Recursive Factorial Acted Out: Environments and When to Recurse

How do four calls each named n avoid colliding — and what signals tell you a problem naturally wants recursion instead of a loop?

Finger exercises

Short drills on what this video just taught. Write the code, run the checks, and reveal the answer only if you are stuck.

0 / 5 passed

Exercise 1 of 5

Write factorial recursively

Define fact_recur(n) so that it returns n factorial for any integer n of 1 or more. Give it the two pieces every recursive function needs: a base case that stops at the smallest input, and a recursive step written in terms of fact_recur itself.

No loop, and no printing — just the returned number.

# Factorial, written in terms of itself.
def fact_recur(n):
    """ n is an int >= 1
        returns n factorial """
    return ...  # your code here

# Factorial, written in terms of itself.
def fact_recur(n):
    """ n is an int >= 1
        returns n factorial """
    return ...  # your code here

Python runs in your browser

⌘/Ctrl + Enter runs

is a function you can call

answers the smallest input outright

matches factorial for larger inputs

gets there by calling itself

←Previous Divide and Conquer: The Regrade Chain and Writing power_recur Next fib_recur: Two Recursive Calls and an Exploding Call Tree →