# L-17 MCS 275 Fri 19 Feb 2010 : evalexpr.py

# evaluation of expressions

def ExpressionStack(e):
   """
   Returns a list to represent
   the stack of the expression.
   Spaces are removed.
   """
   L = []
   operators = ['+','-','*','/']
   brackets = ['(',')']
   operand = ''
   for c in e:
      if c in brackets + operators:
         if operand != '':
            L.append(operand)
            operand = ''
         L.append(c)
      elif c != ' ':
         operand = operand + c
   if operand != '':
      L.append(operand)
   return L

def main():
   """
   prompts the user for a string
   and evaluates it
   """
   e = raw_input('Give an expression : ')
   try:
      v = eval(e)
      print e, 'evaluates to', v
   except:
      print 'exception raised...'
   S = ExpressionStack(e)
   print 'the stack :', S

main()