undirected_dfs: how to get the vectors of edges

Question

NB: The question below is the context of "boost-graph". But the issue is maybe a "c++" issue or "boost-graph" issue.

Using boost-graph and undirected_dfs, I am trying to get the vectors of edges.

The code print correctly back_edge and tree_edge.

My goal is to retrieve the vectors of edges. For this operation i use a vector of vectors of edges.

So, when a tree_edge is found i store it in a vector:

edgeVisited.push_back(e);

When the back_end is found, i store this vector of edges (edgeVisited) in a vector of vectors:

myList.push_back(edgeVisited);

Just after this operation, i check the size of myList. The result is correct:

std::cout << "myList size by back_edge: " << myList.size() << std::endl;

After the call to undirected_dfs, i want to get the myList by

std::vector< std::vector<edge_t> > vctr = vis.GetEdges();

and check it size by:

std::cout << vctr.size() << std::endl;

But the vector is void.

Could you please help me to understand why this vector of vector is null?

Here is the whole code:
#include <iostream>
#include <string>
#include <boost/cstdlib.hpp>
#include <boost/graph/adjacency_list.hpp>
#include <boost/graph/undirected_dfs.hpp>
#include <boost/graph/graphviz.hpp>

using namespace boost;
typedef adjacency_list< 
    vecS, 
    vecS, 
    undirectedS,
    no_property,
    property<edge_color_t, default_color_type> > graph_t;

typedef boost::graph_traits<graph_t>::vertex_descriptor vertex_t;
typedef boost::graph_traits < graph_t>::edge_descriptor edge_t;

struct detect_loops : public boost::dfs_visitor<>
{
    template <class edge_t, class Graph>

    void back_edge(edge_t e, const Graph& g) {
        std::cout << source(e, g) << " -- " << target(e, g) << "\n";
        edgeVisited.push_back(e);
        myList.push_back(edgeVisited);
        edgeVisited.clear();
        std::cout << "myList size by back_edge: " << myList.size() << std::endl;
    }
    template <class Graph>
    void tree_edge(edge_t e, const Graph& g) {
        std::cout << "tree_edge: " << boost::source(e, g) << " --> " << boost::target(e, g) << std::endl;
        edgeVisited.push_back( e );
    }
    //get the vectors.
    std::vector< std::vector<edge_t> >  GetEdges() const { 
        std::cout << "MyList by GetEdges : " << myList.size() << std::endl;
        return myList; 
    }

private:
    std::vector<edge_t> edgeVisited;
    std::vector< std::vector<edge_t> > myList;
};

void make(graph_t &g)
{
    //Create the graph
    boost::add_edge(0, 1, g);
    boost::add_edge(0, 2, g);
    boost::add_edge(1, 3, g);
    boost::add_edge(2, 3, g);
    boost::add_edge(2, 4, g);
    boost::add_edge(3, 5, g);
    boost::add_edge(4, 5, g);
    //print the graph
    std::ofstream f("d:\\tmp\\dot\\s13.dot");
    boost::write_graphviz(f, g);
    std::system(std::string("dot -Tsvg -Grankdir=LR -Nfontsize=24 d:\\tmp\\dot\\s13.dot > d:\\tmp\\dot\\s13.svg").c_str());
}

int main(int, char*[])
{
    graph_t g;
    make(g);

    detect_loops vis;
    undirected_dfs(g, root_vertex(vertex_t(0)).visitor(vis) .edge_color_map(get(edge_color, g)));
    std::vector< std::vector<edge_t> > vctr = vis.GetEdges();
    std::cout << vctr.size() << std::endl;

    return boost::exit_success;
}

Thanks

score 0 · Answer 1 · answered Dec 08 '16 at 10:49

It was more a "c++" issue. Here is the solution: i add the vector of vectors as entry parameter to the class detect_loop

detect_loops(std::vector< std::vector<edge_t> > &vctr )

and call it :

std::vector< std::vector<edge_t> > vctr;
detect_loops vis(vctr);

That's all. Thanks

undirected_dfs: how to get the vectors of edges

1 Answers1