Recursion

Recursion

Recursion is when a function calls itself. It's a clean way to solve problems that break into smaller copies of themselves.

The classic example: factorial

def tri_recursion(k):
    if k > 0:
        result = k + tri_recursion(k - 1)
        print(result)
    else:
        result = 0
    return result

tri_recursion(6)

How it works

tri_recursion(6) calls tri_recursion(5), which calls tri_recursion(4), and so on until k is 0. Then each call returns and adds its value.

A base case is essential

Every recursive function needs a stopping condition:

def countdown(n):
    if n <= 0:   # base case stops the recursion
        print("Done!")
        return
    print(n)
    countdown(n - 1)

Without a base case, you get infinite recursion and a RecursionError.

Recursion depth limit

Python limits how deep recursion can go (about 1000 by default). Very deep recursion errors out.

When recursion shines

• Trees and graphs
• Fractals
• Problems like factorials, Fibonacci, and sorting

TL;DR

• Recursion: a function calls itself.
• Always include a base case that stops it.
• Without one, you hit a RecursionError.
• Great for tree-like or self-similar problems.