Recursion in Python

Functions that call themselves, shrinking toward a base case.

def factorial(n):
    if n <= 1:
        return 1
    return n * factorial(n - 1)


def count_down(n):
    if n == 0:
        return ["liftoff"]
    return [n] + count_down(n - 1)


def digits_sum(n):
    if n < 10:
        return n
    return n % 10 + digits_sum(n // 10)

How it works

1. factorial multiplies down to the base case of 1.
2. count_down builds a list on the way back up.
3. digits_sum peels one digit per call with % and //.

Keywords and builtins used here

• count_down
• def
• digits_sum
• factorial
• if
• return

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Step 5 of 6 in Functions, step 37 of 72 in Language basics.

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Recursion in other languages

• C