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 1 '11 at 1:44

JoshAdel · Accepted Answer · 2011-04-01 01:52:27Z

up vote 6 down vote accepted

According the numpy tutorial:

a[tuple(b)]

is the correct way to do this:

answered Apr 1 '11 at 1:52

JoshAdel

28.3k57596

add a comment |

Matthäus Brandl · Answer 2 · 2013-02-21 09:59:05Z

up vote 0 down vote

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 9:59

Matthäus Brandl

769715

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

add a comment |

2 Answers 2