🪆 Recursion Explained Like You're 5

Russian nesting dolls of code

Day 5 of 149

👉 Full deep-dive with code examples

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The Nesting Dolls

Russian nesting dolls (Matryoshka):

• Open the big doll
• There's a smaller doll inside
• Open that one
• Another smaller doll inside
• Keep going until the smallest one

Recursion is a function calling itself!

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In Code

A function that calls itself to solve smaller pieces:

def countdown(n):
    if n == 0:
        print("Liftoff! 🚀")
    else:
        print(n)
        countdown(n - 1)  # Calls itself!

countdown(3)
# Output: 3, 2, 1, Liftoff! 🚀

Each call handles a smaller problem (n-1) until we hit the base case (n=0).

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The Two Rules

Every recursive function needs:

1. Base Case → When to stop (the smallest doll)
2. Recursive Case → Call itself with a smaller problem

Without a base case, it can behave like an infinite loop = 💥

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Classic Example: Factorial

5! = 5 × 4 × 3 × 2 × 1

But also: 5! = 5 × 4!

def factorial(n):
    if n == 1:           # Base case
        return 1
    else:
        return n * factorial(n-1)  # Recursive case

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In One Sentence

Recursion is solving a problem by breaking it into smaller versions of the same problem.

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