控制流#

# 基本语法
if condition:
    # 如果条件为True，执行这里的代码
    statement

# 示例
age = 20
if age >= 18:
    print("已成年")

# 基本语法
if condition:
    # 如果条件为True，执行这里
    statement1
else:
    # 如果条件为False，执行这里
    statement2

# 示例
age = 15
if age >= 18:
    print("已成年")
else:
    print("未成年")

# 基本语法
if condition1:
    statement1
elif condition2:
    statement2
elif condition3:
    statement3
else:
    statement4

# 示例：成绩等级
score = 85
if score >= 90:
    print("优秀")
elif score >= 80:
    print("良好")
elif score >= 60:
    print("及格")
else:
    print("不及格")

age = 25
has_license = True

if age >= 18:
    if has_license:
        print("可以开车")
    else:
        print("需要先考驾照")
else:
    print("未成年，不能开车")

# 使用逻辑运算符简化
age = 25
has_license = True

if age >= 18 and has_license:
    print("可以开车")

# 使用链式比较
x = 10
if 0 < x < 20:
    print("x在0和20之间")

# 基本语法
for item in sequence:
    # 对每个item执行操作
    statement

# 遍历列表
fruits = ["apple", "banana", "orange"]
for fruit in fruits:
    print(fruit)

# 遍历字符串
for char in "Python":
    print(char)

# 遍历字典
user = {"name": "Alice", "age": 30, "city": "Beijing"}
for key in user:
    print(key, user[key])

for key, value in user.items():
    print(key, value)

# range(stop) - 从0开始到stop-1
for i in range(5):
    print(i)  # 0, 1, 2, 3, 4

# range(start, stop) - 从start开始到stop-1
for i in range(2, 5):
    print(i)  # 2, 3, 4

# range(start, stop, step) - 指定步长
for i in range(0, 10, 2):
    print(i)  # 0, 2, 4, 6, 8

# 反向遍历
for i in range(5, 0, -1):
    print(i)  # 5, 4, 3, 2, 1

# 转换为列表
numbers = list(range(5))  # [0, 1, 2, 3, 4]

# 基本用法
fruits = ["apple", "banana", "orange"]
for index, fruit in enumerate(fruits):
    print(index, fruit)
# 输出：
# 0 apple
# 1 banana
# 2 orange

# 指定起始索引
for index, fruit in enumerate(fruits, start=1):
    print(index, fruit)
# 输出：
# 1 apple
# 2 banana
# 3 orange

names = ["Alice", "Bob", "Charlie"]
ages = [25, 30, 35]

# 同时遍历两个列表
for name, age in zip(names, ages):
    print(f"{name} is {age} years old")
# 输出：
# Alice is 25 years old
# Bob is 30 years old
# Charlie is 35 years old

# 多个序列
cities = ["Beijing", "Shanghai", "Guangzhou"]
for name, age, city in zip(names, ages, cities):
    print(f"{name}, {age}, {city}")

# zip() 返回的是迭代器，可以转换为列表
pairs = list(zip(names, ages))  # [('Alice', 25), ('Bob', 30), ('Charlie', 35)]

# 基本语法
while condition:
    # 只要条件为True，就执行这里的代码
    statement

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

# 示例：用户输入
while True:
    user_input = input("输入'quit'退出: ")
    if user_input == "quit":
        break
    print(f"你输入了: {user_input}")

# 在for循环中使用break
for i in range(10):
    if i == 5:
        break  # 当i等于5时退出循环
    print(i)  # 只打印 0, 1, 2, 3, 4

# 在while循环中使用break
count = 0
while True:
    if count >= 5:
        break
    print(count)
    count += 1

# 跳过偶数
for i in range(10):
    if i % 2 == 0:
        continue  # 跳过偶数
    print(i)  # 只打印 1, 3, 5, 7, 9

# 跳过空值
items = [1, None, 2, None, 3]
for item in items:
    if item is None:
        continue
    print(item)  # 只打印 1, 2, 3

# 在循环中使用pass
for i in range(5):
    if i == 2:
        pass  # 什么都不做，继续执行
    print(i)

# 在条件语句中使用pass
if condition:
    pass  # 暂时不实现，避免语法错误
else:
    print("else block")

# for循环的else子句
for i in range(5):
    print(i)
else:
    print("循环正常结束")
# 输出：
# 0
# 1
# 2
# 3
# 4
# 循环正常结束

# 如果使用break，else不会执行
for i in range(5):
    if i == 3:
        break
    print(i)
else:
    print("这行不会执行")
# 输出：
# 0
# 1
# 2

# while循环的else子句
count = 0
while count < 5:
    print(count)
    count += 1
else:
    print("循环正常结束")

# 实际应用：查找元素
numbers = [1, 2, 3, 4, 5]
target = 6

for num in numbers:
    if num == target:
        print(f"找到了 {target}")
        break
else:
    print(f"没有找到 {target}")
# 输出：没有找到 6

# 如果target = 3，输出：找到了 3（else不会执行）

# 嵌套for循环：打印乘法表
for i in range(1, 4):
    for j in range(1, 4):
        print(f"{i} × {j} = {i*j}", end="  ")
    print()  # 换行
# 输出：
# 1 × 1 = 1  1 × 2 = 2  1 × 3 = 3
# 2 × 1 = 2  2 × 2 = 4  2 × 3 = 6
# 3 × 1 = 3  3 × 2 = 6  3 × 3 = 9

# 嵌套循环中使用break和continue
for i in range(3):
    for j in range(3):
        if j == 1:
            break  # 只退出内层循环
        print(f"i={i}, j={j}")
# 输出：
# i=0, j=0
# i=1, j=0
# i=2, j=0

# 循环变量在循环外也可以访问
for i in range(3):
    pass
print(i)  # 2（最后的值）

# Python 3中，循环变量不会创建新的作用域
# 这与某些语言不同

# 基本条件过滤
evens = [x for x in range(10) if x % 2 == 0]  # [0, 2, 4, 6, 8]

# 条件表达式
result = [x if x > 0 else 0 for x in range(-3, 4)]  # [0, 0, 0, 0, 1, 2, 3]

# 多重条件
result = [x for x in range(20) if x % 2 == 0 if x % 3 == 0]  # [0, 6, 12, 18]

# 错误的方式（不推荐）
numbers = [1, 2, 3, 4, 5]
for num in numbers:
    if num % 2 == 0:
        numbers.remove(num)  # 危险！在遍历时修改列表
print(numbers)  # 可能不是预期结果

# 正确的方式1：创建新列表
numbers = [1, 2, 3, 4, 5]
odds = [num for num in numbers if num % 2 != 0]

# 正确的方式2：使用切片复制
numbers = [1, 2, 3, 4, 5]
for num in numbers[:]:  # 遍历副本
    if num % 2 == 0:
        numbers.remove(num)

# 正确的方式3：反向遍历
numbers = [1, 2, 3, 4, 5]
for i in range(len(numbers) - 1, -1, -1):
    if numbers[i] % 2 == 0:
        del numbers[i]

# 使用while True创建无限循环
while True:
    user_input = input("输入命令: ")
    if user_input == "quit":
        break
    # 处理命令

# 使用enumerate
items = ["a", "b", "c"]
for index, item in enumerate(items):
    print(f"{index}: {item}")

# 手动计数
count = 0
for item in items:
    print(f"{count}: {item}")
    count += 1

# 避免在循环中进行不必要的计算
# 不好的做法
for i in range(1000):
    result = len(my_list) * 2  # len()在每次迭代都计算

# 好的做法
list_length = len(my_list) * 2
for i in range(1000):
    result = list_length