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 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 = "<

On Mar 6, 2006, at 12:23 AM, Alejandro Aragón wrote:

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);

I think you can change this line to: v_new = add_vertex(get(vertex_all, *gPtr, *vi), *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);

And then change this line to: tie(e_new, inserted) = add_edge(source(*ei, *gPtr), target(*ei, *gPtr), get(edge_all, *gPtr, *ei), *copy);

weightmap[e_new] = weightmap[*ei]; anglemap[e_new] = anglemap[*ei]; }

std::cout<<"Number of edges in original graph = "<

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On Mar 10, 2006, at 4:06 PM, Alejandro Aragón wrote:

It works perfectly! Thanks for the changes Doug. I would like to go further with this. I'm not an expert in c++ programming but I will try to make this funciton a template and I will provide a couple of examples. Maybe I will see it implemented in the library some day! =)

Great! If you manage to clean it up and submit it, we'd love to add it into the BGL. Doug