學習內容：

1.if語句
2.使用者輸入和while迴圈

一.if語句

（1）1.簡單的if語句：
格式：

if conditional_test:
 do something 

在第1行中，可包含任何條件測試，而在緊跟在測試後面的縮排程式碼塊中，可執行任何操作。
如果條件測試的結果為True，Python就會執行緊跟在if語句後面的程式碼；否則Python將忽略這些程式碼。
例如：

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

執行結果為:

You are old enough to vote!

2.if-else語句：
經常需要在條件測試通過了時執行一個操作，並在沒有通過時執行另一個操作；在這種情況下，可使用Python提供的if-else語句。
例如：

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!")

執行結果為：

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

3.if-elif-else語句：
Python只執行if-elif-else結構中的一個程式碼塊（即if或elif或else中的為真語句），它依次檢查每個條件測試，直到遇到通過了的條件測試。
例如：

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.")

執行結果為：

Your admission cost is $5.

也可以使用多個elif的程式碼塊
例如：

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) + ".") 

執行結果為：

Your admission cost is $5.

同時模組中也可以省略else改用elif。
if-elif-else結構功能強大，但僅適合用於只有一個條件滿足的情況：遇到通過了的測試後，Python就跳過餘下的測試。這種行為很好，效率很高，讓你能夠測試一個特定的條件。
4.測試多個條件：
使用一系列不包含elif和else程式碼塊的簡單if語句。在可能有多個條件為True，且你需要在每個條件為True時都採取相應措施
時，適合使用這種方法。
例如：

 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!") 

執行結果為：

Adding mushrooms.
Adding extra cheese.
Finished making your pizza!

if-elif-else情況下：

requested_toppings = ['mushrooms', 'extra cheese']
if 'mushrooms' in requested_toppings:
 print("Adding mushrooms.")
elif 'pepperoni' in requested_toppings:
 print("Adding pepperoni.")
elif 'extra cheese' in requested_toppings:
 print("Adding extra cheese.")

print("\nFinished making your pizza!") 

執行結果為：

Adding mushrooms.
Finished making your pizza!

在倆者的對比之下，更易體會何時使用何種語句。
（2）使用if語句處理列表
1.檢查列表中特殊元素：
例如：

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!") 

執行結果為：

Adding mushrooms.
Sorry, we are out of green peppers right now.
Adding extra cheese.
Finished making your pizza!

這樣就可以找出列表中的特殊元素。
2.確定列表不為空：
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True，並在列表為空時返回False。
例如：

requested_toppings = []
if requested_toppings:
 print("\nFinished making your pizza!")
else:
 print("Are you sure you want a plain pizza?")

執行結果為：

Are you sure you want a plain pizza? 

二.使用者輸入和while迴圈

（1）input（）函式
1.input（）函式原理：函式input()讓程式暫停執行，等待使用者輸入一些文字。獲取使用者輸入後，Python將其儲存在一個變數中，以方便你使用。
函式input()接受一個引數：即要向使用者顯示的提示或說明，讓使用者知道該如何做。
例如：

message = input("Tell me something, and I will repeat it back to you: ")
print(message) 

執行結果為：

Tell me something, and I will repeat it back to you: Hello everyone!
Hello everyone! 

2.使用int（）來獲取數值輸入：
使用函式input()時，Python將使用者輸入解讀為字串。然後要得到我們需要的值（整型，浮點型等）就需要強制轉換。
用法：int（input（））
（2）while迴圈
1.首先迴圈是什麼結構組成，其次如何使用迴圈，迴圈可以幹什麼。
迴圈結構：測試條件+迴圈體
例如：

current_number = 1
while current_number <= 5:
 print(current_number)
 current_number += 1 

執行結果：

1
2
3
4
5 

以上為一個列印1到5的簡單迴圈。
2.while迴圈break語句：
當程式執行遇到break時將會退出迴圈結構。

prompt = "\nPlease enter the name of a city you have visited:"
prompt += "\n(Enter 'quit' when you are finished.) "
while True:
 city = input(prompt)

 if city == 'quit':
 break 
else:
 print("I'd love to go to " + city.title() + "!") 

執行結果為：

Please enter the name of a city you have visited:
(Enter 'quit' when you are finished.) New York
I'd love to go to New York!
Please enter the name of a city you have visited:
(Enter 'quit' when you are finished.) San Francisco
I'd love to go to San Francisco!
Please enter the name of a city you have visited:
(Enter 'quit' when you are finished.) quit 

3.使用while迴圈處理列表和字典：
在列表之間移動元素
例如：

unconfirmed_users = ['alice', 'brian', 'candace']
confirmed_users = []
while unconfirmed_users: 
   current_user = unconfirmed_users.pop()
   print("Verifying user: " + current_user.title())
   confirmed_users.append(current_user)

print("\nThe following users have been confirmed:")
for confirmed_user in confirmed_users:
 print(confirmed_user.title())

輸出結果為：

Verifying user: Candace
Verifying user: Brian
Verifying user: Alice
The following users have been confirmed:
Candace
Brian
Alice

刪除包含特定值的所有列表元素
例如：

pets = ['dog', 'cat', 'dog', 'goldfish', 'cat', 'rabbit', 'cat']
print(pets)
while 'cat' in pets:
      pets.remove('cat')

print(pets) 

執行結果為：

['dog', 'cat', 'dog', 'goldfish', 'cat', 'rabbit', 'cat']
['dog', 'dog', 'goldfish', 'rabbit'] 

使用使用者輸入來填充字典
例如：

responses = {}
polling_active = True
while polling_active:
      name = input("\nWhat is your name? ")
      response = input("Which mountain would you like to climb someday? ")

      responses[name] = response


      repeat = input("Would you like to let another person respond? (yes/ no) ")
       if repeat == 'no':
          polling_active = False

print("\n--- Poll Results ---")
for name, response in responses.items():
     print(name + " would like to climb " + response + ".")

執行結果為：

What is your name? Eric
Which mountain would you like to climb someday? Denali
Would you like to let another person respond? (yes/ no) yes

What is your name? Lynn
Which mountain would you like to climb someday? Devil's Thumb
Would you like to let another person respond? (yes/ no) no
--- Poll Results ---
Lynn would like to climb Devil's Thumb.
Eric would like to climb Denali.