BGL : get vertex descriptor with data - c++

Question

I want to get the vertex descriptor with the composant of the vertex, like this :
struct WayPoint{
std::pair<float, float> pos; // with this composant
};
the adjency list :
typedef boost::adjacency_list<
boost::listS,
boost::vecS,
boost::undirectedS,
WayPoint,
WayPointConnection
> WayPointGraph;
typedef WayPointGraph::vertex_descriptor WayPointID;
typedef WayPointGraph::edge_descriptor WayPointConnectionID;
I built my graph and created all the vertices / edges .... the aim is to apply an astar on the graph.
void PathFinding::findMeAPath(std::pair<float, float>begin, std::pair<float, float>end)
{
std::vector<WayPointID> p(boost::num_vertices(graphe));
std::vector<float> d(boost::num_vertices(graphe));
WayPointID start = // I want to find the WayPointID with begin
WayPointID goal = //same with end;
shortest_path.clear();
try {
boost::astar_search
(
graphe,
start,
boost::astar_heuristic<WayPointGraph, float>(),
boost::predecessor_map(&p[0]).distance_map(&d[0]).visitor(astar_goal_visitor(goal)).weight_map(boost::get(&WayPointConnection::dist, graphe))
);
} catch(found_goal fg) {
for(WayPointID v = goal;; v = p[v]) {
shortest_path.push_front(v);
if(p[v] == v)
break;
}
}
}

Answer 1

You need to write a function to find a vertex given a position. The graph type that you have defined uses std::vector to store vertices, so the function will have to iterate through it and compare the queried position to each WayPoint. Something like this could do:
std::pair<WayPointID, bool> find_vertex(const WayPoint& wp, const WayPointGraph& graph)
{
for (WayPointID id = 0; id < boost::num_vertices(graph); ++id)
{
if (equal(graph[id], wp))
return std::make_pair(id, true);
}
return std::make_pair(0, false);
}
Note that the function returns a pair (Id + boolean flag) to indicate whether the search succeeded or not, so you would use it as follows:
bool vertex_found;
WayPointID start;
std::tie (start, vertex_found) = find_vertex(begin, graphe);
if (!vertex_found)
// do something about it
Also the function uses the following to compare positions:
bool equal(const std::pair<float, float>& p1, const std::pair<float, float>& p2)
{
const float EPS = 1e-6;
return (std::fabs(p1.first - p2.first) < EPS &&
std::fabs(p1.second - p2.second) < EPS);
}