python中for-in循环、while循环、if语句、input()函数、流程控制语句break和continue

循环 for-in和while

for-in循环

语法结构 ：
for 自定义的变量 in 可迭代的对象：
条件执行体
注意：python中不用大括号括起来表示一个整体，而是用缩进。
练习：遍历列表元素

bicycles = ['trek','cannondale','redline','specialized']
for bicycle in bicycles:
    print(bicycle)

trek
cannondale
redline
specialized

练习：通过以下代码，注意缩进与不缩进的区别

magicians = ['alice','david','carolina']
for magician in magicians:
    print(f"{magician.title()},that was a great trick")
    print(f"I can't wait to see your next trick,{magician.title()}.\n")
print("thank you,everyone.that was a great magic show!")

Alice,that was a great trick
I can't wait to see your next trick,Alice.

David,that was a great trick
I can't wait to see your next trick,David.

Carolina,that was a great trick
I can't wait to see your next trick,Carolina.

thank you,everyone.that was a great magic show!

while循环

while循环不断运行，直到条件不满足为止
语法结构
while 条件表达式： 条件执行体（循环体）
练习：使用while循环来数数，从1数到5

a = 1
while a<6:
    print(a)
    a=a+1

1
2
3
4
5

练习：

prompt = '\n Tell me sth,and I will repeat it back to you :'
prompt += '\n Enter "quit" to end the program '
message = ''
while message!='quit':
    message=input(prompt)
    print(message)


Tell me sth,and I will repeat it back to you :
 Enter "quit" to end the program hello world
hello world

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

进程已结束，退出代码 0

流程控制语句

break用于结束循环结构，常与if一起使用

while（条件）：
······
if ····：
break
练习：用户指出去过的地方

prompt = '\n please enter a city you have ever visited'
prompt += '\n Enter "quit" to end the program '
message = ''
while True:
    message=input(prompt)
    if message =='quit':
        break
    else:
        print(message)

please enter a city you have ever visited
 Enter "quit" to end the program Beijing
Beijing

 please enter a city you have ever visited
 Enter "quit" to end the program Shanghai
Shanghai

 please enter a city you have ever visited
 Enter "quit" to end the program quit

进程已结束，退出代码 0

continue语句用于结束当前循环，进入下一次循环

练习：要求输出1~30所有5的倍数

num = 0
while num <=30:
    num+=1
    if num % 5!=0:
        continue
    else:
        print(num)

5
10
15
20
25
30

注意：
1、使用if语句、while、for循环时，着重注意不要忘记冒号导致语法出错
2、在循环时，不要忘记自加条件，例如上例中num+=1，一开始忘记加，造成死循环。

if语句

补充：input()函数
input（）函数让程序暂停运行，等待用户输入一些文本。
注意input（）输出为字符串类型，数值输入要进行运算和比较前需要进行类型转换

age = input('How old are you?')
print(age)
print(type(age))

How old are you?22
22
<class 'str'>

以下代码进行了类型转换为int型，否则不能进行数值比较

age = int(input('How old are you?'))
print(age>=18)

How old are you?22
True

单分支结构

如果…就… 语法结构： if 条件表达式： 条件执行体 如果条件满足就执行，反之如果条件不满足，则忽略。
练习：银行卡余额

a = 2000
num = int(input('请输入你想取的钱'))
if(num<a):
    a=a-num
    print(f"你的余额为{a}")

请输入你想取的钱300
你的余额为1700

双分支结构

双分支结构 如果…不满足 就…
语法结构：
if 条件表达式：条件执行体1 else：条件执行体2
练习：输入一个数，判断是奇数还是偶数

num = int(input('please input a number'))
if(num%2 ==0):
    print('这是一个偶数')
else:
    print('这是一个奇数')

please input a number22
这是一个偶数

多分支结构

多分支结构
if 条件表达式1：
条件执行体1
elif条件表达式2：
条件执行体2…
练习：按年龄收费的游乐场，4岁以下免费，4~18岁收费25美元，18岁及以上收费40美元

age = int(input('please input your age'))
if(age<4):
    print('免费')
elif(age>4 and age<18):
    print('收费25美元')
else:
    print('收费40美元')

please input your age25
收费40美元