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

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

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python中，值不为空时，判断条件为True，如

>>> str='flag'
>>> if str:
...     print("not empty")
... else:
...     print("empty")
... 
not empty
>>> str=''
>>> if str:
...     print("not empty")
... else:
...     print("empty")
... 
empty

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单个判断条件

>>> tup=('dog','cat','water','bj')
>>> for val in tup:
...      if len(val) < 3:
...          print("the length of " + val + " is less than 3")
...      elif len(val) == 3:
...          print("the length of " + val + " is 3")
...      else:
...          print ("the length of " + val + " is more than 3")
... 
the length of dog is 3
the length of cat is 3
the length of water is more than 3
the length of bj is less than 3

多个判断条件可以使用and或者or

>>> name="xiao"
>>> if name != "" and len(name) > 3:
...     print ("long name")
... else:
...     print ("short name")
... 
long name

>>> weight=100
>>> if weight > 150 or weight < 60:
...     print ("no normal weight")
... else:
...     print ("normal weight")
... 
normal weight

 判断某个值是否在列表中存在

>>> ls=['car','cat','dog']
>>> if 'car' in ls:
...     print("yes, it is car")
... else:
...     print("no car")
... 
yes, it is car

while循环

>>> i=5
>>> while(i>1):
...     i-=1
...     print(i)
... 
4
3
2
1

for循环

略