2D Sorting with NumPy - sort 1 row and have the other follow the sorting

Question

Say I have a NumPy array:

[[4 9 2]
 [5 1 3]]

I want to sort the bottom row of this array, but have the top row follow the sorting, such that I get:

[[9 2 4]
 [1 3 5]]

I know that you can sort like this using the sorted() function, but that requires input and output of lists.

Any ideas? Thanks so much!

Answer 1

import numpy as np
a = np.array([[4,9,2],[5,1,3]])
idx = np.argsort(a[1])

Now you can use idx to index your array:

b=a[:,idx]

Answer 2

The only (efficient) solution I can think of needs a copy of the original array.

import numpy as np
a = np.array([[4,9,2],[5,1,3]])
idx = np.argsort(a[1])

So, that idx is the index of the sorted column.

c = a.copy()
for i in range(len(idx)):
  a[:,i] = c[:,idx[i]]

That should be reasonably fast, but, of course, wastes some memory.