Convolving two arrays in python without for loops

I have two arrays(arr_1,arr_2), and need to generate an output(arr_out) as follows:

arr_1 = [21, 28, 36, 29, 40]
arr_2 = [0, 225, 225, 0, 225]

arr_out = [-1, 28, 36, -1, 40]

The outputarr_out should have -1 at an index if the product of the elements in arr_1 and arr_2 at that index is 0. Otherwise, the value from arr_1 should be in the output array.

I have implemented a solution using a for loop:

 arr_out = [0] * 5

 for index, value in enumerate(arr_1):
    if (value * arr_2[index]) == 0:
        arr_out[index] = -1
    else:
        arr_out[index] = value

Is it possible to achieve this without using a for loop?

You can achieve this using list comprehensions along with the zip function:

arr_out = [-1 if (x * y) == 0 else x for x, y in zip(arr_1, arr_2)]

This will remove the need for for loops as well as enumerate. We're using the zip function, which combines two lists element-wise. Then, we use a list comprehension to loop through the combined elements and perform the desired operation.

You can use [tag:numpy] and convert list to numpy.array then use numpy.where.

import numpy as np
arr_1 = [21, 28, 36, 29, 40]
arr_2 = [0, 225, 225, 0, 225]

np_arr_1 = np.array(arr_1)
np_arr_2 = np.array(arr_2)
arr_out = np.where(np_arr_1 * np_arr_2 == 0, -1, np_arr_1)
print(arr_out)

Output:

[-1 28 36 -1 40]