How can I get input edges of a given vertex on a directed graph?

Question

I have a directed graph and I want to fetch the parent of a given vertex.

Say I have the graph 1 -> 2 -> 3 , I hold the vertex 2 and I want to get vertex 1 .

My vertex and graph definitions:

struct TreeVertex   {  int id = -1;  };

typedef boost::adjacency_list<
    boost::vecS,
    boost::vecS,
    boost::directedS,
    TreeVertex
    > tree_t;

An MVCE showing what I want to achieve (see online here ):

int main() {
    tree_t tree;
    auto v1 = boost::add_vertex( tree );    
    auto v2 = boost::add_vertex( tree );    
    auto v3 = boost::add_vertex( tree );    
    boost::add_edge( v1, v2, tree );
    boost::add_edge( v2, v3, tree );

// attempt to get the input edge of v2
    auto pair_it_edge = boost::in_edges( v2, tree ); // FAILS TO BUILD  
    auto v = boost::source( *pair_it_edge.first ); // should be v1
}

Another answer suggests transforming the graph into a BidirectionalGraph but I need to keep it directed.

Question: Is this possible ? How can I get the incoming edge of v2 , so that I can extract v1 ?

Answer 1

Without using a bidirectional graph, you will have to do a brute force search of all the nodes, looking for one that has vertex 2 as its child.

#include <iostream>
#include <boost/graph/adjacency_list.hpp>
using namespace std;
   using namespace boost;

struct TreeVertex   {  int id = -1;  };

typedef boost::adjacency_list<
    boost::vecS,
    boost::vecS,
    boost::directedS,
    TreeVertex
    > tree_t;

    tree_t tree;


int main() {


    auto v1 = boost::add_vertex( tree );    
    auto v2 = boost::add_vertex( tree );    
    auto v3 = boost::add_vertex( tree );    
    boost::add_edge( v1, v2, tree );
    boost::add_edge( v2, v3, tree );

    int looking_for = 2;

    typename graph_traits < tree_t >::out_edge_iterator ei, ei_end;
    for( int v = 0; v < num_edges( tree ); v++ )
    for (boost::tie(ei, ei_end) = out_edges(v, tree); ei != ei_end; ++ei) {
    auto source = boost::source ( *ei, tree );
    auto target = boost::target ( *ei, tree );
    if( target == looking_for )
        std::cout << "There is an edge from " << source <<  " to " << target << std::endl;

// create an inverted edge tree to search for parents
tree_t invtree;
boost::add_edge( v2, v1, invtree );
boost::add_edge( v1, v3, invtree );
typename graph_traits < tree_t >::adjacency_iterator it, it_end;
for (tie(it, it_end) = adjacent_vertices(v2, invtree ); it != it_end; ++it) 
{
    std::cout << "There is an inv edge from " <<  v2
        << " to " << *it << std::endl;
}


    return 0;
}

It may be worthwhile to create a temporary tree with inverted edges as a 'copy' of your graph to simplify the search for parents. Something like invtree at the end of the code posted.