Invert undirected graph edge

Question

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.

@BenIzd This is not safe when dealing with edge tagged graphs. — Szabolcs
– Szabolcs, Commented Jul 30, 2022 at 8:57
@Szabolcs Thanks for pointing that out. For that case, we could map SubsetMap[Reverse, ;; 2] — Ben Izd
– Ben Izd, Commented Jul 30, 2022 at 9:04

Szabolcs · Accepted Answer · 2022-07-30 08:59:28Z

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.

Ben Izd · Accepted Answer · 2022-08-01 04:44:01Z

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.

2 Answers 2