python —— 分支循环语句

一、分支语句

每条 if 语句的核心都是一个值为True或False的表达式，这种表达式也称为条件测试
if 和 else 语句以及各自的缩进部分都是一个代码块
（1）if 语句
条件测试结果为True，，执行 if 语句后面所有缩进的代码块，否则忽略他们

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

（2）if - else 语句
条件测试通过后执行 if 语句后面缩进的代码块，不通过时执行 else 语句后面缩进的代码块

age = 14
if age >= 18:
    print("you are enough to vote!")
else:
    print("Sorry,you are too young to vote!")
    
输出：Sorry,you are too young to vote!

（3）if - elif - else 语句

elif 的应用场景：同时判断多个条件，所有的条件是平级的，可以根据需要使用任意数量的 elif 代码块，同时 if - elif - else 语句结构中else代码块也可以省略。
if - elif - else 语句结构仅适合于只有一个条件满足的情况

age = 14
if age < 4:
    print("your admisson cost is $0!")
elif age < 18:
    print("your admisson cost is $10!")
else:
   print("your admission cost is $20!")
   
输出：your admission cost is $10!

elif 代码行其实是另一个if测试，仅在前面的测试未通过时才会执行这行代码，elif代码行测试通过后，会跳过余下的else语句代码块。

(4) if 嵌套

if 语句可以用来测试多个条件

requested_topping = ['mushrooms','extra cheese']

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

输出：Adding mushrooms.
     Adding extra cheese.

if 嵌套语句是在之前条件满足的情况下，再增加额外的判断

age = 14
if age <= 18:
    if age > 3:
        print("you are enough to vote!")

输出：you are enough to vote!

二、循环语句

循环：从0开始，让特定代码重复执行

1、for 循环

针对集合中的每个元素都执行一个代码块，是一种遍历列表的有效方式。
for 循环操作列表

2、while循环

最常用的场景就是让指定的代码按照制定的次数重复执行

while 条件（判断 计数器 是否达到 目标次数）：
条件满足时，做的事情1
条件满足时，做的事情2
…

处理条件（计数器 + 1）
break和continue：是专门在循环中使用的关键字

break 某一条件满足时，退出循环，不再执行后续重复的代码

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(f"I'd love to go to {city.title()}!")

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

Please enter the name of a city you have visited:
(Enter 'quit' when you are finished.)quit

continue 某一条件满足时，不执行后续重复的代码
要返回循环开头，并根据条件测试结果决定是否继续执行循环，就可以使用continue语句

current_number = 0
while current_number < 10:
    current_number += 1
    if current_number %2 == 0:
        continue          //偶数时跳出该循环，即不输出

    print(current_number)
   
输出：
1
3
5
7
9

break和continue只针对当前所在循环有效，每个循环都必须要有停止运行的条件，在写循环语句时务必对每个循环进行测试，避免写出死循环（程序无休止的运行下去），如果陷入无限循环，可以按Ctrl+C，也可以通过关闭程序输出的终端窗口。

循环嵌套：
while嵌套就是：while里面还有while

3、while 循环处理列表和字典
while 循环可以在遍历列表的同时对其进行修改

（1）列表之间移动元素

# 首先，创建一个待验证用户列表
# 和一个用于存储已验证用户的空列表

unconfirmed_users = ['alice','brian','candace']
confirmed_users = []

# 验证每个用户，直到没有未验证用户为止
#  将每个经过验证的用户都移到已验证用户列表中
while unconfirmed_users:
    current_user = unconfirmed_users.pop()   # 方法pop()以每次一个的方式从列表unconfirmed_users末尾删除未验证的用户，最先删除的是Candace，其名字首先被赋给变量current_user

    print(f"Verifying user:{current_user.title()}")
    confirmed_users.append(current_user)

# 显示所有已验证的用户
print("\nThe following users have been confirmed:")
for confirmed_user in confirmed_users:
    print(confirmed_user.title())

输出：
Verifying user:Candace
Verifying user:Brian
Verifying user:Alice

The following users have been confirmed:
Candace
Brian
Alice

（2）删除为特定值的所有列表元素

使用函数remove()来删除列表中的特定值
pets = ['dog','cat','dog','goldfish','rabbit','cat']
print(pets)

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

print(pets)

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

（3）使用用户来填充字典

responses = {}  #定义一个空字典

# 设置一个标志，指出调查是否继续
polling_active = True

while polling_active:
# 提示输入被调查者的名字和回答
    name = input("\nWhat is your name?")
    response = input("Which mountain would you like to climb someday?")

# 将回答存储在字典中
    responses[name] = response

# 看看是否还有人要参与调查
    repeat = input("Would you like to let another person respond?(Yes/No)")
    if repeat == 'No':
        polling_active = False

# 调查结束，显示结果
print("\n--- Poll Results ---")
for name,response in responses.item():
    print(f"{name} would like to climb {response}.")

输出：
What is your name?KK
Which mountain would you like to climb someday?NN
Would you like to let another person respond?(Yes/No)Yes

What is your name?MM
Which mountain would you like to climb someday?LO
Would you like to let another person respond?(Yes/No)No

--- Poll Results ---
KK would like to climb NN.
MM would like to climb LO.