How to quickly inverse a permutation by using PyTorch?

[KFrank]

Hi Bin!

@111414

Example #1:
[x, y, z] shuffled by P: [2, 0, 1], we will obtain [z, x, y],
the corresponding inverse should be P^-1: [1, 2, 0].

Example #2:
[a, b, c, d, e, f] shuffled by P: [5, 2, 0, 1, 4, 3], then we will get [f, c, a, b, e, d],
the corresponding inverse should be P^-1: [2, 3, 1, 5, 4, 0].

If you represent your permutation as a vector of permuted integers
(as you do), you may use argsort() to obtain the inverse permutation:

>>> import torch
>>> torch.__version__
'1.7.1'
>>> p1 = torch.tensor ([2, 0, 1])
>>> torch.argsort (p1)
tensor([1, 2, 0])
>>> p2 = torch.tensor ([5, 2, 0, 1, 4, 3])
>>> torch.argsort (p2)
tensor([2, 3, 1, 5, 4, 0])

Best.

K. Frank