循环控制 #

# 跳出for循环
for i in range(10):
    if i == 5:
        break
    print(i)
# 输出：0 1 2 3 4

# 跳出while循环
count = 0
while True:
    print(count)
    count += 1
    if count >= 5:
        break
# 输出：0 1 2 3 4

# 查找第一个满足条件的元素
numbers = [1, 3, 5, 7, 8, 9, 10]

for n in numbers:
    if n % 2 == 0:
        print(f"找到第一个偶数：{n}")
        break
# 输出：找到第一个偶数：8

# 限制尝试次数
max_attempts = 3
for attempt in range(max_attempts):
    password = input("请输入密码：")
    if password == "secret":
        print("登录成功！")
        break
    else:
        remaining = max_attempts - attempt - 1
        print(f"密码错误，还剩{remaining}次机会")
else:
    print("尝试次数过多，账户已锁定")

# break只跳出最内层循环
for i in range(3):
    for j in range(3):
        if j == 1:
            break  # 只跳出内层循环
        print(f"({i}, {j})")
# 输出：
# (0, 0)
# (1, 0)
# (2, 0)

# 跳出多层循环（使用标志位）
found = False
for i in range(5):
    for j in range(5):
        if i * j > 10:
            found = True
            break
    if found:
        break
print(f"找到：i={i}, j={j}")

# 使用函数返回
def find_target():
    for i in range(5):
        for j in range(5):
            if i * j > 10:
                return i, j
    return None, None

i, j = find_target()

# break会跳过else块
for i in range(5):
    if i == 3:
        break
    print(i)
else:
    print("循环正常结束")  # 不会执行

# 正常结束时执行else
for i in range(3):
    print(i)
else:
    print("循环正常结束")  # 会执行
# 输出：0 1 2 循环正常结束

# 跳过特定值
for i in range(5):
    if i == 2:
        continue
    print(i)
# 输出：0 1 3 4

# 跳过奇数
for i in range(10):
    if i % 2 != 0:
        continue
    print(i)
# 输出：0 2 4 6 8

# 只处理有效数据
data = [1, None, 2, None, 3, 4, None, 5]

for item in data:
    if item is None:
        continue
    print(f"处理：{item}")
# 输出：处理：1, 处理：2, 处理：3, 处理：4, 处理：5

# 过滤无效输入
inputs = ["hello", "", "world", "  ", "python"]

for text in inputs:
    stripped = text.strip()
    if not stripped:  # 空字符串
        continue
    print(f"处理：'{stripped}'")
# 输出：处理：'hello', 处理：'world', 处理：'python'

# 验证用户数据
users = [
    {"name": "Tom", "age": 25, "email": "tom@example.com"},
    {"name": "", "age": 30, "email": "user2@example.com"},
    {"name": "Jerry", "age": -5, "email": "jerry@example.com"},
    {"name": "Alice", "age": 28, "email": ""},
    {"name": "Bob", "age": 35, "email": "bob@example.com"}
]

for user in users:
    # 验证名字
    if not user["name"]:
        print(f"跳过：名字为空")
        continue
    
    # 验证年龄
    if user["age"] <= 0:
        print(f"跳过：{user['name']} 年龄无效")
        continue
    
    # 验证邮箱
    if not user["email"]:
        print(f"跳过：{user['name']} 邮箱为空")
        continue
    
    print(f"有效用户：{user['name']}")
# 输出：
# 有效用户：Tom
# 跳过：名字为空
# 跳过：Jerry 年龄无效
# 跳过：Alice 邮箱为空
# 有效用户：Bob

# 使用continue减少嵌套
# 不推荐：深层嵌套
def process_items(items):
    for item in items:
        if item.is_valid:
            if item.has_data:
                if item.is_ready:
                    process(item)

# 推荐：提前continue
def process_items(items):
    for item in items:
        if not item.is_valid:
            continue
        if not item.has_data:
            continue
        if not item.is_ready:
            continue
        process(item)

# continue不会影响else
for i in range(5):
    if i == 2:
        continue
    print(i)
else:
    print("循环正常结束")
# 输出：0 1 3 4 循环正常结束

# else在循环正常结束时执行
for i in range(3):
    print(i)
else:
    print("循环结束")
# 输出：0 1 2 循环结束

# while循环的else
count = 0
while count < 3:
    print(count)
    count += 1
else:
    print("循环结束")
# 输出：0 1 2 循环结束

# 查找元素，找不到时执行else
def find_user(users, name):
    for user in users:
        if user["name"] == name:
            return user
    return None

