CODEWITHSUNDEEP
pythonnumpynumpy-slicing
under_the_ground
under_the_ground

Reputation: 27

Why is slicing using "colon and comma" different than using a collection of indexes

Why is slicing using "colon and comma" different than using a collection of indexes?

Here is an example of what I expected to yield the same result but but it does not:

import numpy as np

a = np.array([[[1,2,3],[4,5,6]],[[7,8,9],[10,11,12]]])

print(a[[0,1],[0,1]])
# Output
# [[ 1  2  3]
#  [10 11 12]]

print(a[:,[0,1]])
# Output
# [[[ 1  2  3]
#   [ 4  5  6]]
#  [[ 7  8  9]
#   [10 11 12]]]

Why are they not equivalent?

Upvotes: 1

Views: 456

Answers (1)

OriolAbril
OriolAbril

Reputation: 8783

In the first case, you are indexing the array a with 2 lists of the same length, which would be equivalent to indexing with 2 arrays of the same shape (see numpy docs on arrays as indices).

Therefore, the output is a[0,0] (which is the same as a[0,0,:]) and a[1,1], the elementwise combinations of the index array. This is expected to return an array of shape 2,3. 2 because it is the length of the index array, and 3 because it is the axis that is not indexed.

In the second case however, the result is a[:,0] (equivalent to a[:,0,:]) and a[:,1]. Thus, here the expected result is an array with the first and third dimensions equivalent to the original array, and the second dimension equal to 2, the length of the index array (which here is the same as the original size of the second axis).

To show clearly that these two operations are clearly not the same, we can try to assume equivalence between : and a range of the same length as the axis to the third axis, which will result in:

print(a[[0,1],[0,1],[0,1,2]])
IndexError                                Traceback (most recent call last)
<ipython-input-8-110de8f5f6d8> in <module>()
----> 1 print(a[[0,1],[0,1],[0,1,2]])

IndexError: shape mismatch: indexing arrays could not be broadcast together with shapes (2,) (2,) (3,)

That is because there is no elementwise combination of the index arrays possible. Opposite to that, a[:,:,:] would return the whole array, and a[[0,1],[0,1],[0,2]] returns [ 1 12] which as expected is an array of one dimension with length 2, like the index array.

Upvotes: 1

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