第 5 章练习了以下内容
简单的 if 判断语句
判断字符串是否相等，还是不等
进行数字的大小比较
and，or 比较
检查列表中是否存在指定的元素
if，if-else，if-elif-else 语句写法
if 判断列表是否为空
使用多个列表进行比较判断
这一章的内容也比较简单，感觉和 shell 差不多，但还是多练习吧。
希望路过的大牛指出不足，小弟在此谢过了。
一个简单的 if 判断语句
循环打印 cars 列表中的元素，如果其中的元素等于 bmw，就全部大写打印
否则只是将元素的首字母大写
-------------------------------------------------
cars = ['audi', 'bmw', 'subaru', 'toyota']
for car in cars:
if car == 'bmw':
print(car.upper())
else:
print(car.title())
-----------------------------------------------------
Audi
BMW
Subaru
Toyota
判断是否相等
大小写不一样，也会不等
--------------------------
car = 'Audi'
print(car == 'Audi')
--------------------------
True
--------------------------
car = 'Audi'
print(car == 'audi')
---------------------------
False
转换大小写进行比较
------------------------------------
car = 'Audi'
print(car.lower() == 'audi')
------------------------------------
True
检查是否不相等
不过两个值不相等，就打印
-----------------------------------------------
requested_topping = 'mushrooms'
if requested_topping != 'anchovies':
print("Hold the anchovies!")
------------------------------------------------
Hold the anchovies!
数字的比较
----------------------
age = 18
print(age == 18)
----------------------
True
进行 if 语句判断，如果两个数字不等，就打印
-----------------------------------------------------------------------------
answer = 17
if answer != 18:
print("That is not the correct answer. Please try again!")
-----------------------------------------------------------------------------
That is not the correct answer. Please try again!
不光可以进行比较是否相等，还可以比较大小
直接在 python 的 IDE 下进行比较是不是看着更方便。哈哈哈
>>> age = 19
>>> age < 21
True
>>> age <= 21
True
>>> age > 21
False
>>> age >= 21
False
检查多个条件
and 当两个条件都满足的情况下，打印 True 否则打印 False
>>> age_0 = 22
>>> age_1 = 18
>>> age_0 >= 21 and age_1 >=21
False
>>> age_1 = 22
>>> age_0 >= 21 and age_1 >=21
True
or 至少满足一个条件，打印 True 否则打印 False
>>> age_0 = 22
>>> age_1 = 18
>>> age_0 >= 21 or age_1 >= 21
True
>>> age_0 = 18
>>> age_0 >= 21 or age_1 >= 21
False
检查指定的值是否包含在列表中
----------------------------------------------------------------------------
requested_toppings = ['mushrooms', 'onions', 'pineapple']
print('mushrooms' in requested_toppings)
print('pepperoni' in requested_toppings)
----------------------------------------------------------------------------
True
False
给要查找的值指定一个变量并查找，如果不存就打印出来
----------------------------------------------------------------------------
banned_users = ['andrew', 'carolina', 'david']
user = 'marie'
if user not in banned_users:
print(user.title() + ", you can post a response if you wish.")
----------------------------------------------------------------------------
Marie, you can post a response if you wish.
简单的 if 语句
设定年龄为19，进行 if 语句判断，如果大于18就打印
---------------------------------------------------
age = 19
if age >= 18:
print("You are old enough to vote!")
----------------------------------------------------
You are old enough to vote!
if-else 语句
设定年龄为 17，与 18 进行比较，如果大于等于 18 就打印 if 下的语句，
否则打印 else 中的语句
----------------------------------------------------------------------------
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!")
------------------------------------------------------------------------------
Sorry, you are too young to vote.
Please register to vote as soon as you turn 18!
if-elif-else 语句
年龄设定为 17，分别进行判断，小于 4 多少钱，小于 18 多少钱，其他多少钱进行打印
--------------------------------------------------
age = 17
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.")
----------------------------------------------------
Your admission cost is $5.
简化一下上面的写法，将判断的值定义一个变量，最后打印
----------------------------------------------------------------
age = 12
if age < 4:
price = 0
elif age < 18:
price = 5
else:
price = 10
print("Your admission cost is $" + str(price) + ".")
-----------------------------------------------------------------
Your admission cost is $5.
使用多个 elif 代码块
判断条件当然不止3个，这时候就用到了多个 elif 代码块
python 并不要求必须有 else 代码块，虽然书里是这么写的，但我作为小白的我还是倔强的认为这个习惯不太好，
所以自作主张不练这个 ’省略代码块‘ 了
----------------------------------------------------------------
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) + ".")
----------------------------------------------------------------
Your admission cost is $5.
检查特殊元素
判断列表中的元素并指定某个元素，进行判断
-------------------------------------------------------------------------------------------
requsested_toppings = ['mushrooms', 'green peppers', 'extra cheese']
for requsested_topping in requsested_toppings:
if requsested_topping == 'green peppers':
print("Sorry, we are out of green peppers right now.")
else:
print("Adding " + requsested_topping + ".")
print("\nFinished making your pizza!")
-------------------------------------------------------------------------------------------
Adding mushrooms.
Sorry, we are out of green peppers right now.
Adding extra cheese.
Finished making your pizza!
在 循环之前先进行一个 if 判断
if requested_toppings 意识是对列表进行判断，列表中至少有一个元素时，返回 True，
现在这个列表为空，返回值为 False，打印 else 代码块中的内容
--------------------------------------------------------------
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?")
----------------------------------------------------------------
Are you sure you want a plain pizza?
使用多个列表
对 requested_toppings 进行遍历，和 available_toppings 列表中的元素进行比较
-------------------------------------------------------------------------------------
available_toppings = ['mushrooms', 'olives', 'green peppers',
'pepperoni', 'pineapple', 'extra cheese']
requested_toppings = ['mushrooms', 'french fries', 'extra cheese']
for requested_topping in requested_toppings:
if requested_topping in available_toppings:
print("Adding " + requested_topping + ".")
else:
print("Sorry, we don't have " + requested_topping + ".")
print("\nFinished making your pizza!")
--------------------------------------------------------------------------------------
Adding mushrooms.
Sorry, we don't have french fries.
Adding extra cheese.
Finished making your pizza!
本文转自 mlwzby 51CTO博客，原文链接:http://blog.51cto.com/aby028/1964929
