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

Here you go:

In [17]: a = np.array([0, 10, 20])

In [18]: b = np.array([20, 30, 40, 50])  

In [19]: (a.reshape(-1, 1) + b).ravel()
Out[19]: array([20, 30, 40, 50, 30, 40, 50, 60, 40, 50, 60, 70])

Here are the details.

a.reshape(-1, 1) converts a to an array with shape (3, 1):

In [20]: a.reshape(-1, 1)
Out[20]: 
array([[ 0],
       [10],
       [20]])

When b is added to that, broadcasting applies, which in effect does an "outer sum" (i.e. adds all the pairwise combinations), forming an array with shape (3, 4):

In [21]: a.reshape(-1, 1) + b
Out[21]: 
array([[20, 30, 40, 50],
       [30, 40, 50, 60],
       [40, 50, 60, 70]])

The ravel() method flattens the result into a one-dimensional array:

In [22]: (a.reshape(-1, 1) + b).ravel()
Out[22]: array([20, 30, 40, 50, 30, 40, 50, 60, 40, 50, 60, 70])

See @HYRY's answer for an even more concise version.