Python中循环及判断语句

循环判断条件是编程语言中一个很重要的部分，python也不例外，循环判断条件一般结合continue，return，break关键字来判断，这些关键字用法与java中基本一致

一、if判断语句

判断条件返回的结果为布尔值，在python中，布尔值为True/False，首字母必须大写，否则将出现如下异常

1>>> ls=false
2Traceback (most recent call last):
3  File "<stdin>", line 1, in <module>
4NameError: name 'false' is not defined

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python中，值不为空时，判断条件为True，如

1>>> str='flag'
2>>> if str:
3...     print("not empty")
4... else:
5...     print("empty")
6... 
7not empty
8>>> str=''
9>>> if str:
10...     print("not empty")
11... else:
12...     print("empty")
13... 
14empty

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单个判断条件

1>>> tup=('dog','cat','water','bj')
2>>> for val in tup:
3...      if len(val) < 3:
4...          print("the length of " + val + " is less than 3")
5...      elif len(val) == 3:
6...          print("the length of " + val + " is 3")
7...      else:
8...          print ("the length of " + val + " is more than 3")
9... 
10the length of dog is 3
11the length of cat is 3
12the length of water is more than 3
13the length of bj is less than 3

多个判断条件可以使用and或者or

1>>> name="xiao"
2>>> if name != "" and len(name) > 3:
3...     print ("long name")
4... else:
5...     print ("short name")
6... 
7long name
8
9>>> weight=100
10>>> if weight > 150 or weight < 60:
11...     print ("no normal weight")
12... else:
13...     print ("normal weight")
14... 
15normal weight

 判断某个值是否在列表中存在

1>>> ls=['car','cat','dog']
2>>> if 'car' in ls:
3...     print("yes, it is car")
4... else:
5...     print("no car")
6... 
7yes, it is car

while循环

1>>> i=5
2>>> while(i>1):
3...     i-=1
4...     print(i)
5... 
64
73
82
91

for循环

略