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I'm trying to modify this algorithm: http://demonstrations.wolfram.com/ConnectingTownsUsingKruskalsAlgorithm/

The thing is that I want to receive a Graph (declared outside, with all its vertex, edges and weights) in the Kruskal's function and start from there, using maybe a WeightedAdjacencyMatrix[G] and sort the weights and returning them. It's just a practice I want to do, because I'm learning Mathematica, so I'm still a noob with this. To be more specific with my question, I just want to know where and how I can replace parts from the code, with the things I mentioned above. This is my idea, of course without the Kruskal's implementation: 

G = Graph[{a \[UndirectedEdge] b, a \[UndirectedEdge] d, 
a \[UndirectedEdge] f, b \[UndirectedEdge] c, 
b \[UndirectedEdge] d, b \[UndirectedEdge] e, 
c \[UndirectedEdge] e, c \[UndirectedEdge] g, 
d \[UndirectedEdge] e, d \[UndirectedEdge] f, 
d \[UndirectedEdge] g, e \[UndirectedEdge] g, 
f \[UndirectedEdge] g}, 
EdgeWeight -> {5, 6, 7, 5, 5, 4, 3, 3, 2, 2, 3, 1, 2}, 
VertexLabels -> "Name", ImagePadding -> 10] 
Kruskal[G]
J. M.'s missing motivation
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José Del Valle
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  • Sorry José, but I can't understand "I just want to know where and how I can replace parts from the code" Can you explain that further? – Dr. belisarius Oct 18 '12 at 02:58
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    And welcome! I think you're our first Tico around! – Dr. belisarius Oct 18 '12 at 03:00
  • Yeah sorry for not explaining it well, and thanks for your welcome. I think I did a mess actually. I want to change the implementation of the function, I want it to receive the Graph, with its vertex, edges, and weights. And find the MinimumSpanningTree. It's like implementing the MinimumSpanningTree function actually, step by step. I was looking for the Combinatorica implemntation but I need to have it into one same function. Sorry if I'm not explaining it well! Thanks for your time! – José Del Valle Oct 18 '12 at 03:05
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    Have you seen this? – J. M.'s missing motivation Oct 18 '12 at 03:47
  • Yes, and thanks ! But what does pts mean? – José Del Valle Oct 18 '12 at 03:57
  • I have read it all, and now I know what pts mean, I'll see what can I do to it...Because I'm creating the graph with its weights, so I just need it to receive the graph – José Del Valle Oct 18 '12 at 04:03
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    If you solve your own question, please remember to post the answer – Dr. belisarius Oct 18 '12 at 04:15
  • I hope to solve it! I have a sample of a Prim's algorithm the same way I want the Kruskal's, but it is long as hell...Do you speak spanish Belisarius? Maybe I can send it to you by mail if you want (the prim's sample the teacher gave us) – José Del Valle Oct 18 '12 at 04:19

1 Answers1

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I don't exactly know that I understand you correctly. You have some graph G in structure Graph. Then you want to use MST (Prim/Kruskal) algorithm to obtain changed G which is MST graph. I tried that but it didn't work in M8.

Here is how I did that the other way:

  1. I prepared graph in nested list graph (matrix nxn), where graph[[i,j]] are weights between i and j vertices. Of course that matrix, in principle, should be symmetric and weights graph[[i,i]] should be 0.

  2. I use MST algorithm on that matrix, for example Kruskal:

     kruskal[pts_] := 
  Module[{n = Length[pts[[2]]], vpairs, jj = 0, hh, pair, dist, c1, c2,
c1c2}, Do[hh[k] = {k}, {k, n}];
vpairs=Sort[Flatten[
 Table[{pts[[k, l]], {k, l}}, {k, 1, n - 1}, {l, k + 1, n}], 1]];
 First[Last[Reap[While[jj < Length[vpairs], jj++;
  {dist, pair} = vpairs[[jj]];
  {c1, c2} = {hh[pair[[1]]], hh[pair[[2]]]};
  If[c1 =!= c2, Sow[vpairs[[jj, 2]]];
   c1c2 = Union[c1, c2];
   Do[hh[c1c2[[k]]] = c1c2, {k, Length[c1c2]}];
   If[Length[hh[pair[[1]]]] == n, Break[]];];]]]]]

  3. Use that Kruskal function on your matrix: kruskal@graph. Function return list of pairs, something like: {{2,5},{5,7},...}

  4. Then you must change that list to object that can be understand by Graph function in Mathematica. Use that function:

    mstListToEdge[mstList_] := mstList /. {x_, y_} :> x \[UndirectedEdge] y

  5. In the end use list from 4. and graph matrix to generate Graph. You will obtain MST graph picture with weights.

    graphMst[edges_, graph_] :=
 Graph[edges, VertexLabels -> "Name", ImagePadding -> 10, 
  GraphLayout -> "SpringElectricalEmbedding",
  EdgeWeight -> 
   Array[graph[[edges[[#, 1]], 
      edges[[#, 2]]]] & (*mstWeights *), Length@edges]]
Bartek
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