# 使用for-else实现
def find_user_v2(users, name):
    for user in users:
        if user["name"] == name:
            print(f"找到用户：{user}")
            break
    else:
        print(f"未找到用户：{name}")

users = [{"name": "Tom"}, {"name": "Jerry"}]
find_user_v2(users, "Tom")    # 找到用户：{'name': 'Tom'}
find_user_v2(users, "Alice")  # 未找到用户：Alice

# 验证所有元素都满足条件
numbers = [2, 4, 6, 8, 10]

for n in numbers:
    if n % 2 != 0:
        print("发现奇数")
        break
else:
    print("全是偶数")
# 输出：全是偶数

# 使用all更简洁
if all(n % 2 == 0 for n in numbers):
    print("全是偶数")

def is_prime(n):
    """检查n是否为素数"""
    if n < 2:
        return False
    
    for i in range(2, int(n ** 0.5) + 1):
        if n % i == 0:
            return False  # 找到因子，不是素数
    else:
        return True  # 没找到因子，是素数

print(is_prime(7))   # True
print(is_prime(10))  # False

import random

def try_operation(max_retries=3):
    """尝试操作，成功返回True"""
    for attempt in range(max_retries):
        print(f"第{attempt + 1}次尝试...")
        # 模拟操作
        if random.random() > 0.7:  # 30%成功率
            print("操作成功！")
            break
    else:
        print("所有尝试都失败了")
        return False
    return True

try_operation()

def menu_system():
    """简单的菜单系统"""
    while True:
        print("\n=== 菜单 ===")
        print("1. 新建")
        print("2. 打开")
        print("3. 保存")
        print("0. 退出")
        
        choice = input("请选择：")
        
        if choice == '0':
            print("再见！")
            break
        elif choice == '1':
            print("新建文件...")
        elif choice == '2':
            print("打开文件...")
        elif choice == '3':
            print("保存文件...")
        else:
            print("无效选择，请重试")
            continue
        
        print("操作完成")

# menu_system()

def process_data(data):
    """数据处理管道"""
    results = []
    
    for item in data:
        # 跳过空值
        if item is None:
            continue
        
        # 跳过无效值
        try:
            value = float(item)
        except (ValueError, TypeError):
            continue
        
        # 跳过负数
        if value < 0:
            continue
        
        # 处理有效数据
        results.append(value ** 2)
    
    return results

data = [1, None, "2", "invalid", -3, 4, "5.5"]
print(process_data(data))  # [1.0, 4.0, 16.0, 30.25]

def game_loop():
    """简单的游戏循环"""
    score = 0
    lives = 3
    
    while lives > 0:
        # 游戏逻辑
        print(f"\n分数：{score}，生命：{lives}")
        action = input("输入'action'得分，'quit'退出：")
        
        if action == 'quit':
            print("游戏结束！")
            break
        elif action == 'action':
            score += 10
            print(f"得分！当前分数：{score}")
        else:
            lives -= 1
            print(f"操作错误，失去一条生命")
    else:
        print("游戏结束，生命耗尽！")
    
    print(f"最终分数：{score}")

# game_loop()

# 不能在finally块中使用break/continue
# for i in range(5):
#     try:
#         pass
#     finally:
#         break  # SyntaxError

# 循环变量在循环外仍可访问
for i in range(5):
    pass
print(i)  # 4（循环变量的最后值）

# while循环
while False:
    x = 1
# print(x)  # NameError（如果循环一次都没执行）

# 使用continue时注意资源清理
def process_files(filenames):
    for filename in filenames:
        f = open(filename, 'r')
        if should_skip(filename):
            continue  # 文件没有关闭！
        process(f)
        f.close()

# 正确方式
def process_files_v2(filenames):
    for filename in filenames:
        with open(filename, 'r') as f:
            if should_skip(filename):
                continue  # with会自动关闭文件
            process(f)

# 不推荐
found = False
for item in items:
    if condition(item):
        found = True
        break

if found:
    handle_found()

# 推荐
for item in items:
    if condition(item):
        handle_found()
        break
else:
    handle_not_found()

# 不推荐：深层嵌套
for item in items:
    if condition1(item):
        if condition2(item):
            if condition3(item):
                process(item)

# 推荐：使用continue
for item in items:
    if not condition1(item):
        continue
    if not condition2(item):
        continue
    if not condition3(item):
        continue
    process(item)

# else适合"没找到"的情况
for item in items:
    if is_target(item):
        handle_found(item)
        break
else:
    handle_not_found()

# 不适合复杂的后处理
for item in items:
    process(item)
else:
    # 这里容易被忽略
    cleanup()