Python 循环语句

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

# 遍历字符串
for char in "Python":
    print(char, end=" ")
# P y t h o n

# 遍历字典
user = {"name": "Alice", "age": 25, "city": "Beijing"}
for key in user:
    print(f"{key}: {user[key]}")

for key, value in user.items():
    print(f"{key} = {value}")

# range(stop)        0 到 stop-1
for i in range(5):
    print(i, end=" ")    # 0 1 2 3 4

# range(start, stop)  start 到 stop-1
for i in range(2, 6):
    print(i, end=" ")    # 2 3 4 5

# range(start, stop, step)  带步长
for i in range(0, 10, 2):
    print(i, end=" ")    # 0 2 4 6 8

# 倒序
for i in range(5, 0, -1):
    print(i, end=" ")    # 5 4 3 2 1

# range 返回的是可迭代对象，不是列表
r = range(5)
print(type(r))       # <class 'range'>
print(list(r))       # [0, 1, 2, 3, 4]

# 支持索引和切片
print(r[2])          # 2
print(list(r[1:4]))  # [1, 2, 3]

# 大范围不占用大量内存
big = range(1000000)
print(len(big))      # 1000000

# 基本用法
count = 0
while count < 5:
    print(count, end=" ")
    count += 1
# 0 1 2 3 4

# 用户输入验证
password = ""
while password != "secret":
    password = input("请输入密码: ")
print("登录成功!")

# 无限循环 + break
while True:
    command = input("输入命令 (q 退出): ")
    if command == "q":
        break
    print(f"执行: {command}")

# break：立即终止整个循环
for i in range(10):
    if i == 5:
        break
    print(i, end=" ")
# 0 1 2 3 4

# continue：跳过当前迭代，继续下一次
for i in range(10):
    if i % 2 == 0:
        continue
    print(i, end=" ")
# 1 3 5 7 9

# 查找列表中第一个负数
numbers = [3, 7, 2, -1, 5, -3, 8]

for num in numbers:
    if num < 0:
        print(f"找到第一个负数: {num}")
        break

# 跳过处理空值
items = [10, None, 20, "", 30, 0]
total = 0
for item in items:
    if not item:        # None、空字符串、0 都被跳过
        continue
    total += item
print(f"总和: {total}")   # 60

# 判断是否为质数
n = 17

for i in range(2, n):
    if n % i == 0:
        print(f"{n} 不是质数")
        break
else:
    print(f"{n} 是质数")

# 对比：不用 else 的写法
found = False
for i in range(2, n):
    if n % i == 0:
        found = True
        break
if not found:
    print(f"{n} 是质数")

fruits = ["apple", "banana", "cherry"]

# 普通写法
for i in range(len(fruits)):
    print(f"{i}: {fruits[i]}")

# 使用 enumerate
for index, fruit in enumerate(fruits):
    print(f"{index}: {fruit}")

# 指定起始索引
for index, fruit in enumerate(fruits, start=1):
    print(f"{index}. {fruit}")
# 1. apple
# 2. banana
# 3. cherry

names = ["Alice", "Bob", "Charlie"]
ages = [25, 30, 35]
cities = ["Beijing", "Shanghai", "Shenzhen"]

for name, age, city in zip(names, ages, cities):
    print(f"{name}, {age}岁, {city}")

# 长度不一致时以最短为准
a = [1, 2, 3]
b = ["a", "b"]
for x, y in zip(a, b):
    print(x, y)
# 1 a
# 2 b

# zip 配合 enumerate
for i, (name, age) in enumerate(zip(names, ages)):
    print(f"{i}: {name} - {age}")

# 打印乘法表
for i in range(1, 10):
    for j in range(1, i + 1):
        print(f"{j}×{i}={i*j:2}", end="  ")
    print()

# 输出：
# 1×1= 1
# 1×2= 2  2×2= 4
# 1×3= 3  2×3= 6  3×3= 9
# ...

# 遍历二维列表
matrix = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]

for row in matrix:
    for item in row:
        print(item, end=" ")
    print()
# 1 2 3
# 4 5 6
# 7 8 9

# 普通循环
squares = []
for x in range(10):
    squares.append(x ** 2)

# 列表推导式
squares = [x ** 2 for x in range(10)]
print(squares)
# [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

# 带条件的列表推导式
evens = [x for x in range(20) if x % 2 == 0]
print(evens)
# [0, 2, 4, 6, 8, 10, 12, 14, 16, 18]

# 双重循环的列表推导式
pairs = [(x, y) for x in [1, 2] for y in ["a", "b"]]
print(pairs)
# [(1, 'a'), (1, 'b'), (2, 'a'), (2, 'b')]

"""
猜数字游戏
"""
import random

target = random.randint(1, 100)
attempts = 0
max_attempts = 7

print("=" * 30)
print("      猜 数 字 游 戏")
print(f"我想了一个 1-100 的整数，你有 {max_attempts} 次机会")
print("=" * 30)

while attempts < max_attempts:
    guess_input = input(f"\n第 {attempts + 1} 次猜测: ")

    # 输入验证
    if not guess_input.isdigit():
        print("请输入有效的整数!")
        continue

    guess = int(guess_input)
    attempts += 1

    if guess < target:
        print("太小了! ↑")
    elif guess > target:
        print("太大了! ↓")
    else:
        print(f"\n🎉 恭喜你猜对了! 答案就是 {target}")
        print(f"你用了 {attempts} 次")
        break
else:
    print(f"\n😔 机会用完了，正确答案是 {target}")

# 陷阱 1：修改正在遍历的列表
items = [1, 2, 3, 4, 5]
for item in items:
    if item % 2 == 0:
        items.remove(item)   # 危险！会跳过元素
print(items)   # [1, 3, 5]（碰巧正确，但不建议）

# 正确做法：遍历副本
items = [1, 2, 3, 4, 5]
for item in items[:]:
    if item % 2 == 0:
        items.remove(item)
print(items)   # [1, 3, 5]

# 或者用列表推导式
items = [1, 2, 3, 4, 5]
items = [x for x in items if x % 2 != 0]

# 陷阱 2：while 忘记更新条件变量
# i = 0
# while i < 5:
#     print(i)   # 死循环！i 没有增加

# 陷阱 3：for 循环变量泄露到外部
for i in range(5):
    pass
print(i)   # 4（i 在循环外仍可用）