Array values from list containing indices using Python

Question

I would like to get array values of A based on indices mentioned in list B. The desired output is attached.

import numpy as np
A=np.array([[1,2,3],[4,5,6],[7,8,9]])
B=[(0,0),(1,0),(2,1),(2,2)]

The desired output should look like

[1,4,8,9]

Answer 1

The first step of the trick to be used here is:

• create a Numpy array from B,
• transpoose it,
• convert to a tuple of tuples.

The code to do it is:

tuple(map(tuple, np.array(B).T))

For your source data the result is:

((0, 1, 2, 2), (0, 0, 1, 2))

The first inner tuple contains row indices, the second - column indices.

Then you can use it to retrieve indicated elements of A, using A[…].

So the whole code to get your expected elements is:

A[tuple(map(tuple, np.array(B).T))]

Another, shorter option is:

A[tuple(np.array(B).T)]

This time tuple(np.array(B).T) yields a tuple, made of 1-D Numpy arrays, made of rows of np.array(B).T.