numpy sorting by entire row/column

Question

I'm trying to sort a 2d array in numpy by its entire rows: that is, the elements in each row should be kept in their order within that row, but the entire row itself can be swapped with another. For each row, their order is determined by their elements: going through each element, if the ith element in row 1 is the same as row 2, look at the ith + 1 element. Else, take the row whose ith element is smaller. :

import numpy as np
a = array([[1,2,3,4],
           [0,3,4,5],
           [1,2,4,5],
           [0,2,4,5],])

return:

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

Is there a prebuilt numpy function that does this?

Seb · Accepted Answer · 2019-11-27 01:31:29Z

1

There is np.lexsort. Pass in the columns of a (as rows, hence a.T) as the sorting keys, being aware that the last key defines the primary sort order (hence reverse the keys with a.T[::-1]):

>>> a[np.lexsort(a.T[::-1])]
array([[0, 2, 4, 5],
       [0, 3, 4, 5],
       [1, 2, 3, 4],
       [1, 2, 4, 5]])

edited Nov 27, 2019 at 1:31

answered Nov 26, 2019 at 23:09

Seb

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