5  Control structure

5.1 if 语句

5.1.1 if语句

if conditional_test1:
    do something
if conditional_test2:
    do something
......
if conditional_testn:
    do something

5.1.2 if-else 语句

if conditional_test:
    do something
else:
    do anotherthing

5.1.3 if-elif-else 语句

else:可省略

if conditional_test1:
    do something
elif conditional_test2:
    do something
......
elif condtional_testn:
    do something
else:
    do otherthings
Code 
cars = ["audi","bmW","subaru","toyota"] 
for i in cars: 
    if i.lower()=='bmw': 
        i.upper() 
    else: 
        i.title()

与 and、或or

Code 
ages = [17,34,29,18,46,55,9,60,35,14,56,64] 
state = [] 
for i in ages: 
    if i <18: 
        state.append("young") 
    elif i>=18 and i<60: 
        state.append("adult") 
    else: 
        state.append("elder") 
state
['young',
 'adult',
 'adult',
 'adult',
 'adult',
 'adult',
 'young',
 'elder',
 'adult',
 'young',
 'adult',
 'elder']

在或不在 in、not in

Code 
users = ["小米","小亮","小美","小明","小帅"]

user1 = "小美" 
if user1 in users:
    print(f"欢迎光临,{user1}!")
user2 = "小花"
if user2 not in users:
    print(f"请加入我们，{user2}!")
欢迎光临,小美!
请加入我们，小花!

5.1.4 确定列表非空

对于数值0，空值None，单引号空字符串''，双引号空字符串""，空列表[]，空元组()，空字典{}，python都会返回False。

Code 
users = []

if users:
    for i in users:
        print(f"欢迎光临,{i}!")
else:
    print("无人使用")
无人使用

5.1.5 使用多个列表

Code 
invited = ["A","S","D","F","G","H","J","K"]
present = ["A","F","Z","X","K","C"]

for pre in present:
    if pre in invited:
        print(f"欢迎您的到来，{pre}先生/女士！")
    else:
        print(f"抱歉，{pre}先生/女士，你没有请柬，无法进入。")
print("\n 欢迎")
欢迎您的到来，A先生/女士！
欢迎您的到来，F先生/女士！
抱歉，Z先生/女士，你没有请柬，无法进入。
抱歉，X先生/女士，你没有请柬，无法进入。
欢迎您的到来，K先生/女士！
抱歉，C先生/女士，你没有请柬，无法进入。

 欢迎

5.2 while循环

Code 
i=0
while i<=9:
    print(i)
    i=i+1
0
1
2
3
4
5
6
7
8
9

5.2.1 用户选择退出循环

Code 
prompt = "\n 输入一条消息，我将会复述一遍:\n 输入 quit 结束程序。"
message = ""
while message != 'quit':
    message = input(prompt)
    if message !='quit':
        print(message)

5.2.2 使用标志flag

Code 
prompt = "\n 输入一条消息，我将会复述一遍:\n 输入 quit 结束程序。"
active = True
while active:
    message = input(prompt)
    if message !='quit':
        print(message)
    else:
        active =False

5.2.3 使用break退出循环

Code 
prompt = "\n 输入一条消息，我将会复述一遍:\n 输入 quit 结束程序。"
while True:
    message = input(prompt)
    if message !='quit':
        print(message)
    else:
        break

5.2.4 在循环中使用continue

Code 
i=0
while i <10:
    i=i+1
    if i%2 ==0:
        continue
    print(i)
1
3
5
7
9

5.2.5 使用while循环处理列表和字典

5.2.5.1 在列表之间移动元素

Code 
# 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() 
    print(f"Verifying user: {current_user.title()}")
    confirmed_users.append(current_user) 
    
# 显示所有的已验证用户 
print("\nThe following users have been confirmed:") 
for confirmed_user in confirmed_users: 
    print(confirmed_user.title())
Verifying user: Candace
Verifying user: Brian
Verifying user: Alice

The following users have been confirmed:
Candace
Brian
Alice

5.2.5.2 删除所有特定值

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

5.2.5.3 让用户填充字典

Code 
# 调查程序
responses = {}
flag = True


while flag:
    # 提示被调查者输入性名和回答,存储在字典中
    name = input("\n What is your name?")
    response = input("What is your favorite fruit?")
    responses[name] = response
    
    repeat = input("Whether to move on to the next person (yes/no)")
    if repeat == "no":
        flag = False

print("\n ---Results---")
for i,j in responses.items():
        print(f"{i}'s favorite fruit is {j}.")