Sort array by other array sort indexing in Python

Question

I'm having a bit of a difficulty. I'm trying to vectorize some code in python in order to make it faster. I have an array which I sort (A) and get the index list (Ind). I have another array (B) which I would like to sort by the index list, without using loops which I think bottlenecks the computation.

A = array([[2, 1, 9],
           [1, 1, 5],
           [7, 4, 1]])
Ind = np.argsort(A)

This is the result of Ind:

Ind = array([[1, 0, 2],
             [0, 1, 2],
             [2, 1, 0]], dtype=int64)

B is the array i would like to sort by Ind:

B = array([[ 6,  3,  9],
           [ 1,  5,  3],
           [ 2,  7, 13]])

I would like to use Ind to rearrange my elements in B as such (B rows sorted by A rows indexes):

B = array([[ 3,  6,  9],
           [ 1,  5,  3],
           [13,  7,  2]])

Any Ideas? I would be glad to get any good suggestion. I want to mention I am using millions of values, I mean arrays of 30000*5000.

Cheers, Robert

you are creating Ind and then immediately overwriting it on the next line, just FYI. — Erik Kaplun
– Erik Kaplun, Commented Mar 11, 2015 at 12:09
I don't know... use comments or smth; it's confusing when you're obviously overwriting something you just created; also while we're at it, check out python.org/dev/peps/pep-0008 — you might find it useful in the long term. — Erik Kaplun
– Erik Kaplun, Commented Mar 11, 2015 at 12:12
I would have overwritten it with the same values as the results...im not here to pick a fight im here to solve a problem...i appretiate the comments and thanks for the heads up — user3612505
– user3612505, Commented Mar 11, 2015 at 12:17

fjarri · Accepted Answer · 2015-03-11 12:24:15Z

1

I would do something like this:

import numpy as np
from numpy import array

A = array([[2, 1, 9],
           [1, 1, 5],
           [7, 4, 1]])
Ind = np.argsort(A)

B = array([[ 3,  6,  9],
           [ 1,  5,  3],
           [13,  7,  2]])

# an array of the same shape as A and B with row numbers for each element
rownums = np.tile(np.arange(3), (3, 1)).T

new_B = np.take(B, rownums * 3 + Ind)
print(new_B)
# [[ 6  3  9]
#  [ 1  5  3]
#  [ 2  7 13]]

You can replace the magic number 3 with the array shape.

answered Mar 11, 2015 at 12:24

fjarri

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2 Comments

ali_m Over a year ago

You can use broadcasting instead of np.tile in order to get the row indices: rownums = np.arange(3); B[rownums[:, None], Ind]

fjarri Over a year ago

Good point, it'll be useful if the arrays are huge. And your way of indexing looks clearer than mine.

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