How to sort a numpy array based on the values in a specific row?

Question

I was wondering how I would be able to sort a whole array by the values in one of its columns.

I have :

array([5,2,8,2,4])

and:

array([[ 0,  1,  2,  3,  4],
       [ 5,  6,  7,  8,  9],
       [10, 11, 12, 13, 14],
       [15, 16, 17, 18, 19],
       [20, 21, 22, 23, 24]])

I want to append the first array to the second one like this:

array([[ 0,  1,  2,  3,  4],
       [ 5,  6,  7,  8,  9],
       [10, 11, 12, 13, 14],
       [15, 16, 17, 18, 19],
       [20, 21, 22, 23, 24],
       [5,  2,  8,  2,  4]])

And then sort the array by the appended row to get either this:

array([[1,  3,  4,  0,  2],
       [6,  8,  9,  5,  7],
       [11, 13, 14, 10, 12],
       [16, 18, 19, 15, 17],
       [21, 23, 24, 20, 22],
       [2,  2,  4,  5,  8]])

or this:

array([[ 2,  1,  3,  4,  0],
       [ 7,  6,  8,  9,  5],
       [12, 11, 13, 14, 10],
       [17, 16, 18, 19, 15],
       [22, 21, 23, 24, 20],
       [ 8,  5,  4,  2,  2]])

And then remove the appended column to get:

array([[1,  3,  4,  0,  2],
       [6,  8,  9,  5,  7],
       [11, 13, 14, 10, 12],
       [16, 18, 19, 15, 17],
       [21, 23, 24, 20, 22]])

or:

array([[ 2,  1,  3,  4,  0],
       [ 7,  6,  8,  9,  5],
       [12, 11, 13, 14, 10],
       [17, 16, 18, 19, 15],
       [22, 21, 23, 24, 20]])

Is there a code to carry out this procedure. I am very new to python. Thanks a lot!

Answer 1

You can use numpy.argsort to get a list with the sorted indices of your array. Using that you can then rearrange the columns of the matrix.

import numpy as np

c = np.array([5,2,8,2,4])    
a = np.array([[ 0,  1,  2,  3,  4],
              [ 5,  6,  7,  8,  9],
              [10, 11, 12, 13, 14],
              [15, 16, 17, 18, 19],
              [20, 21, 22, 23, 24]])

i = np.argsort(c)
a = a[:,i]