Python, The fastest way to find numbers indexes in np. array

Question

The fastest way to find numbers indexes in np. array in Python is?

Suppose we have a list of numbers from 0 to 20, and we want to know the indexes of digits 2 and 5

What have you tried? Did you measure the time? What does a profiler show? — Robert
– Robert, Commented Apr 6, 2020 at 17:59
Does this answer your question? How to return the index of numpy ndarray based on search? — Lydia van Dyke
– Lydia van Dyke, Commented Apr 6, 2020 at 18:01

Lydia van Dyke · Accepted Answer · 2020-04-06 20:16:37Z

1

The canonical way would be to use numpy's where method:

a = np.array(range(20))
np.where((a == 2) | (a == 5))

Note that in order to combine the two terms (a == 2) and (a == 5) we need the bitwise or operator |. The reason is that both (a == 2) and (a == 5) return a numpy array of dtype('bool'):

>>> a == 2
array([False, False,  True, False, False, False, False, False, False,
       False, False, False, False, False, False, False, False, False,
       False, False])

>>> (a == 5)
array([False, False, False, False, False,  True, False, False, False,
       False, False, False, False, False, False, False, False, False,
       False, False])

>>> (a == 2) | (a==5)
array([False, False,  True, False, False,  True, False, False, False,
       False, False, False, False, False, False, False, False, False,
       False, False])

edited Apr 6, 2020 at 20:16

answered Apr 6, 2020 at 18:00

Lydia van Dyke

2,5263 gold badges15 silver badges26 bronze badges

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

Collectives™ on Stack Overflow

Python, The fastest way to find numbers indexes in np. array

1 Answer 1

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related