Recursion Basics 10 XP Easy

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Recursion is when a function calls itself! It's like looking into two mirrors facing each other — images within images within images... 🪞

💡 Think of it like this: Russian nesting dolls (matryoshka). To find the smallest doll, you open the big one, find a smaller one inside, open that, find an even smaller one... until you reach the tiniest doll (that's your base case!).

Every recursive function needs:

🛑 Base case: When to STOP (without this → infinite loop → crash!)
🔄 Recursive case: The function calls itself with a SMALLER problem
📉 Progress: Each call must get closer to the base case

# Countdown — simplest recursion example
def countdown(n):
    if n <= 0:         # Base case: stop at 0
        print('Go! 🚀')
        return
    print(n)
    countdown(n - 1)   # Recursive case: smaller problem

countdown(3)
# Output: 3, 2, 1, Go! 🚀

# Factorial: 5! = 5 × 4 × 3 × 2 × 1 = 120
def factorial(n):
    if n <= 1:           # Base case
        return 1
    return n * factorial(n - 1)  # n! = n × (n-1)!

# How it works:
# factorial(4)
# = 4 * factorial(3)
# = 4 * (3 * factorial(2))
# = 4 * (3 * (2 * factorial(1)))
# = 4 * (3 * (2 * 1))
# = 4 * (3 * 2)
# = 4 * 6
# = 24 ✓

Pro tip: When stuck with recursion, ask yourself: "If I had the answer to a smaller version of this problem, how would I use it to solve the bigger problem?"

📋 Instructions

Write a recursive function `sum_to_n(n)` that returns the sum of all numbers from 1 to n. Test with n=5 (15) and n=10 (55).

Base case: if n <= 0, return 0. Recursive case: return n + sum_to_n(n-1). sum_to_n(5) = 5 + sum_to_n(4) = 5 + 4 + sum_to_n(3) ... = 15.

🧪 Test Cases

Input

sum_to_n(5)

Expected

15

Test case 1

Input

sum_to_n(10)

Expected

55

Test case 2

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main.py

Hi! I'm Rex 👋

Output

Ready. Press ▶ Run or Ctrl+Enter.

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