2
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In the following code, I expected that only the edge between vertices 0 and 1 would be removed.

EdgeContract[
 Graph[{0, 2, 
   1}, {Null, {{1, 3}, {1, 2}, {1, 2}, {1, 2}, {1, 2}}, {Null, 1, 2, 
    3, 4}}, 
     {FormatType -> TraditionalForm, 
   ImageSize -> {407.25, Automatic}, EdgeLabels -> {"EdgeTag"}, 
   GraphLayout -> {"Dimension" -> 2}, 
       ImageSize -> {396., Automatic}, VertexLabels -> {"Name"}}], 
 UndirectedEdge[0, 1]]

enter image description here

Is this a bug? Is there a workaround on this?

(13.3.0 for Linux x86 (64-bit))

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5
  • $\begingroup$ Yes, most likely a bug. It seems that SimpleGraph is applied first. $\endgroup$ Commented May 1 at 12:04
  • $\begingroup$ Please do report it. $\endgroup$ Commented May 1 at 14:51
  • 1
    $\begingroup$ @Szabolcs Okay, I just filled the form. $\endgroup$ Commented May 1 at 22:39
  • $\begingroup$ On version 13.0.1 the bug is not present. $\endgroup$ Commented May 4 at 9:35
  • $\begingroup$ This phenomenon also occurs in version 14.0.0 $\endgroup$ Commented May 4 at 10:27

1 Answer 1

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0
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Here is a workaround, assuming all edges are undirected:

g = Graph[{0, 2, 1}, {Null, {{1, 3}, {1, 2}, {1, 2}, {1, 2}, {1, 2}}, {Null, 1, 2, 3, 4}}, {FormatType -> TraditionalForm, 
   ImageSize -> {407.25, Automatic}, EdgeLabels -> {"EdgeTag"}, 
   GraphLayout -> {"Dimension" -> 2}, ImageSize -> {396., Automatic}, 
   VertexLabels -> {"Name"}}]

0

ClearAll[myEdgeContract];
myEdgeContract[g_Graph, UndirectedEdge[v1_, v2_]] := 
  Module[{newvertices, newedges, options},
   newvertices = Union[VertexList[g] /. v1 -> v2]; 
   newedges = 
    DeleteCases[EdgeList[g], 
      UndirectedEdge[v1, v2, ___] | UndirectedEdge[v2, v1, ___]] /. 
     UndirectedEdge[u_, v_, t___] :> 
      UndirectedEdge[u /. v1 -> v2, v /. v1 -> v2, t];
   options = Options[g];
   Graph[newvertices, newedges, options]];

myEdgeContract[g, UndirectedEdge[0, 1]]

1

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