Indexing a multi dimensional array in Numpy

Question

I'm a little confused on indexing in python. I have the following array

[[ 2  4]
 [ 6  8]
 [10 12]
 [14 16]]

and I want to obtain array([4, 2]) as my output. I tried using

Q4= [Q3[0,1],Q3[0,0]]

and my output come out as [4, 2]. I'm missing "array ("Any pointers on indexing in Python ? Thanks!!

RomanPerekhrest · Accepted Answer · 2023-01-29 20:24:06Z

2

Just slice the 1st row reversed:

Q3[0,::-1]

array([4, 2])

answered Jan 29, 2023 at 20:24

RomanPerekhrest

93.1k4 gold badges75 silver badges112 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Roy Smart · Accepted Answer · 2023-01-29 20:30:16Z

1

While one option would be to just wrap your result in another call to numpy.array(),

np.array([Q3[0,1],Q3[0,0]])

it would be better practice and probably more performant to use integer advanced indexing. In that case, you can just use your indices from before to make one vectorized call to numpy.ndarray.__getitem__ (instead of two independent calls).

Q3[[0, 0], [1, 0]]

Edit: RomanPerekhrest's answer is definitely better in this situation, my answer would only be useful for arbitrary array indices.

edited Jan 29, 2023 at 20:30

answered Jan 29, 2023 at 20:22

Roy Smart

7245 silver badges15 bronze badges

2 Comments

Chrysophylaxs Over a year ago

I think your example is more insightful for OP. +1

Roy Smart Over a year ago

Thanks @Chrysophylaxs. It's definitely important to know both methods and recognize when you can use slicing vs. when to resort to integer indexing.

Collectives™ on Stack Overflow

Indexing a multi dimensional array in Numpy

2 Answers 2

Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related