CODEWITHSUNDEEP
pythonrecursion
user5438924
user5438924

Reputation:

Recursively sum a list and compare it to a key in python

I need to write a recursive function sums_to(nums, k) that takes a list of integers and returns True if the sum of all the elements in the list is equal to k and returns False otherwise.

I can't use sum function in any form, nor sum the list and then at the end check whether it equals k. In addition, I have to write sums_to as a single recursive function.

So far I have this:

def sums_to(nums, k):
    if nums == []:
        if k == 0
            return True
        return 0
    else:
        return nums[0] + sums_to(nums[1:], k) == k

Upvotes: 0

Views: 501

Answers (1)

Óscar López
Óscar López

Reputation: 236114

Your approach doesn't seem correct, in particular the recursive step isn't right - you have to decrease the expected k value until we reach the end of the list (and only test k at this point!), instead of adding/comparing at each point of the list. Also, the base case is missing something - what happens if k is not zero? you should return False, not 0. Better try this:

def sums_to(nums, k):
    if nums == []:
        return k == 0
    else:
        return sums_to(nums[1:], k - nums[0])

It works as expected:

sums_to([1, 2, 3, 4, 5], 14)
=> False
sums_to([1, 2, 3, 4, 5], 16)
=> False
sums_to([1, 2, 3, 4, 5], 15)
=> True

Upvotes: 3

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