score = 85

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
elif score >= 60:
    grade = "D"
else:
    grade = "F"

print(f"成绩等级: {grade}")
                      

# 创建列表
numbers = [1, 2, 3, 4, 5]
mixed = [1, "hello", True, 3.14]
empty = []

# 访问元素（索引从0开始，负数从后向前）
print(numbers[0])   # 1
print(numbers[-1])  # 5（最后一个）

# 切片 [start:end:step]
print(numbers[1:3])      # [2, 3]
print(numbers[:3])       # [1, 2, 3]
print(numbers[2:])       # [3, 4, 5]
print(numbers[::2])      # [1, 3, 5] (步长2)

# 修改
numbers[0] = 100

# 添加元素
numbers.append(6)         # 末尾添加 O(1)
numbers.insert(0, 0)      # 指定位置插入 O(n)

# 删除
numbers.remove(3)         # 删除第一个匹配值 O(n)
popped = numbers.pop()    # 弹出最后一个 O(1)
del numbers[0]            # 删除指定索引 O(n)

# 常用方法
print(len(numbers))       # 长度
print(2 in numbers)       # 包含判断 O(n)
numbers.sort()            # 排序 O(n log n)
numbers.reverse()         # 反转 O(n)
print(numbers.index(2))   # 查找索引 O(n)
print(numbers.count(2))   # 统计次数 O(n)
                      

# 创建字典
student = {
    "name": "张三",
    "age": 20,
    "major": "计算机科学"
}

# 访问
print(student["name"])           # 张三
print(student.get("age", 0))     # 20（get带默认值）
print(student.get("gender"))     # None（无默认值）

# 添加/修改
student["grade"] = "大二"        # 添加
student["age"] = 21              # 修改

# 删除
del student["major"]              # O(1)
age = student.pop("age")          # O(1)，返回被删除的值

# 常用操作
print(student.keys())            # dict_keys(['name', 'age', 'grade'])
print(student.values())          # dict_values(['张三', 21, '大二'])
print(student.items())           # dict_items([...])

# 遍历
for key, value in student.items():
    print(f"{key}: {value}")

# 字典推导式
squares = {x: x**2 for x in range(5)}
                      

# 创建元组
point = (3, 4)
colors = ("红", "绿", "蓝")
single = (1,)  # 单元素元组需要逗号

# 访问
print(point[0])  # 3

# 元组解包
x, y = point
print(x, y)  # 3 4

# 用途：函数返回多个值
def get_size():
    return 100, 200

width, height = get_size()
                      

# 创建集合
s1 = {1, 2, 3, 3, 4}  # {1,2,3,4}（自动去重）
s2 = set([3, 4, 5, 6])

# 添加删除
s1.add(5)
s1.remove(1)

# 集合运算
print(s1 | s2)  # 并集 {2,3,4,5,6}
print(s1 & s2)  # 交集 {3,4}
print(s1 - s2)  # 差集 {2}
print(s1 ^ s2)  # 对称差 {2,5,6}

# 去重应用
lst = [1, 2, 2, 3, 3, 3, 4]
unique = list(set(lst))  # [1,2,3,4]
                      

# 定义函数
def greet(name):
    """这是文档字符串，说明函数用途"""
    return f"你好, {name}"

# 调用函数
message = greet("张三")
print(message)

# 带默认参数
def power(x, n=2):
    return x ** n

print(power(3))      # 9 (默认n=2)
print(power(3, 3))   # 27

# 可变参数 *args
def sum_all(*numbers):
    total = 0
    for num in numbers:
        total += num
    return total

print(sum_all(1, 2, 3, 4, 5))  # 15

# 关键字参数 **kwargs
def print_info(**kwargs):
    for key, value in kwargs.items():
        print(f"{key}: {value}")

print_info(name="张三", age=20)
                      

# 创建字符串
s1 = "Hello"
s2 = 'World'
s3 = """多行
字符串"""

# 访问与切片
s = "Python"
print(s[0])    # P
print(s[-1])   # n
print(s[1:4])  # yth

# 拼接与重复
print("Hello" + " " + "World")
print("Hi" * 3)

# 长度
print(len("Hello"))  # 5

# 成员检查
print("th" in "Python")  # True
                      

s = "  Hello, World!  "

# 大小写
print(s.upper())  # HELLO, WORLD!
print(s.lower())  # hello, world!
print(s.title())  # Hello, World!

# 去除空白
print(s.strip())   # "Hello, World!"
print(s.lstrip())  # "Hello, World!  "
print(s.rstrip())  # "  Hello, World!"

# 分割与连接
print(s.split(","))  # ["  Hello", " World!  "]
print("-".join(["a", "b", "c"]))  # a-b-c

# 替换
print(s.replace("World", "Python"))

# 查找
print(s.find("World"))  # 8
print(s.count("l"))     # 3

# 判断
print(s.startswith("He"))
print(s.endswith("!"))
print(s.isdigit())
print(s.isalpha())
                      

# 基本输入
name = input("请输入姓名: ")
print(f"你好, {name}!")

# 获取数字
age = int(input("请输入年龄: "))
height = float(input("请输入身高(米): "))

# 多个输入
nums = input("请输入3个数字,用空格分隔: ").split()
x, y, z = map(int, nums)
print(f"和: {x + y + z}")
                      

# print函数
print("Hello")
print(1, 2, 3)
print(1, 2, 3, sep="-")  # 1-2-3
print("Hello", end=" ")   # 不换行
print("World")

# 格式化输出
name = "Alice"
print(f"Name: {name}")
print("Name: {}".format(name))
                      

# 定义类
class Person:
    # 构造方法
    def __init__(self, name, age):
        self.name = name  # 属性
        self.age = age
    
    # 方法
    def greet(self):
        return f"你好, 我是{self.name}"
    
    def have_birthday(self):
        self.age += 1
        return f"{self.name}今年{self.age}岁了！"

# 创建对象
p = Person("张三", 20)
print(p.name)  # 张三
print(p.greet())  # 你好, 我是张三
print(p.have_birthday())  # 张三今年21岁了！
                      

class Student(Person):  # 继承Person
    def __init__(self, name, age, student_id):
        super().__init__(name, age)  # 调用父类构造
        self.student_id = student_id
    
    def study(self):
        return f"{self.name}正在学习，学号{self.student_id}"

s = Student("李四", 19, "2024001")
print(s.greet())  # 继承的方法
print(s.study())  # 自己的方法