CODEWITHSUNDEEP
pythonnumpy3dslice
Michael Sohnen
Michael Sohnen

Reputation: 980

Numpy: trouble obtaining a 2D slice of 3D array

Similar questions have been asked on SO, but never in the form that I needed.

I seem to be having trouble understanding NumPy slicing behavior.

Lets say I have an Numpy Array of shape 512x512x120

vol1=some_numpy_array
print(x.shape) #result: (512,512,120)

I want to take a "z-slice" of this array, however I end up with a 512x120 array instead of a 512x512 one I try the following code for instance

zSlice=(vol1[:][:][2]).squeeze()
print(zSlice.shape()) #result: (512,120)

Why is the resulting array shape (512,120) and not (512,512)? How can I fix this problem?

Upvotes: 1

Views: 1475

Answers (2)

ThePyGuy
ThePyGuy

Reputation: 18466

The problem with vol1[:][:][2] is: vol1[:] will give the entire array, then again [:] gives the entire array, and finally [2] gives the array at index 2 to in the outer axis, eventually vol1[:][:][2] is nothing but vol1[2] which is not what you want.

You need to take numpy array slice:

vol1[:, :, 2].

Now, it will take all the items from outermost, and outer axes, but only item at index 2 for the inner most axis.

Upvotes: 1

U13-Forward
U13-Forward

Reputation: 71610

You have to slice at once:

vol1[:, :, 2].squeeze()

>>> vol1[:, :, 2].squeeze().shape
(512, 512)

Because doing [:] repeatedly doesn't do anything:

>>> (vol1[:][:] == vol1).all()
True
>>>

It's because [:] gets the whole of the list... doing it multiple times does not change anything.

Upvotes: 1

Related Questions