Python - push pop 구현

1. 후위 표기법(RPN) 

: n1 ? n2  > n1 n2 ? 로 표기하는 기법

ex1)  10 5 * 7 3 + /

> (10 * 5) / (7 + 3)

ex2)  2 3 2 3 7 + + + +

> 2 + (3 + (2 + (3 + 7)))

 

2. push/pop을 이용한 후위표기법 계산 프로그램

>>> the_stack = []
>>> 
>>> def main():
>>>     s = input('Enter RPN string : ')
>>>     a_list = s.split()
>>> 
>>>     for item in a_list :
>>>         if item.isdisit() :
>>>             the_stack.append(float(item))
>>>         else :
>>>             if the_stack.count() < 2
>>>                 print('ERROR : Too few stack')
>>>                 return
>>> 
>>>             op2 = the_stack.pop()
>>>             op1 = the_stack.pop()
>>>             if item == '+' :
>>>                 the_stack.append(op1 + op2)
>>>             elif item == '-' :
>>>                 the_stack.append(op1 - op2)
>>>             elif item == '*' :
>>>                 the_stack.append(op1 * op2)
>>>             else item == '/' :
>>>                 the_stack.append(op1 / op2)
>>>             else :
>>>                 print('ERROR : Operation out of range')
>>>                 return 
>>> 
>>>     print(the_stack.pop())
>>> 
>>> main()