Python學習之路4-if語句

VPointer發表於2018-05-29
Python

《Python程式設計：從入門到實踐》筆記。

本章主要講述條件語句if, if-else, if-elif, if-elif-else等結構。

1. 條件測試

包括了“相等”，“不等”，“大於”，“小於”，“大於等於”，“小於等於”，“存在於”，“與或非”等判斷。值得注意的是，Python對大小寫敏感：

>>> car = "Audi"
>>> car == "audi"
False

>>> car.lower() == "audi"
True

>>> car != "audi"
True

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

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

>>> requested_toppings = ['mushrooms', 'onions', 'pineapple']
>>> 'mushrooms' in requested_toppings
True
>>> 'mushrooms' not in requested_toppings
False
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2. if 語句

2.1 簡單的if語句

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

# 結果：
You are old enough to vote!
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2.2 if-else 語句

# 程式碼：
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!")

# 結果：
Sorry, you are too young to vote.
Please register to vote as soon as you turn 18!
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2.3 if-elif-else 結構

# 程式碼：
age = 12

if age < 4:
    price = 0
elif age < 18:
    price = 5
else:
    price = 10

print("Your admission cost is $" + str(price) + ".")

# 結果：
Your admission cost is $5.
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還可以根據需要使用任意數量的elif程式碼塊：

# 程式碼：
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) + ".")

# 結果：
Your admission cost is $5.
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其次，Python並不要求if-elif結構後面必須有else程式碼塊。else是一條包羅永珍的語句，只要不滿足前面的條件，其中的程式碼就會執行，這可能會引入無效甚至惡意的資料。所以如果知道最終要測試的條件，應考慮使用一個elif程式碼塊來代替else程式碼塊，使程式碼更清晰，如下：

# 程式碼：
age = 12

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

# 結果：
Your admission cost is $5.
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2.4 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，即只要其中一個條件滿足，其餘條件都會被跳過，這保證了程式的高效性。然而有時必須檢查你關心的所有條件，這時則應該使用一系列不包含elif和else程式碼塊的簡單if語句：

# 程式碼：
requested_toppings = ["mushrooms", "extra cheese"]

if "mushrooms" in requested_toppings:
    print("Adding mushrooms.")
if "pepperoni" in requested_toppings:
    print("Adding pepperoni.")
if "extra cheese" in requested_toppings:
    print("Adding extra cheese.")

print("\nFinished making your pizza!")

# 結果：
Adding mushrooms.
Adding extra cheese.

Finished making your pizza!
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總之：如果你只想執行一個程式碼塊，就用if-elif-else結構；如果要執行多個程式碼塊，就使用一系列獨立的if語句。

3. 使用if語句處理列表

if語句常和迴圈結構配合使用。

3.1 檢查特殊元素

# 程式碼：
requested_toppings = ["mushrooms", "extra cheese", "green peppers"]

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!")

# 結果：
Adding mushrooms.
Adding extra cheese.
Sorry, we are out of green peppers right now.

Finished making your pizza!
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3.2 確定列表不是空的

到目前為止，對於處理的每個列表都做了一個簡單的假設，即它們非空，然而實際工程中，在遍歷一個列表前需要先判斷該列表是否為空：

# 程式碼：
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?")

# 結果：
Are you sure you want a plain pizza?
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Python學習之路4-if語句

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