python3學習筆記6--條件判斷if語句

• 一個簡單示例

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

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

• 條件測試

每條if 語句的核心都是一個值爲True 或False 的表達式，這種表達式被稱爲條件測試 。

1、檢查是否相等

將變量當前值與特定值進行比較。

2、檢查是否相等時不考慮大小寫

當大小寫不是很重要，可將變量的值轉換爲小寫後比較。

3、檢查是否不相等

判斷兩個值是否不相等，可結合使用感嘆號和等號（!=）；!表示   不  。

4、比較數字

age = 18
if age == 18:
    print(age == 18)

5、檢查多個條件

and or 

6、檢查特定值是否包含在列表中

requested_toppings = ['mushrooms', 'onions', 'pineapple']
if 'mushrooms' in requested_toppings:
    print('mushrooms' in requested_toppings)

7、檢查特定值是否不包含在列表中

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.")

8、布爾表達式

條件測試的別名

• if 語句

age = 19
if age >= 18:
    print("You are old enough to vote!")
    print("Have you registered to vote yet?")

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!")

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.")

• 使用 if 語句處理列表

1、檢查特殊元素

requested_toppings = ['mushrooms', 'green peppers', 'extra cheese']
for requested_topping in requested_toppings:
    if requested_topping == 'green peppers':
        print("Sorry, we are out of green peppers right now.")
    else:
        print("Adding " + requested_topping + ".")
print("\nFinished making your pizza!")

2、確定列表不是空

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?")

3、使用多個列表

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!")

• 設置 if 語句的格式

在條件測試的格式設置方面，PEP 8提供的唯一建議是，在諸如== 、>= 和<= 等比較運算符兩邊各添加一個空格，例如，if

age < 4: 要比if age<4: 好。 這樣的空格不會影響Python對代碼的解讀，而只是讓代碼閱讀起來更容易。