Python: use numpy array of indices to "lookup" values from another matrix

Question

What is the fastest way of creating a new matrix that is a result of a "look-up" of some numpy matrix X (using an array of indices to be looked up in matrix X)? Example of what I want to achieve:

indices = np.array([[[1,1],[1,1],[3,3]],[[1,1],[5,8],[6,9]]]) #[i,j]
new_matrix = lookup(X, use=indices)

Output will be something like:

new_matrix = np.array([[3,3,7],[3,4,9]])

where for example X[1,1] was 3. I'm using python 2.

X is just some matrix I want to perform a lookup on. So if indices have [1,1] it means I want to look up index [1,1] in X. — ru111, Feb 09 '18 at 18:49

Divakar · Accepted Answer · 2018-02-09T19:02:05.043

1

Use sliced columns for indexing into X -

X[indices[...,0], indices[...,1]]

Or with tuple -

X[tuple(indices.T)].T # or X[tuple(indices.transpose(2,0,1))]

Sample run -

In [142]: X
Out[142]: 
array([[0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
       [0, 3, 0, 0, 0, 0, 0, 0, 0, 0],
       [0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
       [0, 0, 0, 7, 0, 0, 0, 0, 0, 0],
       [0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
       [0, 0, 0, 0, 0, 0, 0, 0, 4, 0],
       [0, 0, 0, 0, 0, 0, 0, 0, 0, 9]])

In [143]: indices
Out[143]: 
array([[[1, 1],
        [1, 1],
        [3, 3]],

       [[1, 1],
        [5, 8],
        [6, 9]]])

In [144]: X[indices[...,0], indices[...,1]]
Out[144]: 
array([[3, 3, 7],
       [3, 4, 9]])

edited Feb 09 '18 at 19:02

answered Feb 09 '18 at 18:55

Divakar

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Thanks! Is there a way to do this without copy & pasting if i had 700 lines instead to 2? – ru111 Feb 09 '18 at 18:59
@ru111 Without copy & pasting what exactly? – Divakar Feb 09 '18 at 19:00
Ah nevermind - I mistakenly thought you did ...0 and ...1 because there's two rows in my example :) – ru111 Feb 09 '18 at 19:01
Could you also explain why ..., is needed in [...,0]? – ru111 Feb 09 '18 at 19:03
@ru111 https://stackoverflow.com/a/40383002/, https://stackoverflow.com/questions/772124/what-does-the-python-ellipsis-object-do – Divakar Feb 09 '18 at 19:05

Python: use numpy array of indices to "lookup" values from another matrix

1 Answers1