我正在阅读boost :: graph文档以供将来使用。我对A *算法特别感兴趣。
看看boost :: graph :: astar_search用法示例,似乎停止算法的方法是抛出异常并将其捕获到算法之外。
因为我不想抛出任何异常,导致C ++中的异常处理非常复杂且效率不高,我想知道boost :: graph是否提出了另一种方法来在达到目标时停止算法。
有没有人有另一个例子没有使用任何例外?
答案 0 :(得分:5)
根据FAQ,从BFS提前退出的唯一方法是抛出异常并“查看提升邮件列表以获取详细信息”,但我从未发现过此类详细信息。
要不在A *中使用例外,您必须修改算法并为您自己的访问者引入停止条件:
#include <boost/graph/astar_search.hpp>
namespace boost { namespace graph {
template < class Visitors = null_visitor >
struct stopable_astar_visitor : public astar_visitor<Visitors> {
stopable_astar_visitor() {}
stopable_astar_visitor(Visitors vis) : astar_visitor<Visitors>(vis) {}
template < class Vertex, class Graph >
bool should_stop(Vertex &v, Graph &g) {
return true;
}
};
template <typename VertexListGraph, typename AStarHeuristic,
typename AStarVisitor, typename PredecessorMap,
typename CostMap, typename DistanceMap,
typename WeightMap,
typename CompareFunction, typename CombineFunction,
typename CostZero>
inline void
stoppable_astar_search_no_init_tree
(const VertexListGraph &g,
typename graph_traits<VertexListGraph>::vertex_descriptor s,
AStarHeuristic h, AStarVisitor vis,
PredecessorMap predecessor, CostMap cost,
DistanceMap distance, WeightMap weight,
CompareFunction compare, CombineFunction combine,
CostZero zero)
{
typedef typename graph_traits<VertexListGraph>::vertex_descriptor
Vertex;
typedef typename property_traits<DistanceMap>::value_type Distance;
typedef d_ary_heap_indirect<
std::pair<Distance, Vertex>,
4,
null_property_map<std::pair<Distance, Vertex>, std::size_t>,
function_property_map<graph_detail::select1st<Distance, Vertex>, std::pair<Distance, Vertex> >,
CompareFunction>
MutableQueue;
MutableQueue Q(
make_function_property_map<std::pair<Distance, Vertex> >(graph_detail::select1st<Distance, Vertex>()),
null_property_map<std::pair<Distance, Vertex>, std::size_t>(),
compare);
int counter = 0;
vis.discover_vertex(s, g);
Q.push(std::make_pair(get(cost, s), s));
while (!Q.empty()) {
counter++;
Vertex v;
Distance v_rank;
boost::tie(v_rank, v) = Q.top();
Q.pop();
if (vis.should_stop(v, g)) {
return;
}
vis.examine_vertex(v, g);
BGL_FORALL_OUTEDGES_T(v, e, g, VertexListGraph) {
Vertex w = target(e, g);
vis.examine_edge(e, g);
Distance e_weight = get(weight, e);
if (compare(e_weight, zero))
BOOST_THROW_EXCEPTION(negative_edge());
bool decreased =
relax(e, g, weight, predecessor, distance,
combine, compare);
Distance w_d = combine(get(distance, v), e_weight);
if (decreased) {
vis.edge_relaxed(e, g);
Distance w_rank = combine(get(distance, w), h(w));
put(cost, w, w_rank);
vis.discover_vertex(w, g);
Q.push(std::make_pair(w_rank, w));
} else {
vis.edge_not_relaxed(e, g);
}
}
vis.finish_vertex(v, g);
}
}
}} // end namespace boost::graph