How to access edge_descriptor with given vertex_descriptor in boost::grid_graph

Question

I\'m trying to change the weight of edges in my grid_graph but fail to access the edge_descriptor with:

std::pair ed_right = boo         


        

Answer 1

You can get the in/out edges for any particular node:

http://www.boost.org/doc/libs/1_63_0/libs/graph/doc/grid_graph.html#indexing

// Get the out-edge associated with vertex and out_edge_index
Traits::edge_descriptor
out_edge_at(Traits::vertex_descriptor vertex,
            Traits::degree_size_type out_edge_index,
            const Graph& graph);

// Get the out-edge associated with vertex and in_edge_index
Traits::edge_descriptor
in_edge_at(Traits::vertex_descriptor vertex,
           Traits::degree_size_type in_edge_index,
           const Graph& graph);

Demo using a 3d 4x4x4 grid with non-wrapping dimensions (this show-cases that the degree of nodes on the edges is lower):

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#include <boost/graph/grid_graph.hpp>
#include <boost/graph/adjacency_list.hpp>
#include <iostream>

using Grid = boost::grid_graph<3>;
using Traits = boost::graph_traits<Grid>;
using vertex_descriptor = Grid::vertex_descriptor;
using edge_descriptor = Grid::edge_descriptor;

static inline std::ostream& operator<<(std::ostream& os, vertex_descriptor const& vd) {
    return os << "(" << vd[0] << ", " << vd[1] << ", " << vd[2] << ")"; 
}

void print_in_edges(vertex_descriptor vd, Grid const& grid) {
    for (Traits::degree_size_type ei = 0; ei < in_degree(vd, grid); ++ei) {
        auto ed_left = in_edge_at(vd, ei, grid);
        std::cout << "Detected in edge: " << ed_left.first << " -> " << ed_left.second << "\n";
    }
}

void print_out_edges(vertex_descriptor vd, Grid const& grid) {
    for (Traits::degree_size_type ei = 0; ei < out_degree(vd, grid); ++ei) {
        auto ed_left = out_edge_at(vd, ei, grid);
        std::cout << "Detected out edge: " << ed_left.first << " -> " << ed_left.second << "\n";
    }
}

int main() {
    Grid grid({ { 4, 4, 4 } }, false);

    print_in_edges({{ 2, 2, 2 } }, grid);
    print_out_edges({{ 2, 2, 2 } }, grid);
    std::cout << "----\n";
    print_in_edges({{ 0, 0, 0 } }, grid);
    print_out_edges({{ 0, 0, 0 } }, grid);
}

Prints:

Detected in edge: (1, 2, 2) -> (2, 2, 2)
Detected in edge: (3, 2, 2) -> (2, 2, 2)
Detected in edge: (2, 1, 2) -> (2, 2, 2)
Detected in edge: (2, 3, 2) -> (2, 2, 2)
Detected in edge: (2, 2, 1) -> (2, 2, 2)
Detected in edge: (2, 2, 3) -> (2, 2, 2)
Detected out edge: (2, 2, 2) -> (1, 2, 2)
Detected out edge: (2, 2, 2) -> (3, 2, 2)
Detected out edge: (2, 2, 2) -> (2, 1, 2)
Detected out edge: (2, 2, 2) -> (2, 3, 2)
Detected out edge: (2, 2, 2) -> (2, 2, 1)
Detected out edge: (2, 2, 2) -> (2, 2, 3)
----
Detected in edge: (1, 0, 0) -> (0, 0, 0)
Detected in edge: (0, 1, 0) -> (0, 0, 0)
Detected in edge: (0, 0, 1) -> (0, 0, 0)
Detected out edge: (0, 0, 0) -> (1, 0, 0)
Detected out edge: (0, 0, 0) -> (0, 1, 0)
Detected out edge: (0, 0, 0) -> (0, 0, 1)