- 4.1 一個簡單的數列

給定一個汽車列表，將其中每一輛汽車的名稱打印出來，要求列印 ‘bmw’ 時所有字母都要大寫，其餘名稱只需要首字母大寫：

 cars = ['audi' , 'bmw' , 'subaru' , 'toyota']
 for car in cars:
 	if car == 'bmw':
		print(car.upper())
	else:
		print(car.title())

輸出結果如下：

Audi
BMW
Subaru
Toyota

- 4.1.1 檢查特定值是否包含在列表當中

要判斷特定的值是否已包含在列表當中，可使用關鍵字 in

 user_names = ['andia' , 'david' , 'liwa']
 user = 'andia'
 if user in user_names:
 	print(user.title() + "is in user_name.")

輸出結果如下：

Andiais in user_name.

要判斷特定的值是否不包含在列表當中，可使用關鍵字 not in

 user_names = ['andia' , 'david' , 'liwa']
 user = 'kivle'
 if user not in user_names:
 	print(user.title(
 
) + "is not in user_name.")

輸出結果如下：

Kivleis not in user_name.

- 4.2 if-else 語句

 age = input("請輸入你的年齡檢視是否可以去網咖：")
 if int(age) >= 18:
     print("You are old enough to go to the net bar!")
     print("You should go to net bar less,study more!")
 else:
     print("You are too young to go to the net bar!"
 
)
     print("Wait until you are 18 to go to the net bar!")

分別輸入19和15，輸出結果如下：

 請輸入你的年齡檢視是否可以去網咖：19
 You are old enough to go to the net bar!
 You should go to net bar less,study more!

 請輸入你的年齡檢視是否可以去網咖：15
 You are too young to go to the net bar!
 Wait until you are 18 to go to the net bar!

- 4.3 if-elif-else 結構

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

輸出結果如下：

 Your admission cost is $5.

- 4.3.1 使用多個 elif 程式碼塊

age = 20
if age < 4:
	price = 0
elif age < 18:
	price = 5
elif age < 65:
	price = 15
else:
	price = 10
print("Your admission cost is $" + str(price) + ".")

輸出結果如下：

Your admission cost is $15.

- 4.3.2 省略 else 程式碼塊

Python並不要求 if-elif 結構後面必須有 else 程式碼塊：

age = 20
if age < 4:
	price = 0
elif age < 18:
	price = 5
elif age < 65:
	price = 15
elif age >= 65:
	price = 10
print("Your admission cost is $" + str(price) + ".")

輸出結果仍與3.3.1一樣

- 4.4 測試多個條件

if-elif-else結構功能強大，但僅適用於只有一個條件滿足的情況：遇到通過了的測試後，Python就會跳過餘下的測試：

 names = ['Zhangshan' , 'Wanger']
if 'Zhangshan' in names:
    print("Zhangshan is here!")
if 'Wanger' in names:
    print("Wanger is here!")
if 'Xiaoming' in names:
    print("Xiaoming is here!")
print("All the students are here!")

輸出結果如下：

Zhangshan is here!
Wanger is here!
All the students are here!

相同的程式，如果使用 if-elif-else 結構，程式碼將不能正確執行：

names = ['Zhangshan' , 'Wanger']
if 'Zhangshan' in names:
    print("Zhangshan is here!")
elif 'Wanger' in names:
    print("Wanger is here!")
elif 'Xiaoming' in names:
    print("Xiaoming is here!")
print("All the students are here!")

輸出結果如下：

Zhangshan is here!
All the students are here!

總之，如果我們只想執行一個程式碼塊，就使用 if-elif-else 結構；如果要執行多個程式碼塊，就必須使用一系列獨立的 if 語句！

- 4.5 使用 if 語句處理列表

- 4.5.1 檢查特殊元素

對3.4例子改版，加入姓名 ‘Xiaoming’，當檢索到Xiaoming時告訴他，他媽媽叫他回家吃飯

names = ['Zhangshan' , 'Wanger' , 'Xiaoming']
for name in names:
    if name == 'Xiaoming':
        print("Xiaoming,Your mother told you to go home for dinner!")
    else:
        print(name +"is here!")
print("All the students are here!")

輸出結果如下：

Zhangshanis here!
Wangeris here!
Xiaoming,Your mother told you to go home for dinner!
All the students are here!

- 4.5.2 確定列表不是空的

在檢索姓名前檢查姓名是否為空，不為空則打印出所有姓名，為空則提示沒有姓名：

names = []
if names:
    for name in names:
        print(name +" is here!")
    print("All the students are here!")
else:
    print("There is no students!")

輸出結果如下：

There is no students!

在if語句中將列表名用在條件表示式中時，Python將在列表至少包含一個元素時返回Ture，並在列表為空時返回False

- 4.5.3 使用多個列表

兩個列表names_1和names_2，要求輸出既在names_2中又在names_1中的元素：

names_1 = ['Zhangshan' , 'Liyang'  , 'Wanger' , 'Tangyang' , 'Xiaoming']
names_2 = ['Liyang' , 'Zhangwei' , 'Tangyang']
for names in names_2:
    if names in names_1:
        print(names +" is here!")
print("All the students are here!")

輸出結果如下：

Liyang is here!
Tangyang is here!
All the students are here!