CODEWITHSUNDEEP
pythonnumpy3dslice
Joe McG
Joe McG

Reputation: 167

Accessing slice of 3D numpy array

I have a 3D numpy array of floating point numbers. Am I indexing the array improperly? I'd like to access slice 124 (index 123) but am seeing this error:

>>> arr.shape
(31, 285, 286)
>>> arr[:][123][:]
Runtime error 
Traceback (most recent call last):
  File "<string>", line 1, in <module>
IndexError: index 123 is out of bounds for axis 0 with size 31

What would be the cause of this error?

Upvotes: 8

Views: 17310

Answers (5)

hpaulj
hpaulj

Reputation: 231335

arr[:][123][:] is processed piece by piece, not as a whole.

arr[:]  # just a copy of `arr`; it is still 3d
arr[123]  # select the 123 item along the first dimension
# oops, 1st dim is only 31, hence the error.

arr[:, 123, :] is processed as whole expression. Select one 'item' along the middle axis, and return everything along the other two.

arr[12][123] would work, because that first select one 2d array from the 1st axis. Now [123] works on that 285 length dimension, returning a 1d array. Repeated indexing [][].. works just often enough to confuse new programmrs, but usually it is not the right expression.

Upvotes: 14

Mir Ilias
Mir Ilias

Reputation: 515

import numpy as np

s=np.ones((31,285,286)) # 3D array of size 3x3 with "one" values
s[:,123,:] # access index 123 as you want

Upvotes: 0

Anton Glukhov
Anton Glukhov

Reputation: 3707

Look up at some slice examples of a xD array. You can try this:

a[:,123,:]

Upvotes: 0

fourwood
fourwood

Reputation: 51

I think you might just want to do arr[:,123,:]. That gives you a 2D array with a shape of (31, 286), with the contents of the 124th place along that axis.

Upvotes: 5

ericthegreat
ericthegreat

Reputation: 303

Is this what you want?

arr.flatten()[123]

Upvotes: 1

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