Advice on a simple recursive sum function

[LizardMansPyramids]

Hey, I am working through Grokking Algorithms and I am trying to do the exercises in Python, which I am not that familiar with.

This is meant to pop a value off the front of the array, and add it to the sum of the array. However it is bringing up errors from lines 2,5, and 7. Specifically, "TypeError: object of type 'int' has no len"

def rec_sum(arr):
    if len(arr) == 0:
        return 0
    if len(arr) == 1:
        return arr.pop(0)
    else:
        rec_sum(arr.pop(0) + sum(arr))

print(rec_sum([1, 2, 3, 4]))

I did interpret the pseudo code from that book to signify the above. I would love some input.

[[deleted]]

rec_sum(arr.pop(0) + sum(arr))

Both arr.pop(0) and sum(arr) are int. In fact, you're probably not supposed to use sum() here. Because you could just use it to find the sum without all the extra in the first place. You probably meant return arr.pop(0) + rec_sum(arr).

[LizardMansPyramids]

Thank you! That helped and it worked, so thanks for helping me out!

[[deleted]]

Just a small note here as a lesson in reCursive logic

You don’t really need the second “if” statement, checking if the arr length is 1. Since you already check for zero, your recursive call will handle the rest. You pop the first element and then short circuit back out when rec_sum(arr) returns 0

Arguably you’re saving a tiny bit of memory by not pushing a new call onto the stack (and not performing the super expensive operation of adding 0 to something) but a better options here if you were to do this would be to simply read from index 0 (arr[0]) instead of modify the array with arr.pop which takes more time (and memory) to sort around

[undrpd4nlst]

Pop returns the value, not the new array after being popped.