select elements of different columns at different rows of numpy array

Question

In [62]: a
Out[62]: 
array([[1, 2],
       [3, 4]])

Is there an easy way to get [2,3], i.e. the second element of the first row, and the first element of the second row? I have the list of the indices for each row, i.e. [1,0] in this case. I have tried a[:,[1,0]], but it doesn't work.

Bi Rico · Accepted Answer · 2015-05-07 23:10:56Z

4

You need to specify both i and j for all the elements you want. For example:

import numpy as np
a = np.array([[1, 2],
              [3, 4]])
i = [0, 1]
j = [1, 0]
print(a[i, j])
# [2, 3]

If you need one item from each row, you can use i = np.arange(a.shape[0])

answered May 7, 2015 at 23:10

Bi Rico

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