How to index a numpy array element with an array

Question

I've got a numpy array, and would like to get the value at a specific element. For example, I might like to access the value at [1,1]

import numpy as np
A = np.arange(9).reshape(3,3)
print A[1,1]
# 4

Now, say I've got the coordinates in an array:

i = np.array([1,1])

How can I index A with my i coordinate array. The following doesn't work:

print A[i]
# [[3 4 5]
#  [3 4 5]]

Ahh I figured it'd be simple.. I don't really understand why that works though. Could you perhaps create an answer pointing to the relevant docs? — ajwood
– ajwood, Commented Sep 2, 2013 at 23:46

Pavel Anossov · Accepted Answer · 2013-09-02 23:50:47Z

4

http://docs.scipy.org/doc/numpy/reference/arrays.indexing.html

In Python, x[(exp1, exp2, ..., expN)] is equivalent to x[exp1, exp2, ..., expN]; the latter is just syntactic sugar for the former.

So to get the same result as with A[1,1], you have to index with a tuple.

If you use an ndarray as the indexing object, advanced indexing is triggered:

http://docs.scipy.org/doc/numpy/reference/arrays.indexing.html#advanced-indexing

answered Sep 2, 2013 at 23:50

Pavel Anossov

63.3k16 gold badges156 silver badges125 bronze badges

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

Comments

user2357112 · Accepted Answer · 2013-09-02 23:48:58Z

0

Your best bet is A[tuple(i)]. The tuple(i) call just treats i as a sequence and puts the sequence items into a tuple. Note that if your array has more than one dimension, this won't make a nested tuple. It doesn't matter in this case, though.

answered Sep 2, 2013 at 23:48

user2357112

286k32 gold badges490 silver badges569 bronze badges

Collectives™ on Stack Overflow

How to index a numpy array element with an array

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related