掌握 Python 条件控制：从基础语法到高级应用

if condition:
    # 执行语句块

if condition:
    # 条件为真时执行的语句块
else:
    # 条件为假时执行的语句块

if condition1:
    # 条件1为真时执行的语句块
elif condition2:
    # 条件2为真时执行的语句块
else:
    # 上述条件都不为真时执行的语句块

if condition1:
    if condition2:
        # 条件1和条件2都为真时执行的语句块
    else:
        # 条件1为真但条件2为假时执行的语句块
else:
    # 条件1为假时执行的语句块

age = 18
if age >= 18:
    print("成年人")

age = 16
if age >= 18:
    print("成年人")
else:
    print("未成年人")

score = 85
if score >= 90:
    print("优秀")
elif score >= 80:
    print("良好")
elif score >= 70:
    print("中等")
elif score >= 60:
    print("及格")
else:
    print("不及格")

num = 10
if num > 0:
    if num % 2 == 0:
        print("正偶数")
    else:
        print("正奇数")
else:
    if num == 0:
        print("零")
    else:
        print("负数")

def validate_user_input():
    user_input = input("请输入一个整数：")
    if user_input.isdigit():
        number = int(user_input)
        print(f"您输入的数字是：{number}")
    else:
        print("输入无效，请输入一个整数。")
validate_user_input()

def process_data(data):
    if not data:
        print("数据为空")
        return
    if all(isinstance(item, int) for item in data):
        print("所有数据项都是整数")
        print("数据的总和是：", sum(data))
    elif all(isinstance(item, str) for item in data):
        print("所有数据项都是字符串")
        print("数据拼接结果是：", " ".join(data))
    else:
        print("数据包含不同类型")
data_list = [1, 2, 3, 4]
process_data(data_list)
data_list = ["hello", "world"]
process_data(data_list)
data_list = [1, "hello", 3]
process_data(data_list)

def control_flow_example(state):
    if state == "start":
        print("程序开始")
    elif state == "process":
        print("程序处理中")
    elif state == "end":
        print("程序结束")
    else:
        print("未知状态")
states = ["start", "process", "end", "unknown"]
for state in states:
    control_flow_example(state)

def divide_numbers(a, b):
    try:
        result = a / b
    except ZeroDivisionError:
        print("错误：除数不能为零")
    except TypeError:
        print("错误：输入必须是数字")
    else:
        print(f"结果是：{result}")
    finally:
        print("操作完成")
divide_numbers(10, 2)
divide_numbers(10, 0)
divide_numbers(10, "a")

def check_number(num):
    if num > 0:
        print("这是一个正数")
    elif num < 0:
        print("这是一个负数")
    else:
        print("这是零")
# 正确缩进
check_number(10)
# 错误缩进（会导致 IndentationError）
# def check_number(num):
# if num > 0:
# print("这是一个正数")
# elif num < 0:
# print("这是一个负数")
# else:
# print("这是零")

def evaluate_condition(a, b):
    if a > b:
        print(f"{a} 大于 {b}")
    elif a < b:
        print(f"{a} 小于 {b}")
    else:
        print(f"{a} 等于 {b}")
evaluate_condition(5, 3)
evaluate_condition(2, 4)
evaluate_condition(7, 7)

def categorize_age(age):
    if age < 0:
        print("年龄不能为负数")
    elif age < 13:
        print("儿童")
    elif age < 18:
        print("青少年")
    elif age < 65:
        print("成年人")
    else:
        print("老年人")
categorize_age(-1)
categorize_age(5)
categorize_age(14)
categorize_age(30)
categorize_age(70)

def check_number(num):
    if num == 0:
        print("这是零")
    elif num > 0:
        print("这是一个正数")
    else:
        print("这是一个负数")
check_number(0)
check_number(5)
check_number(-3)