[英]Can't remove an edge after having removed a vertex with Boost Graph Library
我開始研究BGL ,它看起來很棒,但是我發現很難使用。
當我玩這個圖形示例時,事情開始變得更加清晰。 但是我現在遇到了一個問題:移除頂點后,無法移除圖形中的邊。 到目前為止,這似乎使我對BGL的理解無效:(
這是我必須使問題清楚顯示的最短代碼。
(我確定了具有整數[1..5]的頂點和具有字母[a..g]的邊)
#include <iostream>
#include <boost/graph/adjacency_list.hpp>
using namespace boost;
//==============================================================
// TL;DR : Skip to the end of `main()` where the real problem is
//==============================================================
// lazy iterate over the graph: // {{{
#define VERTEXITERATE(nameV, graph, ...) \
{ \
auto GRAPHITERATOR(vertices(graph)); \
auto& nameV(GRAPHITERATOR.first); \
for(; GRAPHITERATOR.first != GRAPHITERATOR.second; ++GRAPHITERATOR.first) {\
__VA_ARGS__ \
} \
}
#define EDGEITERATE(nameE, graph, ...) \
{ \
auto GRAPHITERATOR(edges(graph)); \
auto& nameE(GRAPHITERATOR.first); \
for(; GRAPHITERATOR.first != GRAPHITERATOR.second; ++GRAPHITERATOR.first) {\
__VA_ARGS__ \
} \
}
// }}}
// Properties of the graph // {{{
struct VertexProperties {
VertexProperties() : id(0) {};
VertexProperties(int id) : id(id) {};
int id;
};
struct EdgeProperties {
EdgeProperties() : id(0) {};
EdgeProperties(char id) : id(id) {};
char id;
};
struct GraphProperties {
GraphProperties() : id(0) {};
GraphProperties(std::string id) : id(id) {};
std::string id;
};
// }}}
// Type definitions // {{{
typedef adjacency_list<
hash_setS // vecS would allow parallel edges, which I don't want.
, vecS
, bidirectionalS
, VertexProperties
, EdgeProperties
, GraphProperties
> Graph;
typedef graph_traits<Graph>::vertex_descriptor Vertex;
typedef graph_traits<Graph>::edge_descriptor Edge;
// }}}
// Glance at the state of the graph
void printGraph(Graph const& g) { // {{{
// Print graph properties
std::cout << get_property(g).id << " : |V| = " << num_vertices(g);
std::cout << ", |E| = " << num_edges(g) << std::endl;
std::cout << "vertices: " << std::endl;
VERTEXITERATE(v, g,
Vertex vertex(*v);
std::cout << g[vertex].id << " :";
std::cout << " in " << in_degree(vertex, g);
std::cout << ", out " << out_degree(vertex, g);
std::cout << std::endl;
);
std::cout << "edges: " << std::endl;
EDGEITERATE(e, g,
Edge edge(*e);
std::cout << g[edge].id << " :";
std::cout << g[source(edge, g)].id << " \u2192 ";
std::cout << g[target(edge, g)].id;
std::cout << std::endl;
);
std::cout << std::endl;
}
// }}}
int main() {
// Build the graph
Graph g(GraphProperties("Graph"));
const int nV(5); // number of vertices
const int nE(7); // number of edges
std::cout << "Created." << std::endl;
// should be empty:
printGraph(g);
// Vertices
std::array<Vertex, nV> V;
int vId(0);
for (Vertex& v : V)
v = add_vertex(VertexProperties(++vId), g);
// Edges
typedef struct { // store here everything we need to define an edge:
Vertex source, target;
EdgeProperties props;
} BuildEdge;
std::array<BuildEdge, nE> builds { {
{V[0], V[1], 'a'} // define the graph topology in this initializer
, {V[0], V[2], 'b'}
, {V[0], V[3], 'c'}
, {V[2], V[3], 'd'}
, {V[1], V[4], 'e'}
, {V[2], V[4], 'f'}
, {V[3], V[4], 'g'}
}};
for(auto p : builds)
add_edge(p.source, p.target, p.props, g);
// See what happened:
std::cout << "Filled up." << std::endl;
printGraph(g); // ok.
//==============================================================
// HERE is the interesting part :
// remove an edge by its vertices:
std::array<Vertex, 2> toRemove {{V[0], V[1]}};
remove_edge(toRemove[0], toRemove[1], g);
std::cout << "Edge removed." << std::endl;
printGraph(g); // ok.
// remove a vertex:
toRemove[0] = V[3];
clear_vertex(toRemove[0], g);
remove_vertex(toRemove[0], g);
std::cout << "Vertex removed" << std::endl;
printGraph(g); // success.
// remove another vertex:
toRemove = {{ V[2], V[4] }};
remove_edge(toRemove[0], toRemove[1], g);
std::cout << "Edge removed." << std::endl;
printGraph(g); // FAIL!
// Why does this fail?
// Is `V` outdated after a call to remove_edge?
//==============================================================
return EXIT_SUCCESS;
}
為什么最后一次清除沒有發生? 當然,刪除中間塊(即保留第四個頂點)將允許正確刪除邊。
我知道在刪除一個邊界的頂點之一之后刪除邊沒有多大意義,但是在這里情況並非如此。
痛擊! 只是在這個主題上找到了一個非常有趣的頁面 。 這是官方的Boost文檔。
用VertexList = vecS
定義adjacency_list
會導致所有頂點描述符通過調用remove_vertex
無效。 選擇其他數據結構(例如hash_setS
將使它們保持有效。
因此:重寫typedef:
typedef adjacency_list<
hash_setS // vecS would allow parallel edges, which I don't want.
, hash_setS // vecS would make the descriptor invalid on graph change
, bidirectionalS
, VertexProperties
, EdgeProperties
, GraphProperties
> Graph;
解決了問題。
它不僅會這樣做,還會影響adjacency_list
其他屬性,例如基礎算法的復雜性。 對我來說,這仍然是相當復雜的,因為我是BGL的新手,但請參閱docs 。 :)
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