python语言之流程控制

if语句：

if 条件1：
    pass
elif 条件2：
   pass
elif 条件3：
   pass
else：
   pass

if 条件语句中，先判断条件1，如果满足条件1，则执行第二行代码，第二行执行完后if条件语句结束。

示例：用户取款机菜单用户选择界面：

from time import sleep
num = input("Please input your chioce:")
if num == "1":
    print("取款中.....")
    sleep(3)
    print("取款完成！")
elif num == "2":
    print("存款中....")
    sleep(3)
    print("存款完成！")
elif num == "3":
    print("查询中....")
    sleep(3)
    print("查询完成！")
elif num == "4":
    print("退出中....")
    sleep(1)
    print("已退出!")
else:
    print("Please input the right nummber!")

——————————————————————————————————————————

while循环 ：while 条件： 用于循环执行某段代码

示例：用户登陆界面

while True:
    name = input("Please input your name :")
    pwd = input("Please input your password :")
    if name == "egon" and pwd == "123":
        print("login  successful")
    else:
        print("name or password Error! Please input again!")

while循环可以嵌套while循环

示例：用户登陆退出

name = "egon"
password = "123"
tag = True
while tag:
    User_name = input("Name >>>")
    User_pwd = input("Password >>>")
    if User_name.lower() == name and User_pwd == password:
        while tag:
            Request = input("Request >>>")
            if Request.lower() == "quit":
                print("Over")
                tag = False

    else:
        print("Name or Password Error,Please input again")

while和break，continue搭配使用

示例1：while和break搭配 用户登陆退出

name = "egon"
password = "123"

while True:
    User_name = input("Name >>>")
    User_pwd = input("Password >>>")
    print(User_name.lower())
    if name.lower() == User_name and password == User_pwd:
        print("login successful!")
        request = input("Request >>>")
        while True:
            if request.lower() =="quit":
                print("Over!")
                break
        break
    else:
        print("Name or Password Error,Please ipnut again!")

循环中遇到break即退出当前循环执行循环外代码。

示例2：while和continue搭配使用，筛选输出值

while True:
    val = input("请输入数据以逗号分隔：")
    val = val.split(",")
    try:
        val = list(map(int,val))
    except ValueError as e:
        continue

    count = 1
    while count < 20:
        if count in val:
            count += 1
            continue
        else:
            print(count)
            count += 1

continue的功能是当执行到continue时不再执行continue以后子代码块语句直接回到循环判断，进行下一次循环

当val = list（map（int，val））语句出错时执行continue语句，当执行到continue语句时立马重新回到当前循环判断条件。

——————————————————————————————————————————

for循环：for循环可以实现的功能while循环统统可以实现，但在某些场景，for循环更简洁，比如取值

如：

L = [1,2,3,4]

for i in L:

   print(i)

>>>

1

2

3

4

for 嵌套

示例1：打印99乘法表

L = []
for i in range(1, 10):
    for j in range(1, i + 1):
        m = "%s * %s = %s" % (i, j, i*j)
        L.append(m)
    print(*L)
    L = []

>>>>

C:UsersDELLAppDataLocalProgramsPythonPython37python3.exe C:/Users/DELL/PycharmProjects/untitled1/ad.py
1 * 1 = 1
2 * 1 = 2 2 * 2 = 4
3 * 1 = 3 3 * 2 = 6 3 * 3 = 9
4 * 1 = 4 4 * 2 = 8 4 * 3 = 12 4 * 4 = 16
5 * 1 = 5 5 * 2 = 10 5 * 3 = 15 5 * 4 = 20 5 * 5 = 25
6 * 1 = 6 6 * 2 = 12 6 * 3 = 18 6 * 4 = 24 6 * 5 = 30 6 * 6 = 36
7 * 1 = 7 7 * 2 = 14 7 * 3 = 21 7 * 4 = 28 7 * 5 = 35 7 * 6 = 42 7 * 7 = 49
8 * 1 = 8 8 * 2 = 16 8 * 3 = 24 8 * 4 = 32 8 * 5 = 40 8 * 6 = 48 8 * 7 = 56 8 * 8 = 64
9 * 1 = 9 9 * 2 = 18 9 * 3 = 27 9 * 4 = 36 9 * 5 = 45 9 * 6 = 54 9 * 7 = 63 9 * 8 = 72 9 * 9 = 81

示例2：金字塔

分析：
        *             #第一行4个空格，1个*
      * * *           #第二行3个空格，3个*
    * * * * *         #第三行2个空格，5个*
  * * * * * * *       # ....
* * * * * * * * *

max_level = 5
L = []
for current_level in range(1 , max_level+1):
    # print(current_level)
    for m in range(max_level-current_level):
        L.append(" ")#将空格装入列表
    for n in range(2*current_level-1):
        L.append("*")#将*装入列表
    print(*L)
    L = [] #里层for循环完一次重新定义L为空列表，准备装下一次的循环的元素

——————————————————————————————————————————

while,for,break等综合应用

from time import sleep

name = "egon"
password = "123"

while True:
    User_name = input("Please input your ID:")
    User_pwd = input("Please input your Password:")
    if User_name == name and User_pwd == password:
        print("login successful")
        print("""
        1 取款
        2 存款
        3 查询
        4 退出
                    """)
        while True:

            num = input("Please input your chioce:")
            if num == "1":
                print("取款中.....")
                sleep(3)
                print("取款完成！")
            elif num == "2":
                print("存款中....")
                sleep(3)
                print("存款完成！")
            elif num == "3":
                print("查询中....")
                sleep(3)
                print("查询完成！")
            elif num == "4":
                print("退出中....")
                sleep(1)
                print("已退出!")
                break
            else:
                print("Please input the right nummber!")
        break

>>>>>

C:UsersDELLAppDataLocalProgramsPythonPython37python3.exe C:/Users/DELL/PycharmProjects/untitled1/ad.py
Please input your ID:egon
Please input your Password:123
login successful

        1 取款
        2 存款
        3 查询
        4 退出
                    
Please input your chioce:1
取款中.....
取款完成！
Please input your chioce:2
存款中....
存款完成！
Please input your chioce:3
查询中....
查询完成！
Please input your chioce:4
退出中....
已退出!

Process finished with exit code 0