How to find maximum negative and minimum positive number in a numpy array?

Question

I am given an array containing both positive and negative numbers.

import numpy as np
arr = np.array([-10.2, -5.3, -2.1, 0, 1.2, 3.4])

I would like to find the index corresponding to the maximum negative number and to a minimum positive number. In the above, my expected outcome is 2 and 4. Is there any numpy trick to achieve this? I have found a solution in this link, but I would like to know how this could be done through numpy functions: Finding index of largest negative and smallest positive element in array

Solution 1

Replace non negative values with -inf, then use argmax to find the largest negative:

np.where(arr < 0, arr, -np.inf).argmax()
# 2

Similarly replace non positive values with inf then use argmin to find the smallest positive:

np.where(arr > 0, arr, np.inf).argmin()
# 4

Solution 2

If your array is always sorted (as in the provided example), then:

# input
Out[66]: array([-10.2,  -5.3,  -2.1,   0. ,   1.2,   3.4])

# obtain a `signed` boolean mask 
In [75]: sign_mask = np.sign(arr)

# compute derivative and obtain index for max_negative element
In [76]: max_neg_idx = np.where(np.diff(sign_mask, append=1) == 1)[0][0]

# add +2 to that to get index for min_positive element
In [77]: min_pos_idx = max_neg_idx + 2

Comments

[John Zwinck]

Is it guaranteed that the input is sorted?

[matttree]

No, it is not guaranteed.

[Warren Weckesser]

Or replace with nan and use nanargmax and nanargmin. But that's not much different than using inf.

[Lost]

@Psidom Is using Numpy functions as in your answer faster or doing it with lists as mentioned in the answer tagged in the question faster?