Indexing numpy array with another numpy array

Question

Suppose I have

a = array([[1, 2],
           [3, 4]])

and

b = array([1,1])

I'd like to use b in index a, that is to do a[b] and get 4 instead of [[3, 4], [3, 4]]

I can probably do

a[tuple(b)]

Is there a better way of doing it?

Thanks

I don't think it is a problem. why you think a[tuple(b)] is bad? — linjunhalida, Apr 01 '11 at 01:44

score 50 · Accepted Answer · edited Oct 02 '21 at 11:42

50

According the NumPy tutorial, the correct way to do it is:

a[tuple(b)]

edited Oct 02 '21 at 11:42

Nicolapps

answered Apr 01 '11 at 01:52

JoshAdel

score 19 · Answer 2 · answered Feb 21 '13 at 09:59

19

Suppose you want to access a subvector of a with n index pairs stored in blike so:

b = array([[0, 0],
       ...
       [1, 1]])

This can be done as follows:

a[b[:,0], b[:,1]]

For a single pair index vector this changes to a[b[0],b[1]], but I guess the tuple approach is easier to read and hence preferable.

answered Feb 21 '13 at 09:59

Brandlingo

4

But which is faster I wonder? Wouldn't tuple() create a copy whereas the views above would not? – Terry Brown Sep 30 '16 at 20:47

score 2 · Answer 3 · answered Feb 01 '22 at 21:20

The above is correct. However, if you see an error like:

IndexError: only integers, slices (`:`), ellipsis (`...`), numpy.newaxis (`None`) and integer or boolean arrays are valid indices

You may have your index array in floating type. Change it to something like this:

arr[tuple(a.astype(int))]

3 Answers3