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])

Answer 2

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.

Answer 3

you can use outer method of ufunc:

np.add.outer(a, b).ravel()

Answer 4

First you need to specify the array type, if you use a constructor like that. For instance for integers, use that:

a = array("i",[0, 10, 20])       # signed int type
b = array("i",[20, 30, 40, 50]) 

Them you might want to use while loops with counters, it is more complex than for, but avoids the for loop.

from array import array

a = array("i",[0, 10, 20])  # signed int

b = array("i",[20, 30, 40, 50]) 

c = array("i",[])

count1 = 0

while count1 < len(a):
   count2 = 0
   while count2 < len(b):
       c.append(a[count1]+b[count2])
       count2 += 1
   count1 += 1

print(c)