Undo sorting of multidimensional numpy array along specific axis

Question

I am sorting a large multidimensional numpy array along an axis. After some computations I'd like to undo the sorting with the values themselves being different to when I did the initial sorting.

Initially I tried by simply reversing the sorting indices, but if the array was already sorted correctly in the first place this of course doesn't make any sense, see below code.

import numpy as np

array = np.random.rand(100,2,4)
array.out = array

arg = array.argsort(axis=1)
arg_rev = arg.argsort(axis=1)

# this solution is based on https://github.com/numpy/numpy/issues/4724

for i in range(0, array.shape[2]):
    tmp = array[:,:,i]
    array.out[:,:,i] = tmp[np.arange(np.shape(tmp)[0])[:, np.newaxis],  arg_ftc_rev[:,:,i]]

I would like to combine this solution with the one from undo or reverse argsort(), python without having to loop over an additional array index.

# ( a = np.random.randint(0,10,10)
# aa = np.argsort(a)
# aaa = np.argsort(aa) )
# e.g.
array.out[:,:,i] = tmp[np.arange(np.shape(tmp)[0])[:, np.newaxis],  arg_ftc[:,:,i][arg_ftc_rev[:,:,i]]

but this does not work, as it results in the wrong shape.

"Undo sorting after changing values" is a pretty nebulous concept. Can you provide an example (with data) of what you would like to accomplish? — Scott Hunter
– Scott Hunter, Commented Sep 13, 2019 at 14:52
Use the inverse_permutation function at the end of this answer to invert the result of argsort. — javidcf
– javidcf, Commented Sep 13, 2019 at 14:52
@jdehesa I tried python import numpy as np array = np.random.rand(100,2,4) arg = array.argsort(axis=1) x = inverse_permutation(arg) but there is an error because it cannot broadcast. — heffalump
– heffalump, Commented Sep 13, 2019 at 15:28
@ScottHunter I mean that the position of each element along the sorting axis should be as to begin with, however the values within these will have been modified. Simply sorting again is thus no option e.g. python x = [1, 3, 1, 6] x = sort(x) # [1, 1, 3, 6] x = x*2 # [2, 2, 6, 12] # undo sorting # [2, 6, 2, 12] — heffalump
– heffalump, Commented Sep 14, 2019 at 21:04

heffalump · Accepted Answer · 2019-09-27 15:07:45Z

0

It was a silly mistake

array.out[:,:,i] = tmp[np.arange(np.shape(tmp)[0])[:, np.newaxis], arg_ftc[:,:,i][arg_ftc_rev[:,:,i]]

needs to be

array.out[:,:,i] = tmp[np.arange(np.shape(tmp)[0])[:, np.newaxis], [arg_ftc_rev[:,:,i]]

answered Sep 27, 2019 at 15:07

heffalump

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