CODEWITHSUNDEEP
pythonrecursion
Aeden Schmidt
Aeden Schmidt

Reputation: 91

How to check every element of list recursively?

I am currently trying to code a function recursively but I am stuck on the base case. Basically, my problem is that I don't know how to create the base case since I don't know how to check every element of the list recursively. If I was to use a for loop, it would go something like

for i in range(len(A)):

where A is a list. How would I changed that into recursion?

Edit: I am now aware that I gave too little details. Lets say I have a list of numbers [5,6,-1,7,-3] I want my output to be just the positive numbers, so by the code it would be [5,6,3]. Now my problem is that I can easily do this using a for loop to check every element of the list, but I want to do it recursively and I don't know how to do that for lists. I hope It is clearer now/makes sense.

Upvotes: 0

Views: 1605

Answers (2)

martineau
martineau

Reputation: 123491

This seems to work. The base case is when there are no more numbers left to check.

def positive_vals(nums, result=None):
    if result is None:
        result = []
    if not nums:
        return result
    else:
        if nums[0] > 0:
            result.append(nums[0])
        return positive_vals(nums[1:], result)


numbers = [5, 6, -1, 7, -3]
print(positive_vals(numbers))  # -> [5, 6, 7]

Upvotes: 0

miquelvir
miquelvir

Reputation: 1757

The base case is when then list element is not a nested list, like follows:

def visit_list_rec(x):
    # base case, we found a non-list element
    if type(x) != list:
        print(x)
        return

    # recursive step, visit each child
    for element in x:
        visit_list_rec(element)


if __name__ == "__main__":
    visit_list_rec(["this", "is", ["a", ["nested", "list"]]])

Upvotes: 1

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