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# Getting elements from a two dimensional array using two dimensional array of indices in NumPy

schedule Aug 11, 2023
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# Solution

To extract elements from a two dimensional array using two dimensional array of indices in NumPy:

x = np.array([[4,5,6],[7,8,9]])
idx = np.array([[0,1],[1,2]])

x[np.arange(x.shape[0])[:, None], idx]
array([[4, 5],
[8, 9]])

# Explanation

Here, we are first extracting the number of rows of x using x.shape[0] (=2), and then using arange(~) to create the following one dimensional array:

np.arange(A.shape[0])
array([0, 1])

We then convert this this into a two-dimensional array like so:

np.arange(A.shape[0])[:, None]
array([[0],
[1]])

This allows us to select the row, while the idx will allow us to select to columns of x:

x = np.array([[4,5,6],[7,8,9]])
idx = np.array([[0,1],[1,2]])

x[np.arange(x.shape[0])[:, None], idx]
array([[4, 5],
[8, 9]])
Published by Isshin Inada
Edited by 0 others
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