Walkthrough: Recursive Countdown

例题: 递归倒计时

≈ 8 min · python-recursion · Open in WolfCode

The textbook's first recursive function. Same output as the Chapter 3 while-loop version, different shape.

Python · runnable

def countdown(n):
    if n <= 0:
        print("Blastoff!")
    else:
        print(n)
        countdown(n - 1)        # ← function calls itself

countdown(3)

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Every recursive function has two pieces

Base case — if n <= 0: print("Blastoff!") — the "stop" condition.
Recursive case — countdown(n - 1) — call self with a smaller / closer-to-base argument.

Why `<= 0`, not `== 0`?

Don't actually run this — it will RecursionError

def countdown_BUG(n):
    if n == 0:
        print("Blastoff!")
    else:
        print(n)
        countdown_BUG(n - 1)

# countdown_BUG(-1)   # → RecursionError: -1, -2, -3, ... never hits 0
print("(skipped — would RecursionError)")

Loading Python runtime (~5 MB, one-time)…

Walkthrough: Recursive Countdown

Every recursive function has two pieces

Why <= 0, not == 0?

Why `<= 0`, not `== 0`?