticesTable[2],graph);
    ds.union_set(verticesTable[1],verticesTable[2]);
    //boost::tie(edge,flag) = add_edge(verticesTable[2],verticesTable[3],graph);
    //ds.union_set(verticesTable[2],verticesTable[3]);
    boost::tie(edge,flag) = add_edge(verticesTable[3],verticesTable[4],graph);
    ds.union_set(verticesTable[3],verticesTable[4]);
    boost::tie(edge,flag) = add_edge(verticesTable[1],verticesTable[5],graph);
    ds.union_set(verticesTable[1],verticesTable[5]);
    boost::tie(edge,flag) = add_edge(verticesTable[5],verticesTable[6],graph);
    ds.union_set(verticesTable[5],verticesTable[6]);
    boost::tie(edge,flag) = add_edge(verticesTable[6],verticesTable[7],graph);
    ds.union_set(verticesTable[6],verticesTable[7]);

    BOOST_FOREACH(Vertex current_vertex, vertices(graph)) 
    {
        std::cout<< "representative["<<current_vertex<<"] = "
        << ds.find_set(current_vertex)<<std::endl;
    }

    std::cout<<std::endl;

    Components components2(parent.begin(), parent.end());

    BOOST_FOREACH(VertexIndex current_index, components2)
    {
        std::cout<<"component "<<current_index<<" contains: ";
        BOOST_FOREACH(VertexIndex child_index, components2[current_index])
        {
              std::cout<<child_index<<" "<<" x,y = "<<graph[verticesTable[child_index]].x<<";"<<graph[verticesTable[child_index]].y<<"||";
        }
        std::cout<<std::endl;
    }

    //write_graphviz(std::cout, graph);

    getchar();

    return 0;
}

I’m still learning boost Graph so please correct my mistakes, make more elegant solutions.
My conclusions also can be not exactly correct.