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Question

def largestNumber(numberList):

    if len(numberList) == 1:
        return numberList[0]
    else:
        largestNumber(numberList[1:])

For this code I am passing in a list, [1,2,3,4,5,6,7,8,9,10] , and the function returns None ... but I am expecting it to call itself until it returns 1 . Can someone help explain?

Answer 1

Remove the last element in each recursion call, and don't forget return the recursion:

def largestNumber(numberList):

    if len(numberList) == 1:
        return numberList[0]
    else:
        numberList = numberList[:-1]
        return largestNumber(numberList)

ls = [1,2,3,4,5,6,7,8,9,10]


print(largestNumber(ls))
print(ls)

=> 1
=> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

Answer 2

Something seems to have gone awry in both this question and the accepted answer. I'm going to try to make sense of it. No one would write a recursive function just to find the first or last element of a list, and certainly wouldn't name that function largestNumber() .

Let's assume the original problem specification was something like:

Without using max() , write a recursive function that emulates max() by finding the largest number in a list of numbers. Called largestNumber() , it takes a list and returns a number.

Once the missing return is added, the remaining problem with the OP's code, and that of the currently accepted answer, is they throw away elements of the list and simply return the base case of the recursion. They never compare the current (first) element of the list with the result of the recursion to determine the largest number:

def largestNumber(numberList):

    a, *rest = numberList

    if not rest:
        return a

    b = largestNumber(rest)

    return a if a > b else b

if __name__ == "__main__":

    import random

    my_list = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

    random.shuffle(my_list)

    print(my_list, '->', largestNumber(my_list))

TEST

> python3 test.py
[2, 4, 6, 3, 5, 7, 8, 1, 9, 10] -> 10
> python3 test.py
[6, 4, 9, 1, 10, 7, 5, 3, 8, 2] -> 10
> python3 test.py
[10, 8, 7, 3, 2, 9, 1, 6, 4, 5] -> 10