Add two arrays without using for. Python

Question

I have these two arrays:

import numpy as np
a = np.array([0, 10, 20])
b = np.array([20, 30, 40, 50])  

I´d like to add both in the followin

Answer 1

One possible and only memory not time intensive solution is using np.repeat and np.resize to repeat and resize the arrays a and b to the size of the resulting shape first and then simply add those two arrays.

Code:

import numpy as np
a = np.array([0, 10, 20])
b = np.array([20, 30, 40, 50])

def extend_and_add(a, b):
    return np.repeat(a, len(b)) + np.resize(b, len(a)*len(b))

So extend_and_add(a, b) returns:

extend_and_add(a, b)
> array([20, 30, 40, 50, 30, 40, 50, 60, 40, 50, 60, 70])

---

Explanation:

Basically np.repeat(a, len(b)) repeats:

a
> array([ 0, 10, 20])

to

np.repeat(a, len(b))
> array([ 0,  0,  0,  0, 10, 10, 10, 10, 20, 20, 20, 20])

after this you need the second array resized with np.resize:

b
> array([20, 30, 40, 50])

is resized to:

np.resize(b, len(a)*len(b))
> array([20, 30, 40, 50, 20, 30, 40, 50, 20, 30, 40, 50])

Now we can simply add the arrays:

array([ 0,  0,  0,  0, 10, 10, 10, 10, 20, 20, 20, 20])
+
array([20, 30, 40, 50, 20, 30, 40, 50, 20, 30, 40, 50])

returns:

array([20, 30, 40, 50, 30, 40, 50, 60, 40, 50, 60, 70])