How to index numpy array with an array

Question

Given an array defined below as:

a = np.arange(30).reshape((3, 10)
col_index = [[1,2,3,5], [3,4,5,7]]
row_index = [2,1]

Is it possible to index a[row_index, col_index], so I can do something like a[row_index, col_index] =1, so then a becomes

[[0,1,2,3,4,5,6,7,8,9], [10,11,12,1,1,1,16,1,18,19], [20,1,1,1,24,1,26,27,28,29]]

So to clarify, in row 2, column 1,2,3, and 5 are set to one, and in row 1, column 3,4,5,7 is also set to 1.

OP, You can only accept one answer, not multiple. When you accept another answer, the first one is automatically unaccepted. — cs95
– cs95, Commented Jan 14, 2018 at 0:19

Paul Panzer · Accepted Answer · 2018-01-13 23:47:10Z

2

Or (if you don't like typing)

a[np.c_[row_index], col_index] = 1

or even shorter but Python 2 only

a[zip(row_index), col_index] = 1

What all these solutions do is to make row and col indices broadcastable to each other. np.c_ is the column concatenation convenience object. It makes columns out of 1D objects. zip used to do essentially the same. Only, since Python 3 it returns an iterator instead of a list and numpy can't handle those. (One could do list(zip(row_index)) but that's not short.)

edited Jan 13, 2018 at 23:47

answered Jan 13, 2018 at 23:34

Paul Panzer

53.3k3 gold badges59 silver badges103 bronze badges

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

Collectives™ on Stack Overflow

How to index numpy array with an array

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related