Lecture Notes on 19 Mar 2014


1) Suppose that A = 7.0, B = 4.0, C = 3.0, and D = -2.0.
   Evaluate the following RPN expressions:

   a) A B + C / D *
   b) A B C D + / *


2) Convert the following infix expressions to RPN:
   
   a) (A + B) / (C + D)
   b) (A - B) * (C - (D + E))


3) Convert the following RPN expressions to infix notation
  
   a) A B C D + - *
   b) A B + C - D E * /



class Stack (object):
  def __init__ (self):
    self.stack = []

  def push (self, item):
    self.stack.append ( item )

  def pop (self):
    return self.stack.pop()

  def peek (self):
    return self.stack[-1]

  def isEmpty (self):
    return (len(self.stack) == 0)

  def size (self):
    return (len(self.stack))

def operate (oper1, oper2, token):
  if (token == '+'):
    return oper1 + oper2
  elif (token == '-'):
    return oper1 - oper2
  elif (token == '*'):
    return oper1 * oper2
  else:
    return oper1 / oper2

def rpn (s):
  stack = Stack()

  operators = ['+', '-', '*', '/']

  tokens = s.split()

  for item in tokens:
    if (item in operators):
      oper2 = stack.pop()
      oper1 = stack.pop()
      stack.push (operate (oper1, oper2, item))
    else:
      stack.push (float(item))

  return stack.pop()

def main():
  inFile = open ('./rpn.txt', 'r')
  for line in inFile:
    line = line.rstrip ('\n')
    value = rpn (line)
    print (line, '=', value)
  inFile.close()

main()