Fill a matrix from a matrix of indices

Question

I want to fill a matrix from an array of indices :

import numpy as np

indx = [[0,1,2],[1,2,4],[0,1,3],[2,3,4],[0,3,4]]
x = np.zeros((5,5))
for i in range(5):
    x[i,indx[i]] = 1.

The result is :

array([[ 1.,  1.,  1.,  0.,  0.],
       [ 0.,  1.,  1.,  0.,  1.],
       [ 1.,  1.,  0.,  1.,  0.],
       [ 0.,  0.,  1.,  1.,  1.],
       [ 1.,  0.,  0.,  1.,  1.]])

As desired.

Question

Is there a way to do this in pure python/numpy without looping ?

Answer 1

Use advanced-indexing after intialization -

x[np.arange(len(indx))[:,None], indx] = 1