Python編程入門到實踐 - 筆記（ 5 章）

第 5 章練習瞭如下內容

簡單的 if 判斷語句

判斷字符串是否相等，仍是不等

進行數字的大小比較

and，or 比較

檢查列表中是否存在指定的元素

if，if-else，if-elif-else 語句寫法

if 判斷列表是否爲空

使用多個列表進行比較判斷

這一章的內容也比較簡單，感受和 shell 差很少，但仍是多練習吧。

但願路過的大牛指出不足，小弟在此謝過了。

一個簡單的 if 判斷語句

循環打印 cars 列表中的元素，若是其中的元素等於 bmw，就所有大寫打印

不然只是將元素的首字母大寫

-------------------------------------------------

cars = ['audi', 'bmw', 'subaru', 'toyota']

for car in cars:  
     if car == 'bmw':    
          print(car.upper())    
     else:    
          print(car.title())

-----------------------------------------------------

Audi  
BMW    
Subaru    
Toyota

判斷是否相等

大小寫不同，也會不等

--------------------------

car = 'Audi'  
print(car == 'Audi')

--------------------------

True

--------------------------

car = 'Audi'  
print(car == 'audi')

---------------------------

False

轉換大小寫進行比較

------------------------------------

car = 'Audi'  
print(car.lower() == 'audi')

------------------------------------

True

檢查是否不相等

不過兩個值不相等，就打印

-----------------------------------------------

requested_topping = 'mushrooms'

if requested_topping != 'anchovies':  
      print("Hold the anchovies!")

------------------------------------------------

Hold the anchovies!

數字的比較

----------------------

age = 18  
print(age == 18)

----------------------

True

進行 if 語句判斷，若是兩個數字不等，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!

不光能夠進行比較是否相等，還能夠比較大小

直接在 python 的 IDE 下進行比較是否是看着更方便。哈哈哈

>>> age = 19  
>>> age < 21    
True    
>>> age <= 21    
True    
>>> age > 21    
False    
>>> age >= 21    
False

檢查多個條件

and 當兩個條件都知足的狀況下，打印 True 不然打印 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 至少知足一個條件，打印 True 不然打印 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

檢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

給要查找的值指定一個變量並查找，若是不存就打印出來

----------------------------------------------------------------------------

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.

簡單的 if 語句

設定年齡爲19，進行 if 語句判斷，若是大於18就打印

---------------------------------------------------

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

----------------------------------------------------

You are old enough to vote!

if-else 語句

設定年齡爲 17，與 18 進行比較，若是大於等於 18 就打印 if 下的語句，

不然打印 else 中的語句

----------------------------------------------------------------------------

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 語句

年齡設定爲 17，分別進行判斷，小於 4 多少錢，小於 18 多少錢，其餘多少錢進行打印

--------------------------------------------------

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.

簡化一下上面的寫法，將判斷的值定義一個變量，最後打印

----------------------------------------------------------------

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.

使用多個 elif 代碼塊

判斷條件固然不止3個，這時候就用到了多個 elif 代碼塊

python 並不要求必須有 else 代碼塊，雖然書裏是這麼寫的，但我做爲小白的我仍是倔強的認爲這個習慣不太好，

因此自做主張不練這個 ’省略代碼塊‘ 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.  

檢查特殊元素

判斷列表中的元素並指定某個元素，進行判斷

-------------------------------------------------------------------------------------------

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 循環以前先進行一個 if 判斷

if requested_toppings 意識是對列表進行判斷，列表中至少有一個元素時，返回 True，

如今這個列表爲空，返回值爲 False，打印 else 代碼塊中的內容

--------------------------------------------------------------

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?

使用多個列表

對 requested_toppings 進行遍歷，和 available_toppings 列表中的元素進行比較

-------------------------------------------------------------------------------------

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!