Walkthrough: Countdown

例题: 倒计时

≈ 8 min · python-loops · Open in WolfCode

The textbook's first while-loop example, in 4 lines. The three-step flow of execution that every loop in CSC 141 follows.

The function

Python · runnable

def countdown(n):
    while n > 0:
        print(n)
        n = n - 1
    print("Blastoff!")

countdown(3)

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The three-step flow

Evaluate the condition (n > 0).
If False → exit, continue after the loop.
If True → run the body, then go back to step 1.

What makes it terminate

The line n = n - 1 (same as n -= 1) is the only reason this loop ever ends. Delete it and you have an infinite loop:

⚠ Don't actually run this — it will hang the browser. Trace it by hand.

def countdown_INFINITE(n):
    while n > 0:
        print(n)
        # n never changes — condition stays True forever
    print("Blastoff!")

# countdown_INFINITE(3)   # ← would loop forever
print("(skipped — would hang)")

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The zero-iteration edge case

What does countdown(0) do?

def countdown(n):
    while n > 0:
        print(n)
        n -= 1
    print("Blastoff!")

countdown(0)
# Condition is False on the FIRST check.
# Body runs zero times. Only "Blastoff!" prints.

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

Sum 1..n uses the same shape — initialize before, update inside — but with a for loop instead of while.

Walkthrough: Countdown

The function

The three-step flow

What makes it terminate

The zero-iteration edge case

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