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python中的语句基础(if语句、while语句)

作者:互联网

一、if语句

if 语句让你能够检查程序的当前状态,并据此采取相应的措施。if语句可应用于列表,以另一种方式处理列表中的大多数元素,以及特定值的元素
1、简单示例

names=['xiaozhan','caiyilin','zhoushen','DAOlang','huangxiaoming']
for name in names:
    if name == 'caiyilin':   #注意:双等号'=='解读为“变量name的值是否为'caiyilin' 
        print(name.upper())
    else:
        print(name.title())

 每条if语句的核心都是一个值为 True 或 False 的表达式,这种表达式被称为条件测试(如上述条件 name == 'caiyilin'),根据条件测试的值为 True 还是 False 来决定是否执行 if 语句中的代码。如果条件测试的值为True ,Python就执行紧跟在 if 语句后面的代码;如果为 False , Python 就忽略这些代码,不执行。

在Python中检查是否相等时区分大小写,例如,两个大小写不同的值会被视为不相等

my_fav_name = 'daolang'
for name in names:
    if name == my_fav_name:
        print('Yes')
    print('No')    
print('\n')
  
for name in names:
    if name.lower() == my_fav_name:
        print('Yes')
    print('No')       
print('\n')
#下方使用  if……else语句  
for name in names:
    if name.lower() != my_fav_name:   #检查是否不相等
        print('NO')
    else:
        print('YES')  

查多个条件:有时候需要两多个条件都为True时才执行操作;或者多个条件中,只满足一个条件为True时就执行操作,在这些情况下,可分别使用关键字and和or

ages=['73','12','60','1','10','55','13']
for age in ages:
    if   age > str(60):  #注意:ages中为列表字符串,所以age也是字符串,无法与整型的数字相比,需要先将数字转化为字符串再比较。
        print("The "+str(age)+" years has retired!")
    elif age > str(18) and age<=str(60):    #两个条件都为True时
        print("The "+str(age)+" years is an Adult!")
    elif age > str(12):
        print("The "+str(age)+" years is a student!")
    else:
        print("The "+str(age)+" years is a child!")

 二、while语句

for 循环用于针对集合中的每个元素都一个代码块,而 while 循环不断地运行,直到指定的条件不满足为止。

例如,while 循环来数数

current_number = 1
while current_number <= 5:
    print(current_number)
    current_number += 1
print("\n")
print(current_number)

当x<=5时,x自动加1,直到大于5时(也即current_number=6),退出while循环,再执行print(current_number)语句。

运行结果:

1
2
3
4
5

6

1、可以用来定义一个标志

定义一个变量,用于判断整个程序是否处于活动状态。这个变量被称为标志,相当于汽车的钥匙,钥匙启动时,汽车所有的电子设备、发动机、空调等均可正常运行,一旦钥匙关闭时,整个汽车熄火状态,全部不能运行。

同理,程序在标志为 True 时继续运行,并在任何事件导致标志的值为 False 时让程序停止运行。

prompt = "\nTell me your secret, and I will repeat it back to you:"
prompt += "\nEnter 'quit' to end the program. "

active = True  #定义一个标志
while active:  #当active为真的时候,执行程序,
    message = input(prompt) 
    if message == 'quit': #当输入信息为quit时,标志active为False,程序停止
        active = False
    else:
        print(message)

2、使用 break 退出循环,要立即退出 while 循环,不再运行循环中余下的代码

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("I'd love to go to " + city.title() + "!")

3、在循环中使用 continue

current_number = 0
while current_number < 10:
    current_number += 1  #以1逐步增加
    if current_number % 2 == 0: #求模运行,是2的倍数,为0
        continue  #忽略并继续运行。
    print(current_number) #打印出数字

4、使用 while 循环来处理列表和字典

1)处理列表中的数据

unconfirmed_users = ['Lucy', 'Bush', 'lincon', 'lucy',
                     'jack', 'lily', 'lucy', 'hanmeimei']  # 首先,创建一个待验证用户列表
confirmed_users = []  # 创建一个用于存储已验证用户的空列表

while unconfirmed_users:  # 验证每个用户,直到没有未验证用户为止
    current_user = unconfirmed_users.pop()  # 注意pop()是从最后一个开始。
    print("Verifying user: " + current_user.title())  # 打印验证的用户,且首字母大写
    confirmed_users.append(current_user)  # 将每个经过验证的列表都移到已验证用户列表中,
    # 相当于将unconfirmed_users倒序保存到current_user中
print("\nThe following users have been confirmed:")  # 显示所有已验证的用户
for confirmed_user in confirmed_users:  # for循环打印每个已验证的用户名
    print(confirmed_user.title())
print("\n")
# 删除包含特定的所有列表元素
while 'lucy' in confirmed_users:  # while 循环,因为lucy在列表中至少出现了一次
    confirmed_users.remove('lucy')  # 删除除最后一个外的其他相同的元素
print(confirmed_users)

运行结果如下:

Verifying user: Hanmeimei
Verifying user: Lucy
Verifying user: Lily
Verifying user: Jack
Verifying user: Lucy
Verifying user: Lincon
Verifying user: Bush
Verifying user: Lucy

The following users have been confirmed:
Hanmeimei
Lucy
Lily
Jack
Lucy
Lincon
Bush
Lucy

['hanmeimei', 'lily', 'jack', 'lincon', 'Bush', 'Lucy']#注意保留的是最后一个Lucy

 2) 处理字典中的数据

responses = {}#定义一个空字典
polling_active = True # 设置一个标志,指出调查是否继续
while polling_active:
    name = input("\nWhat is your name? ")  # 提示输入被调查者的名字和回答
    response = input("Which mountain would you like to climb someday? ")
    responses[name] = response # 将答卷存储在字典中,即name是key,变量response 是值
    repeat = input("Would you like to let another person respond? (yes/ no) ")# 看看是否还有人要参与调查
    if repeat == 'no':  #如无人参与调查,将标志设置为False,退出运行。
        polling_active = False

# 调查结束,显示结果
print("\n--- Poll Results ---")
print(responses)  #把字典打印出来  

for name, response in responses.items():  #访问字典
    print(name.title() + " would like to climb " + response + ".")

运行结果:

What is your name? lilei

Which mountain would you like to climb someday? siguliang

Would you like to let another person respond? (yes/ no) yes


What is your name? Lucy

Which mountain would you like to climb someday? The Alps

Would you like to let another person respond? (yes/ no) no

--- Poll Results ---
{'lilei': 'siguliang', 'Lucy': 'The Alps'}
Lilei would like to climb siguliang.
Lucy would like to climb The Alps.

 实际运行:(为显示全,已删除部分unconfirmed_users中的名字)

 

 

 

 

 

标签:语句,name,python,Lucy,while,user,print,users
来源: https://www.cnblogs.com/codingchen/p/16133182.html