Is there a NumPy function to return the first index of something in an array?

Question

I know there is a method for a Python list to return the first index of something:

>>> l = [1, 2, 3]
>>> l.index(2)
1

Is

Answer 1

Just to add a very performant and handy numba alternative based on np.ndenumerate to find the first index:

from numba import njit
import numpy as np

@njit
def index(array, item):
    for idx, val in np.ndenumerate(array):
        if val == item:
            return idx
    # If no item was found return None, other return types might be a problem due to
    # numbas type inference.

This is pretty fast and deals naturally with multidimensional arrays:

>>> arr1 = np.ones((100, 100, 100))
>>> arr1[2, 2, 2] = 2

>>> index(arr1, 2)
(2, 2, 2)

>>> arr2 = np.ones(20)
>>> arr2[5] = 2

>>> index(arr2, 2)
(5,)

This can be much faster (because it's short-circuiting the operation) than any approach using np.where or np.nonzero.

---

However np.argwhere could also deal gracefully with multidimensional arrays (you would need to manually cast it to a tuple and it's not short-circuited) but it would fail if no match is found:

>>> tuple(np.argwhere(arr1 == 2)[0])
(2, 2, 2)
>>> tuple(np.argwhere(arr2 == 2)[0])
(5,)