Question

I have two permutations represented as Numpy arrays:

A:

     0  1  2  3
A = [1, 3, 2, 0]

and B:

     0  1  2  3
B = [0, 2, 1, 3]

how can I get C = A*B, where

0->1 * 1->2 = 2
1->3 * 3->3 = 3
2->2 * 2->1 = 1
3->0 * 0->0 = 0

C = (2, 3, 1, 0)

efficiently using Numpy?

Answer 1

Quite simply, b[a] will do the trick:

>>> import numpy as np
>>> a = np.array([1, 3, 2, 0])
>>> b = np.array([0, 2, 1, 3])
>>> c = b[a]
>>> print c
[2 3 1 0]