Python -- 4. if()操作 & for操作

1. 条件测试
每条if 语句的核心都是一个值为True 或False 的表达式，这种表达式被称为条件测试 。

• ==, !=, <, <=, >, >=
• and, or
• 检查特定值是否包含在列表中, in / not in

banned_users = ['andrew', 'carolina', 'david']
user = 'marie'
if user not in banned_users:
    print(user.title() + ", you can post a response if you wish.")

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2. if 语句
(1).简单的if 语句

age = 19
if age >= 18:
    print("You are old enough to vote!")

(2).if-else 语句

age = 17
if age >= 18:
    print("You are old enough to vote!")
    print("Have you registered to vote yet?")
else:
    print("Sorry, you are too young to vote.")
    print("Please register to vote as soon as you turn 18!")

(3).if-elif-else 结构

age = 12
if age < 4:
    print("Your admission cost is $0.")
elif age < 18:
    print("Your admission cost is $5.")
else:
    print("Your admission cost is $10.")

(4).使用多个elif 代码块

age = 12
if age < 4:
    price = 0
elif age < 18:
    price = 5
elif age < 65:
    price = 10
else:
    price = 5
print("Your admission cost is $" + str(price) + ".")

(5).省略else 代码块
Python并不要求if-elif 结构后面必须有else 代码块。

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3. 使用if 语句处理列表
确定列表不是空的
在if 语句中将列表名用在条件表达式中时，Python将在列表至少包含一个元素时返回True ，并在列表为空时返回False 。

requested_toppings = []
if requested_toppings:
    for requested_topping in requested_toppings:
        print("Adding " + requested_topping + ".")
    print("\nFinished making your pizza!")
else:
    print("Are you sure you want a plain pizza?")

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4. for 操作

for iterating_var in sequence:
statements(s)

唯一要注意的是，in 后面是个序列：

for i in 100 #是错误的
for i in range(100) #要改成序列的样子
    xxxxxx

初始化，也要注意：

for i=1 in range(100) #也是错误的
#------------------------------
i = 1
for i in range(100) #这样才是对的

for 的步长都是整数，没有小数

for i in range(1, 10, 0.5) #是错误的