Type error when adding 1 to i (Recursion)

April 16, 2022

I am trying to write a recursion that sums the product of items in a list, in the pattern of:

(some_list[i] * some_list[j]) + (some_list[i+1] * some_list[j-1]) +
(some_list[i+2] * some_list[j-2]) + ........

The limit being once every number has been accounted for between i and j.
I believe I have set up the recursion correctly (correct me if I am wrong), however I am getting the following error:

TypeError: unsupported operand type(s) for +: 'int' and 'str'

def sum_product(some_list, i, j):
    limit = len(some_list)//2
    if j <= limit:
        return ' '
    else:
        result = some_list[i] + some_list[j]
        return result + sum_product(some_list, i + 1, j - 1)
print(sum_product([1, 2, 3, 4, 5, 6, 7, 8], 1, 6))

>Solution :

Change to return 0 and change the + to *. The limit should also be adjusted to include all numbers with indexes between i and j. Assuming j > i.

def sum_product(some_list, i, j):
    limit = len(some_list)//2-1
    if j < i:
        return 0
    else:
        result = some_list[i] * some_list[j]
        return result + sum_product(some_list, i + 1, j - 1)
print(sum_product([1, 2, 3, 4, 5, 6, 7, 8], 1, 7))

Output: 86 = 2*8+3*7+4*6+5*5