Is is possible to use recursion to find the sum of each elements of an array using Python?

Question

I am having a problem using recursions to add each element of an array and produce another list that contains their sum.

def add(l1,l2,n,counter): # define new user function named add
    if c >= n: # base case
        # if counter is now greater than the length of the list then return empty array
        return []
    return l1[c] + l2[c], add(l1,l2,n,c+1) # recursion

list1 = [7,8,9] # list 1
list2 = [10,11,12] # list 2
print(add(list1,list2,3,0)) # prompt the output of the add() function

The function of the add() function, in this case, should return a list with the value of [17,19,21]. Instead, it is returning a tuple with the value of (17, (19, (21, [ ]))).

Can someone tell me what I can improve in my code? I appreciate any help you can provide.

Answer 1

First of all, this problem does not require recursion at all. But considering that is your question, what you can do is you can return a list instead of a tuple. So instead of this

return l1[c] + l2[c], add(l1,l2,n,c+1) # recursion

You can return this

return [l1[c] + l2[c], add(l1,l2,n,c+1)] # recursion

But this will give you [17, [19, [21, []]]] as a result, because at each recursion you are returning a list.

To overcome this, you should spread the returned list at each iteration. The final code looks like:

return [l1[c] + l2[c], *add(l1,l2,n,c+1)] # recursion

The * operator spreads the returned list, and you get a single list as a result.