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up vote 1 down vote favorite

Say I have an array

a = np.zeros((3, 3, 3))

and a z-index array

z = np.random.randint(0, 3, (3, 3))

say z is

array([[1, 0, 2],
       [2, 2, 1],
       [1, 1, 0]])

Now I want to select the values of a with coordinates (starting at top left of z and traversing the array row-wise. Column-wise would be fine too.) (0, 0, 1), (0, 1, 0), (0, 2, 2), ... . The bold values are from the z array.

share|improve this question
    
is a supposed to be a 3d array? – M4rtini Apr 4 '14 at 10:06
    
Yes, sorry. Fixed it. – tauran Apr 4 '14 at 10:08
up vote 1 down vote accepted

I think this does what you want. 

import numpy as np 

a = np.arange(3*3*3).reshape(3,3,3)
z = np.array([[1, 0, 2],
       [2, 2, 1],
       [1, 1, 0]])
i = np.arange(a.shape[0]).repeat(a.shape[0]).reshape(a.shape[0], a.shape[1])
j = i.T
#Should do the same as this. possible more efficient, did not test.
#i, j = np.indices(a[...,0].shape)
print a[i,j,z]
share|improve this answer
    
Nope. If I use zeros and then a[i,j,z] = 1, a[0, 0 ,0] is 1 but should be 0. – tauran Apr 4 '14 at 10:21
    
It works if i is array([[0, 0, 0], [1, 1, 1], [2, 2, 2]]) and j is i.T. – tauran Apr 4 '14 at 10:24
    
Ok, a small edit should make it so. – M4rtini Apr 4 '14 at 10:35
1  
i,j = np.ogrid[:3,:3] would work too. – unutbu Apr 4 '14 at 11:42

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