On Apr 3, 11:17 am, Steve Holden <[EMAIL PROTECTED]> wrote:
> [EMAIL PROTECTED] 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)
>
> If you add a print statement here I think you will find that the
> tokenisation here isn't really what you want:
>
> >>> expr = "56 47 + 2 *"
> >>> re.split(" ([^0-9])", expr)
> ['56 47', '+', ' 2', '*', '']
>
>
>
> > 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'd try using
>
> tokenList = split(expr)
>
> instead - this has the added benefit of removing the spaces, so you can
> simplify your code by removing the case that handles empty tokens and
> sapaces, I suspect.
>
> regards
> Steve
> --
> Steve Holden +44 150 684 7255 +1 800 494 3119
> Holden Web LLC/Ltd http://www.holdenweb.com
> Skype: holdenweb http://del.icio.us/steve.holden
> Recent Ramblings http://holdenweb.blogspot.com
Steve,
How do you do "tokenList = split(expr)"? There is no builtin called
"split".
Mike
--
http://mail.python.org/mailman/listinfo/python-list
