比較運算子

1、檢查是否相等

　　相等運算子（==）：用來判斷二者是否“相等”，注意與賦值運算子（=）區分。

car = 'audi'
'''在Python中判斷是否相等時區分大小寫'''
car == 'Audi'    '''False'''
car.title() == 'Audi'    '''True'''

2、檢查是否不相等

　　（1）比較數字：小於(<)、小於等於(<=)、大於(>)、大於等於(>=)。

age = 19

age < 21    '''True'''
age <= 21    '''True'''
 

age > 19    '''False'''
age >= 19    '''True'''

　　（2）條件判斷：不等於(!=)。

requested_topping = 'mushrooms'

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

邏輯運算子

　　使用邏輯運算子and和or可一次檢查多個條件，形成更復雜的判斷式。

age_0 = 22
age_1 = 18
age_0 >= 21 and age_1 >= 21    '''False'''
age_0 
 
>= 21 or age_1 >= 21    '''True'''
age_1 = 22
age_0 >= 21 and age_1 >= 21    '''True'''

if語句

　　上述程式中的判斷語句，均可作為if語句的條件測試。若測試值為真，Python將會執行if語句所包含的程式碼，若為假則不會執行。

1、簡單的if語句

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

2、if-else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!" ) 

輸出為：

Sorry, you are too young to vote.
Please register to vote as soon as you turn 18!

　　和for語句相同，當測試值為真時，Python會執行所有緊跟在if語句之後且縮排的程式碼。

3、if-elif-else結構（elif意為else if）

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) + " dollars." )

使用if語句處理列表

1、判斷列表是否為空

　　當把列表名用作if語句的條件表示式時，Python將在列表至少包含一個元素時返回True，在列表為空時返回False。

requested_toppings = []
if requested_toppings:
    print( "not empty" )
else:
    print( "empty" )
'''列表為空，故列印empty'''

2、判斷元素是否在列表中

　　使用in表示包含於列表，not in表示不包含於列表。

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 not in available_toppings:
        print( "Sorry, we don't have " + requested_topping + "." )
    else:
        print( "Adding " + requested_topping + "." )

print( "\nFinished making your pizza!" )

輸出為：

Adding mushrooms.
Sorry, we don't have french fries.
Adding extra cheese.

Finished making your pizza!

參考書籍：《Python程式設計：從入門到實踐》

2020-07-09