Lecture Notes on 23 Mar 2022

* Evaluate the following postfix expressions:

4 8 7 + 2 * 3 1 - * +
3 4 + 5 *
3 4 5 + *
7 4 + 3 - 2 5 * /

* Python program to process RPN expressions
import sys

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

  # add an item to the top of the stack
  def push (self, item):
    self.stack.append (item)

  # remove an item from the top of the stack
  def pop (self):
    return self.stack.pop()

  # check the item on the top of the stack
  def peek (self):
    return self.stack[-1]

  # check if the stack is empty
  def is_empty (self):
    return (len(self.stack) == 0)

  # return the number of elements in the stack
  def size (self):
    return (len(self.stack))

def operate (oper1, oper2, token):
  expr = str(oper1) + token + str(oper2)
  return eval(expr)

def rpn (s):
  operators = ['+', '-', '*', '/', '//', '%', '**']
  theStack = Stack()
  tokens = s.split()

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

def main():
  for line in sys.stdin:
    line = line.strip()
    value = rpn (line)
    print (line, ' = ', value)

main()