三.python条件循环

1.if语句

1.1.简介

• if语句可以实现条件执行，如果条件判定为真，则后面的语句块执行，如果条件为假，语句块就不会被执行。

1.2.语法

1.2.1.单重条件判断(单分支)

>>> a = 1           
>>> if a==1:        
...     print("True")
... 
True
>>> 

 

1.2.2.多重条件判断(单分支)

>>> a = 1
>>> b = 2
>>> 
>>> if a==1 and b==2:
...     print("True")
... 
True
>>> 

 

1.2.3.多分支条件判断(if...else...)

>>> a = 1
>>> if a==1:
...     print("True")
... else:
...     print("False")
... 
True
>>> 

 

1.2.4.多分支条件判断(if....elif....else)

>>> a = 10
>>> if a > 10:
...     print("a > 10")
... elif a == 10:
...     print("a == 10")
... else:
...     print("a < 10")
... 
a == 10
>>> 

 

2.for语句

2.1.简介

• for是循环可以遍历任何序列的，如列表，元组，字典，字符串，主要作用是迭代。(迭代：每次操作都基于上一次的结果)

2.2.语法

2.2.1.通过序列索引迭代

#列表迭代
>>> a = [1,2,3,4,5,6]
>>> for i in a:      
...     print(i)
... 
1
2
3
4
5
6
>>> 
#字典迭代
>>> Info = {'name':'All is well','age':23}
>>> for k,v in Info.items():
...     print(k,v)          
... 
age 23
name All is well
#序列索引迭代
>>> a
[1, 2, 3, 4, 5, 6]
>>> a = [1, 2, 3, 4, 5, 6]
>>> for i in range(len(a)):
...     print("index %s is %s"%(i,a[i]))
... 
index 0 is 1
index 1 is 2
index 2 is 3
index 3 is 4
index 4 is 5
index 5 is 6

#enumerate
>>> for i,j in enumerate(a):
...     print("index %s is %s"%(i,a[i]))
... 
index 0 is 1
index 1 is 2
index 2 is 3
index 3 is 4
index 4 is 5
index 5 is 6
>>> 

 

2.2.2.break,continue

#break(跳出本层循环)
>>> a = [1,2,3,4,5,6]      
>>> for num in a: 
...     print(num)
...     if num == 3:
...         break
... 
1
2
3
>>> 
#continue(跳出本次循环) 
>>> for num in a:
...     if num == 3:
...         continue
...     print(num)
... 
1
2
4
5
6

 

2.2.3.使用else语句

#在大多数语言中条件循环是没有else的，但在python中是个例外。
#else语句只会在循环完成后执行，因刚刚说的break是跳出循环，break跳出并没有执行完成，所以break语句会跳过else块的内容

#例:break跳出不会执行else
>>> a = [1, 2, 3, 4, 5, 6]
>>> for num in a:
...     print(num)
...     if num == 3:
...         break
... else:
...     print("end...........")
... 
1
2
3
>>> 
#例:正常循环结束会执行else
>>> a = [1, 2, 3, 4, 5, 6]
>>> for num in a:
...     print(num)
... else:
...     print("end...........") 
... 
1
2
3
4
5
6
end...........
#例:continue数据正常结束，也会执行else
>>> a = [1, 2, 3, 4, 5, 6]
>>> for num in a:         
...     if num == 3:      
...         continue      
...     print(num)        
... else:
...     print("end............")
... 
1
2
4
5
6
end............

 

3.while语句

3.1.简介

• Python 编程中 while 语句用于循环执行程序，即在某条件下，循环执行某段程序。(条件：根据特定的条件循环)

3.2.语法

3.2.1 计数循环

>>> n = 0
>>> while n < 10:
...     print('n is %s' %n)
...     n+=1
... 
n is 0
n is 1
n is 2
n is 3
n is 4
n is 5
n is 6
n is 7
n is 8
n is 9
>>> 

3.2.2 无限循环

>>> import time
>>> 
>>> n = 0
>>> 
>>> while True:
...     print('n is %s' %n)
...     time.sleep(1)
...     n+=1
... 
n is 0
n is 1
n is 2
n is 3
n is 4
n is 5
n is 6
n is 7
n is 8
n is 9

 

3.2.3 continue,break,else

• 与for语句的用法相同，原理相同。