Recursion

Sometimes a function can solve a problem by calling itself.

This is called recursion.

It may sound strange at first, but it becomes clear with simple examples.

What Is Recursion?

Recursion happens when a function runs itself.

The function keeps calling itself until a stopping point is reached.

Why a Stopping Point Matters

Without a stopping point, the function would call itself forever.

That would cause an error.

The stopping point is often called the base case.

Simple Example

def countdown(number):
    if number == 0:
        print("Done")
    else:
        print(number)
        countdown(number - 1)

countdown(3)

Output

3
2
1
Done

How This Works

Let’s go step by step:

• countdown(3) prints 3
• Then it calls countdown(2)
• That prints 2
• Then it calls countdown(1)
• That prints 1
• Then it calls countdown(0)
• Now the base case is reached
• It prints Done
• The recursion stops

Another Example: Factorial

Factorial means multiplying numbers down to 1.

Example:

4! = 4 × 3 × 2 × 1 = 24

def factorial(number):
    if number == 1:
        return 1
    else:
        return number * factorial(number - 1)

print(factorial(4))

Output

24

Why Recursion Is Useful

Recursion is useful for problems that repeat in smaller steps.

Examples:

• Countdowns
• Factorials
• Folder search
• Tree structures

Common Beginner Mistake

Forgetting the base case.

def test():
    test()

This never stops and causes an error.

Code Along

def show(number):
    if number == 0:
        print("End")
    else:
        print(number)
        show(number - 1)

show(2)

Output

2
1
End

Mini Challenge

Build a countdown function.

Steps:

• Create a function called countdown
• Give it one parameter called number
• If number is 0, print:
  Go!
• Otherwise:
  • Print the number
  • Call the function again with number - 1

Expected output for countdown(3):

3
2
1
Go!

Real World Use Case

Programs use recursion in file systems, search tools, games, and problems that break into smaller versions of themselves.

Quiz

1. What is recursion?
2. What is a base case?
3. Why is the base case important?
4. What happens if recursion never stops?

Assignment

Create a recursive function that prints numbers from 5 down to 1, then prints Finished.

Summary

You learned that recursion happens when a function calls itself and uses a base case to stop safely.