Re: [Boost-users] stopping dijkstra_shortest_path after target achieved

From: Celso Pitta (celso.pitta_at_[hidden])
Date: 2006-03-14 09:20:28

Yes, it seems to be right.

Is the Daniel´s idea, when the desired vertex is permanently labeled
(marked black) (you check it with the examine_vertex visitor ), you
throw an exception. Sens a little crude to me, but is quite straight
forward to implement. i don't know the performance overhead it will
cause to call the visitor for every labeled vertex...

On 3/14/06, Marcio Paim <marcio.paim_at_[hidden]> wrote:
>
> Hello,
>
> I had the same problem, after gathering some information over the net I got
> this modified version of dijkstra-example.cpp
>
>
> #include <boost/config.hpp>
> #include <iostream>
>
> #include <boost/graph/graph_traits.hpp>
> #include <boost/graph/adjacency_list.hpp>
> #include <boost/graph/dijkstra_shortest_paths.hpp>
>
> using namespace boost;
>
> // define visitor for discover_vertex event
>
> template <class Vertex, class Tag>
> struct target_visitor : public default_dijkstra_visitor
> {
>  target_visitor(Vertex u) : v(u) { }
>
>  template <class Graph>
>  void examine_vertex(Vertex u, Graph& g)
>  {
>   if( u == v ) {
>    throw(-1);
>   }
>  }
> private:
>  Vertex v;
> };
>
> template <class Vertex, class Tag>
> target_visitor<Vertex, Tag>
> target_visit(Vertex u, Tag) {
>  return target_visitor<Vertex, Tag>(u);
> }
>
>
> int main(int argc, char** argv)
> {
>  typedef adjacency_list < listS, vecS, directedS,
>   no_property, property < edge_weight_t, int > > graph_t;
>  typedef graph_traits < graph_t >::vertex_descriptor vertex_descriptor;
>  typedef graph_traits < graph_t >::edge_descriptor edge_descriptor;
>  typedef std::pair<int, int> Edge;
>
>  const int num_nodes = 5;
>  enum nodes { A, B, C, D, E };
>  char name[] = "ABCDE";
>  Edge edge_array[] =
>  {
>   Edge(A, C), Edge(B, B), Edge(B, D), Edge(B, E),
>      Edge(C, B), Edge(C, D), Edge(D, E), Edge(E, A), Edge(E, B)
>  };
>   int weights[] = { 1, 2, 1, 2, 7, 3, 1, 1, 1 };
>  int num_arcs = sizeof(edge_array) / sizeof(Edge);
>
>  graph_t g(edge_array, edge_array + num_arcs, weights, num_nodes);
>  property_map<graph_t, edge_weight_t>::type weightmap = get(edge_weight, g);
>
>  std::vector<vertex_descriptor> p(num_vertices(g));
>  std::vector<int> d(num_vertices(g));
>
>  // choose source and target
>  vertex_descriptor src = vertex(A, g);
>  vertex_descriptor targ = vertex(C, g);
>
>
>  try {
>   dijkstra_shortest_paths(g, src,
>
> predecessor_map(&p[0]).distance_map(&d[0]).visitor(target_visit(targ,
>       on_examine_vertex())));
>  }
>  catch( ... ) {
>  }
>
>
>  std::cout << "distances and parents:" << std::endl;
>  graph_traits < graph_t >::vertex_iterator vi, vend;
>  for (tie(vi, vend) = vertices(g); vi != vend; ++vi) {
>   std::cout << "distance(" << name[*vi] << ") = " << d[*vi] << ", ";
>   std::cout << "parent(" << name[*vi] << ") = " << name[p[*vi]] <<
> std::endl;
>  }
>
>     system("PAUSE");
>  return EXIT_SUCCESS;
> }
> I compiled it with Dev-C++.
> I do not understand it in fully detail but I hopefully expect it to be
> right.
> Is it right?
>
> m.p.a.
>
> On 3/14/06, Daniel Mitchell <danmitchell_at_[hidden]> wrote:
> > > Is there an easy/already implemented way to stop the computation of
> > > the dijkstra_shortest_path algorithm when some target vertex is marked
> > > as black??
> >
> > The recommended method is to use a visitor that throws an exception when
> the
> > target is colored black.
> >
> > Daniel
> > _______________________________________________
> > Boost-users mailing list
> > Boost-users_at_[hidden]
> > http://lists.boost.org/mailman/listinfo.cgi/boost-users
> >
>
>
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