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Suppose I have a list of edges,

 {1 \[UndirectedEdge] 2, 1 \[UndirectedEdge] 3}

How do I get the 'invert' of the list,

 {2 \[UndirectedEdge] 1, 3 \[UndirectedEdge] 1}

I'm aware that both these lists will yield the same graph.

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    $\begingroup$ Map Reverse like Reverse /@ list $\endgroup$
    – Ben Izd
    Commented Jul 30, 2022 at 7:16
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    $\begingroup$ @BenIzd This is not safe when dealing with edge tagged graphs. $\endgroup$
    – Szabolcs
    Commented Jul 30, 2022 at 8:57
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    $\begingroup$ @Szabolcs Thanks for pointing that out. For that case, we could map SubsetMap[Reverse, ;; 2] $\endgroup$
    – Ben Izd
    Commented Jul 30, 2022 at 9:04
  • $\begingroup$ @BenIzd Why don't you post an answer? $\endgroup$
    – Szabolcs
    Commented Jul 30, 2022 at 9:16

2 Answers 2

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You can use

UndirectedEdge[#2, #1, ##3] & @@@ edgelist

Reverse should not be applied to an UndirectedEdge as it may have more than two arguments in EdgeTaggedGraph.

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If you're not working on EdgeTaggedGraph, then you can directly map Reverse like below:

Reverse /@ edgelist

Or with the second argument of Reverse (which is faster than mapping and uses less memory):

Reverse[edgelist, 2]

Or re-order the columns manually:

edgelist[[All, {2, 1}]]

In the case of EdgeTaggedGraph where all the edges are tagged, you can use:

edgelist[[All, {2, 1, 3}]]

Or generally:

SubsetMap[Reverse, ;; 2] /@ edgelist

But it's slower (~45 times) and uses more memory in comparison with Szabolcs's answer.

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