Changing edge weights in a graph using PropertyValue

Question

I encountered a problem trying to change the EdgeWeights in a graph. I am runnning Mathematica 10.1 under Windows 7 Enterprise.

I have an undirected graph with 144 nodes and 576 edges. I first produce a sparce, symmetric adjacency matrix called "waminfful", then use that to define a graph called "myg".

 myg = WeightedAdjacencyGraph[
          Range[144]
        , waminffull
        , DirectedEdges -> False
      ];

Everything seemed to work fine until I tried to change some of the edge weights. I found that some of them would change and others would not.

In the example below, the initial edge weight was 1, and I was able to reset it to 35 without a problem.

PropertyValue[{myg, 52 <-> 53}, EdgeWeight]

PropertyValue[{myg, 52 <-> 53}, EdgeWeight] = 35.0;

PropertyValue[{myg, 52 <-> 53}, EdgeWeight]

1.

35.

However, as the following example shows, this does not always work:

PropertyValue[{myg, 53 <-> 42}, EdgeWeight]

PropertyValue[{myg, 53 <-> 42}, EdgeWeight] = 35.0;

PropertyValue[{myg, 53 <-> 42}, EdgeWeight]

1.41421

1.41421.

Answer 1

I think I have discovered the answer to this problem, but would be interested to hear other solutions.

If one looks at the complete list of Edges using EdgeList[myg], all of the listed edges have the smaller node number listed first. One sees in this list an edge denoted as 42<->53, but no edge listed as 53<->42. My experience indicates that Mathemetica will find the edge weight if given the input 53<->42, but it will not reset the edge weight if it is given an input with the larger node number listed first. If one puts the smaller node number first, then Mathematica resets the edge weight as it should, as illustrated below.

PropertyValue[{myg, 42 <-> 53}, EdgeWeight]

PropertyValue[{myg, 42 <-> 53}, EdgeWeight] = 35.0;

PropertyValue[{myg, 42 <-> 53}, EdgeWeight]

 1.41421

 35.

In my example above where resetting the edge weight worked, the larger node number just happened to be listed first. I had thought that since this was an undirected graph both types of inputs would work, but that is evidently not the case.