摘要：本章主要介紹如何進(jìn)行用戶輸入，循環(huán)，以及與循環(huán)配合使用的語(yǔ)句。函數(shù)在中，使用函數(shù)獲取用戶輸入，這里請(qǐng)注意的返回值為字符串。值得提醒的是，編寫循環(huán)時(shí)應(yīng)避免死循環(huán)，或者叫做無(wú)限循環(huán)，比如循環(huán)忘記了變量自增。

《Python編程：從入門到實(shí)踐》筆記。
本章主要介紹如何進(jìn)行用戶輸入，while循環(huán)，以及與循環(huán)配合使用的break, continue語(yǔ)句。

在Python中，使用input()函數(shù)獲取用戶輸入，這里請(qǐng)注意：input()的返回值為字符串。如果輸入的是數(shù)字，并且要用于后續(xù)計(jì)算，需要進(jìn)行類型轉(zhuǎn)換。
input()函數(shù)可以傳入字符串參數(shù)作為輸入提示，如下：

# 代碼：
number = input()
# 判斷數(shù)據(jù)類型的兩種方法
print(type(number))
print(isinstance(number, str))

print(int(number) ** 2)    # int()函數(shù)將字符串轉(zhuǎn)換成整數(shù)

# 如果提示超過(guò)一行，可以將提示放在變量中，再將變量傳入input()；
# 并且最好在提示后面留一個(gè)空格以區(qū)分提示和用戶輸入
message = input("Tell me something, and I will repeat it back to you: ")
print(message)

# 結(jié)果：
123

True
15129
Tell me something, and I will repeat it back to you: Hello, everyone!
Hello, everyone!

判斷奇偶（作為對(duì)前文常見運(yùn)算的補(bǔ)充）：取模運(yùn)算%，返回余數(shù)

# 代碼：
number = input("Enter a number, and I"ll tell you if it"s even or odd: ")
number = int(number)

if number % 2:
    print("
The number " + str(number) + " is even.")
else:
    print("
The number " + str(number) + " is odd.")

# 結(jié)果：
Enter a number, and I"ll tell you if it"s even or odd: 123

The number 123 is even.

for循環(huán)用于針對(duì)集合中的每個(gè)元素的一個(gè)代碼塊，而while循環(huán)不斷地運(yùn)行，直到指定的條件不滿足為止。比如，讓用戶選擇何時(shí)退出：

# 代碼：
prompt = "
Tell me something, and I will repeat it back to you:"
prompt += "
Enter "quit" to end the program. "
message = ""
while message != "quit":
    message = input(prompt)
    if message != "quit":
        print(message)

# 結(jié)果：
Tell me something, and I will repeat it back to you:
Enter "quit" to end the program. Hello everyone!
Hello everyone!

Tell me something, and I will repeat it back to you:
Enter "quit" to end the program. Hello again.
Hello again.

Tell me something, and I will repeat it back to you:
Enter "quit" to end the program. quit

在上述代碼中我們直接對(duì)輸入數(shù)據(jù)進(jìn)行判斷，這樣做在簡(jiǎn)單的程序中可行，但復(fù)雜的程序中，如果有多個(gè)狀態(tài)同時(shí)決定while循環(huán)的繼續(xù)與否，要是還用上述的方法，則while循環(huán)的條件判斷將很長(zhǎng)很復(fù)雜，這時(shí)可以定義一個(gè)變量作為標(biāo)志來(lái)代替多個(gè)條件。使用標(biāo)志來(lái)改寫上述代碼：

prompt = "
Tell me something, and I will repeat it back to you:"
prompt += "
Enter "quit" to end the program. "

active = True
while active:
    message = input(prompt)
    if message != "quit":
        active = False
    else:
        print(message)

在復(fù)雜的程序中，如很多事件都會(huì)導(dǎo)致程序停止運(yùn)行的游戲中，標(biāo)志很有用：在其中的任何一個(gè)事件導(dǎo)致活動(dòng)標(biāo)志變?yōu)?b>False時(shí)，主游戲循環(huán)將退出。

要立即退出while或者for循環(huán)，不在執(zhí)行循環(huán)中余下的代碼，也不管條件測(cè)試的結(jié)果如何，可使用break語(yǔ)句。再將上述使用標(biāo)志的代碼改寫為break：

prompt = "
Tell me something, and I will repeat it back to you:"
prompt += "
Enter "quit" to end the program. "

while True:
    message = input(prompt)
    if message != "quit":
        break
    print(message)

如果滿足某條件時(shí)要返回循環(huán)開始處，而不是跳出循環(huán)，則使用continue語(yǔ)句。以下是打印1到10中的所有奇數(shù)的代碼：

# 代碼：
count = 0
while count < 10:
    count += 1
    if count % 2 == 0:
        continue
    print(count)

# 結(jié)果：
1
3
5
7
9

break與continue的區(qū)別：break跳過(guò)循環(huán)體內(nèi)余下的所有代碼，并跳出循環(huán)；continue跳過(guò)循環(huán)體內(nèi)余下的所有代碼，回到循環(huán)體開始處繼續(xù)執(zhí)行，而不是跳出循環(huán)體。
值得提醒的是，編寫循環(huán)時(shí)應(yīng)避免死循環(huán)，或者叫做無(wú)限循環(huán)，比如while循環(huán)忘記了變量自增。

將未驗(yàn)證用戶經(jīng)驗(yàn)證后變?yōu)橐羊?yàn)證用戶：

# 代碼：
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("
The following users have been confirmed:")
for confirmed_user in confirmed_users:
    print(confirmed_user.title())

# 結(jié)果：
Verifying user: Candace
Verifying user: Brian
Verifying user: Alice

The following users have been confirmed:
Candace
Brian
Alice

之前的章節(jié)中使用remove()函數(shù)來(lái)刪除列表中的值，但只刪除了列表中的第一個(gè)指定值，以下代碼循環(huán)刪除列表中指定的值：

# 代碼：
pets = ["dog", "cat", "dog", "goldfish", "cat", "rabbit", "cat"]
print(pets)

while "cat" in pets:
    pets.remove("cat")

print(pets)

# 結(jié)果：
["dog", "cat", "dog", "goldfish", "cat", "rabbit", "cat"]
["dog", "dog", "goldfish", "rabbit"]

# 代碼：
responses = {}

# 設(shè)置一個(gè)標(biāo)志，指出調(diào)查是否繼續(xù)
polling_active = True
while polling_active:
    # 提示輸入被調(diào)查者的名字和回答
    name = input("
What is your name? ")
    response = input("Which mountain would you like to climb someday? ")

    # 將回答存入字典
    responses[name] = response
    
    # 是否還有人要參與調(diào)查
    repeat = input("World you like to let another person respond?(yes/ no) ")
    if repeat == "no":
        polling_active = False

# 調(diào)查結(jié)束，輸出結(jié)果
print("
--- Poll Results ---")
for name, response in responses.items():
    print(name + " world like to climb " + response + ".")
    
# 結(jié)果：
What is your name? Eric
Which mountain would you like to climb someday? Denali
World 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
World you like to let another person respond?(yes/ no) no

--- Poll Results ---
Eric world like to climb Denali.
Lynn world like to climb Devil"s Thumb.

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