Python中while循环常见用法与实战代码

while 条件表达式:
    循环体代码块
else:
    循环正常结束（没有被 break 终止）时执行的代码块

count = 0
while count < 5:
    print(count)
    count += 1

a = 1
while a < 3:
    print(f"开始循环，a = {a}")
    a += 1
    print(f"结束本次循环，a = {a}")
print("循环结束")

while True:
    user_input = input("输入 'exit' 退出: ")
    if user_input == 'exit':
        break
    print(f"你输入了: {user_input}")

# 错误：忘记更新循环变量
i = 0
while i < 10:
    print(i)   # 无限循环，因为 i 永远为 0

num = 0
while num < 10:
    if num == 5:
        break
    print(num)
    num += 1
# 输出 0,1,2,3,4

num = 0
while num < 5:
    num += 1
    if num == 3:
        continue
    print(num)
# 输出 1,2,4,5 （跳过了 3）

# 错误示范
i = 0
while i < 5:
    if i == 3:
        continue   # 跳过 i += 1，i 永远为 3，无限循环
    print(i)
    i += 1

# 查找列表中是否包含偶数
numbers = [1, 3, 5, 7]
i = 0
while i < len(numbers):
    if numbers[i] % 2 == 0:
        print(f"找到偶数 {numbers[i]}")
        break
    i += 1
else:
    print("没有找到偶数")

while True:
    age_str = input("请输入年龄（1-120）：")
    if age_str.isdigit():
        age = int(age_str)
        if 1 <= age <= 120:
            break
    print("输入无效，请重新输入")
print(f"您的年龄是 {age}")

while True:
    # 循环体至少执行一次
    response = input("继续吗？(y/n): ")
    if response == 'n':
        break

def sqrt_newton(n, tolerance=1e-6):
    guess = n / 2
    while abs(guess * guess - n) > tolerance:
        guess = (guess + n / guess) / 2
    return guess

print(sqrt_newton(2))   # 约 1.414213562

it = iter([1, 2, 3])
while True:
    try:
        x = next(it)
        print(x)
    except StopIteration:
        break

import time
countdown = 5
while countdown > 0:
    print(countdown)
    time.sleep(1)
    countdown -= 1
print("发射！")

def find_first(predicate, iterable):
    it = iter(iterable)
    while True:
        try:
            item = next(it)
        except StopIteration:
            break
        if predicate(item):
            return item
    else:          # 这里 while-else 中的 else 表示“从未 break”
        return None

print(find_first(lambda x: x > 10, [1,5,3]))   # None
print(find_first(lambda x: x > 10, [1,15,3]))  # 15

with open('large_file.txt') as f:
    while True:
        line = f.readline()
        if not line:
            break
        process(line)

# 不推荐
while i < len(data):
    do_something(i, len(data))
    i += 1

# 推荐
n = len(data)
while i < n:
    do_something(i, n)
    i += 1

# 递归版本（不推荐用于大数据量）
def countdown(n):
    if n <= 0:
        print("发射")
    else:
        print(n)
        countdown(n-1)

# while 版本
def countdown_while(n):
    while n > 0:
        print(n)
        n -= 1
    print("发射")

# 删除列表中所有特定值（原地修改）
lst = [1, 2, 3, 2, 4, 2]
i = 0
while i < len(lst):
    if lst[i] == 2:
        lst.pop(i)
    else:
        i += 1
print(lst)   # [1, 3, 4]