Stack experiment

[kyosohma at gmail.com]

On Apr 3, 10:57 am, t... at finland.com wrote:
> Hi! Im new to Python and doing exercise found from internet. It is
> supposed to evaluate expression given with postfix operator using
> Stack() class.
>
> class Stack:
>      def __init__(self):
>          self.items = []
>
>      def push(self, item):
>          self.items.append(item)
>
>      def pop(self):
>          return self.items.pop()
>
>      def isEmpty(self):
>          return (self.items == [])
>
> def evaluatePostfix(expr):
>      import re
>      tokenList = re.split(" ([^0-9])", expr)
>      stack = Stack()
>      for token in tokenList:
>          if token == '' or token == ' ':
>              continue
>          if token == '+':
>              sum = stack.pop() + stack.pop()
>              stack.push(sum)
>          elif token == '*':
>              product = stack.pop() * stack.pop()
>              stack.push(product)
>          else:
>              stack.push(int(token))
>          return stack.pop()
>
> print evaluatePostfix("56 47 + 2 *")
>
> Errormsg:
> Traceback (most recent call last):
>    File "C:\*\postfix1.py", line 31, in <module>
>      print evaluatePostfix("56 47 + 2 *")
>    File "C:\*\postfix1.py", line 28, in evaluatePostfix
>      stack.push(int(token))
> ValueError: invalid literal for int() with base 10: '56 47'
>
> How can I avoid the error and get desired result?

I don't know why you're using the "re" module. For this, I would skip
it. Change your code so that it looks like this:

def evaluatePostfix(expr):
     tokenList = expr.split(" ")
     stack = Stack()
     for token in tokenList:
         if token == '' or token == ' ':
             continue
         if token == '+':
             sum = stack.pop() + stack.pop()
             stack.push(sum)
         elif token == '*':
             product = stack.pop() * stack.pop()
             stack.push(product)
         else:
             stack.push(int(token))
    return stack.pop()

# this worked for me. There may be something wrong with the "re" code
in your example, but I don't know enough about that to help in that
area.

Mike