Python while 迴圈

while 迴圈會在條件為 True 時重複執行程式碼，直到條件變成 False。

基本語法

while 條件:
    # 條件為 True 時執行的程式碼

count = 0

while count < 5:
    print(count)
    count += 1

輸出：

0
1
2
3
4

# 無窮迴圈！不要執行
while True:
    print("Forever...")

while 和 for 的選擇

• for 迴圈：當你知道要執行多少次，或要遍歷序列時
• while 迴圈：當你不確定要執行多少次，根據條件決定

# 用 for - 已知執行 5 次
for i in range(5):
    print(i)

# 用 while - 不確定執行幾次
import random
num = random.randint(1, 10)
guess = 0

while guess != num:
    guess = int(input("Guess a number (1-10): "))
    
print(f"Correct! The number was {num}")

while...else

while 迴圈可以搭配 else，當條件變成 False 正常結束時執行 else 區塊：

count = 0

while count < 3:
    print(count)
    count += 1
else:
    print("Loop completed")

輸出：

0
1
2
Loop completed

如果迴圈被 break 中斷，else 不會執行：

count = 0

while count < 5:
    if count == 3:
        break
    print(count)
    count += 1
else:
    print("Loop completed")  # 不會執行

輸出：

0
1
2

實際範例

數字猜謎遊戲

import random

secret = random.randint(1, 100)
attempts = 0

while True:
    guess = int(input("Guess a number (1-100): "))
    attempts += 1
    
    if guess < secret:
        print("Too low!")
    elif guess > secret:
        print("Too high!")
    else:
        print(f"Correct! You got it in {attempts} attempts.")
        break

計算總和直到輸入 0

total = 0

while True:
    num = int(input("Enter a number (0 to stop): "))
    if num == 0:
        break
    total += num

print(f"Total: {total}")

等待特定條件

# 模擬等待任務完成
import time

status = "running"
wait_count = 0

while status == "running":
    print("Waiting...")
    time.sleep(1)  # 等待 1 秒
    wait_count += 1
    
    if wait_count >= 5:
        status = "completed"

print("Task completed!")

驗證使用者輸入

while True:
    age = input("Enter your age: ")
    
    if age.isdigit() and int(age) > 0:
        age = int(age)
        break
    else:
        print("Please enter a valid positive number.")

print(f"Your age is {age}")

計算數字的位數

num = 12345
count = 0

while num > 0:
    num //= 10
    count += 1

print(f"Number of digits: {count}")  # 5

計算最大公因數 (GCD)

# 使用輾轉相除法
a, b = 48, 18

while b != 0:
    a, b = b, a % b

print(f"GCD: {a}")  # 6

單行 while（不建議）

技術上可以寫成一行，但不建議這樣寫：

# 不建議
n = 5; fact = 1
while n > 0: fact *= n; n -= 1
print(fact)  # 120

# 建議的寫法
n = 5
fact = 1
while n > 0:
    fact *= n
    n -= 1
print(fact)  # 120