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Re: Convert directed graph to undirected graph

  • To: mathgroup at smc.vnet.net
  • Subject: [mg111228] Re: Convert directed graph to undirected graph
  • From: Murray Eisenberg <murray at math.umass.edu>
  • Date: Sat, 24 Jul 2010 05:07:24 -0400 (EDT)

Of course the original graph-like object was a list of rules and not a 
Combinatorica-type object with head Graph. To get back to the original 
lit-of-rules object, one can use the following function (I believe this 
was posted some time ago to MathGroup, but at the moment I forget its 
author):

   RuleListFromGraph[Graph[edges_, vertices_, ___]] :=
  Module[{vrules},
   vrules = DeleteCases[
     Thread[Range[
        Length[vertices]] -> (VertexLabel /.
         vertices[[All, 2 ;;]])], _ -> VertexLabel];
   Replace[
    Transpose[{Rule @@@ (edges[[All, 1]] /. vrules),
      EdgeLabel /. edges[[All, 2 ;;]]}], {a_, EdgeLabel} -> a, {1}]]

In the example in the post below, one obtains:

   RuleListFromGraph[gr1u]
{1->2,1->3,1->4,2->3,2->4,4->4}

Comment: I really wish WRI would at very long last completely 
rationalize the dichotomy between the newer kernel functions for dealing 
with lists-of-rules as graphs, on the one hand, and Combinatorica 
Graph-objects, on the other hand.

On 7/23/2010 7:09 AM, juan flores wrote:
> Needs["Combinatorica`"];
> Needs["GraphUtilities`"];
> gr1 =
>   ToCombinatoricaGraph[{1 ->  2, 2 ->  1, 3 ->  1, 3 ->  2, 4 ->  1, 4 ->  2,
>      4 ->  4}]
> ShowGraph[gr1]
> gr1u = MakeUndirected[gr1]
> ShowGraph[gr1u]

-- 
Murray Eisenberg                     murray at math.umass.edu
Mathematics & Statistics Dept.
Lederle Graduate Research Tower      phone 413 549-1020 (H)
University of Massachusetts                413 545-2859 (W)
710 North Pleasant Street            fax   413 545-1801
Amherst, MA 01003-9305


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