sorting along multidimensional index in numpy

Question

I have a three dimensional array, that I need to sort. I also have a 2 dimensional index that I got with a lexsort that I would like to use for sorting. I cannot find how to apply the index to the array without extra dimensions sprouting up...

import numpy as np

a = np.array([
    [
        [1, 1, 1],
        [0, 1, 2],
        [3, 1, 1],
        [-1, 1, 1],
        [0, 2, 2],
        [1, 2, 2],
    ],[
        [-1, 1, 1],
        [0, 1, 2],
        [-3, 1, 1],
        [1, 1, 1],
        [0, -2, 2],
        [-1, -2, 2],
    ],[
        [-1, 1, 1],
        [-0, 1, -2],
        [-3, 1, 1],
        [1, 1, 1],
        [-0, -2, -2],
        [-1, -2, -2],
    ],
])
sorted_index = np.lexsort((a[:,:,0], a[:,:,1],a[:,:,2]),axis=1)

sorted_index is

[[3 0 2 1 4 5], [2 0 3 5 4 1], [5 4 1 2 0 3]]

the end result should be

a_sorted = np.array([
    [
        [-1, 1, 1],
        [1, 1, 1],
        [3, 1, 1],
        [0, 1, 2],
        [0, 2, 2],
        [1, 2, 2],
    ],[
        [-3, 1, 1],
        [-1, 1, 1],
        [1, 1, 1],
        [-1, -2, 2],
        [0, -2, 2],
        [0, 1, 2],
    ],[
        [-1, -2, -2],
        [0, -2, -2],
        [0, 1, -2],  
        [-3, 1, 1],
        [-1, 1, 1],
        [1, 1, 1],
    ],
])

I tried.

a[sorted_index]
a[sorted_index, :]
a[:, sorted_index]

I am about to loop through the thing, but I would like to do it properly.

meTchaikovsky · Accepted Answer · 2020-10-26 11:36:18Z

1

You can use np.take_along_axis

np.take_along_axis(a,sorted_index[...,None],axis=1)

the output is

array([[[-1,  1,  1],
        [ 1,  1,  1],
        [ 3,  1,  1],
        [ 0,  1,  2],
        [ 0,  2,  2],
        [ 1,  2,  2]],

       [[-3,  1,  1],
        [-1,  1,  1],
        [ 1,  1,  1],
        [-1, -2,  2],
        [ 0, -2,  2],
        [ 0,  1,  2]],

       [[-1, -2, -2],
        [ 0, -2, -2],
        [ 0,  1, -2],
        [-3,  1,  1],
        [-1,  1,  1],
        [ 1,  1,  1]]])

edited Oct 26, 2020 at 11:36

answered Oct 26, 2020 at 1:32

meTchaikovsky

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1 Comment

Daniele Bernardini Over a year ago

Of course I did a mistake ... I will correct it later in order not to confuse people landing here :D

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