select first column in a 3d ndarray

Question

I got a simple question for python.
Instead of iterating over the first line for a 3D ndarray, I want to select the whole column.
So lets say:instead of:

print test[0][20][33]
print test[1][20][33]
...

I want to put something like:

 print test[:][20][33]

But this does not work. How do I do it?

Asked By: robaatz

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Source

Answer 1

Put all the terms inside the square brackets:

>>> import numpy as np
>>> v = np.array(np.arange(24)).reshape(2,3,4)
>>> v
array([[[ 0,  1,  2,  3],
        [ 4,  5,  6,  7],
        [ 8,  9, 10, 11]],

       [[12, 13, 14, 15],
        [16, 17, 18, 19],
        [20, 21, 22, 23]]])
>>> v[:, 1, 3]
array([ 7, 19])

.. although I’m not sure I’d call that the first column. You can easily shuffle the indices to get whatever you’re after, though:

>>> v[0, :, 0]
array([0, 4, 8])
>>> v[1, 1, :]
array([16, 17, 18, 19])

[relevant doc section on indexing]

Answered By: DSM

select first column in a 3d ndarray

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