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From: Alejandro Aragón (alex_aragon_at_[hidden])
Date: 20060306 00:23:42
Doug Gregor wrote:
> On Feb 26, 2006, at 11:47 PM, Alejandro Aragón wrote:
>> After running Kruskal's algorithm I end up with the list of the edges
>> that form the minimum cost. However, no new graph is created. I need
>> the resulting graph for further computations. How do you create a
>> graph
>> from an existing one but only selecting the edges contained in the
>> minimum spanning tree vector? I've seen that there are some functions
>> to accomplish this but I couldn't find any example of how to do it:
>>
>> template <class EdgeIterator, class EdgePropertyIterator>
>> adjacency_list(EdgeIterator first, EdgeIterator last,
>> EdgePropertyIterator ep_iter,
>> vertices_size_type n,
>> vertices_size_type m = 0,
>> const GraphProperty& p = GraphProperty())
>>
>> The existing graph has properties in edges and vertices that I also
>> need
>> to keep. Can anyone help? Thank you all,
>
> Unfortunately, the above constructor probably won't help. I expect
> you'll need to write the copy routine yourself, because I don't know of
> a function in the Graph library that does exactly what you want. The
> simple way to do this is:
>
>  Create a new, empty graph
>  Create a property map orig2copy that maps from the old graph's
> vertex descriptors to the new graph's vertex descriptors
>  For each vertex v in the original graph, add_vertex on the new graph
> using get(vertex_all, orig, v) for the value of the properties, then
> setup the correspondence orig2copy[v] = the new vertex.
>  For each edge e, add the edge (orig2copy[source(e, orig)],
> orig2copy[target(e, orig)]) using get(edge_all, orig, e) for the value
> of the properties.
>
> Make that a generic function template with a test case and we can add
> it into the BGL :)
>
> Doug
Doug,
I tried to follow your directions but I coundn't find any concrete
example of using the get(vertex_all,orig,v) function. I did something
else, but I think it is not working. Could you give me further
directions to code it your way?
This is what I came up with:
// minimum_spanning_graph
Graph* graph::minimum_spanning_graph(std::vector<Edge>& spanning_tree)
{
// create an empty graph
// the original graph has pointer gPtr
Graph* copy = new Graph();
// iterate over all vertices in the original graph
Vertex v_new;
for (tie(vi, vi_end) = vertices(*gPtr); vi != vi_end; ++vi){
v_new = add_vertex(*copy);
coordmap[v_new] = coordmap[*vi];
}
// iterate over all the edges in the list produced by algorithm
bool inserted;
Edge e_new;
for (std::vector < Edge >::iterator ei = spanning_tree.begin();
ei != spanning_tree.end(); ++ei) {
tie(e_new,inserted) = add_edge(source(*ei,*gPtr),target(*ei,*gPtr),*copy);
weightmap[e_new] = weightmap[*ei];
anglemap[e_new] = anglemap[*ei];
}
std::cout<<"Number of edges in original graph =
"<<num_edges(*gPtr)<<std::endl;
std::cout<<"Number of edges in minimum spanning graph
="<<num_edges(*copy)<<std::endl;
return copy;
}
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