Lecture Notes on 23 Oct 2020

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

  # add an item on 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))

class Queue (object):
  def __init__ (self):
    self.queue = []

  # add an item to the end of the queue
  def enqueue (self, item):
    self.queue.append (item)

  # remove an item from the beginning of the queue
  def dequeue (self):
    return (self.queue.pop(0))

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

  # return the size of the queue
  def size (self):
    return (len(self.queue))


Operators: + - * / // % **

Operands: integers or floats

Infix notation: operator between operands
2 + 3 = 5

9 - 7 = 2

4 * 6 = 24

45 / 15 = 3

Inherent Ambiguity in Infix Notation
2 + 3 * 5 = 17

2 + 3 * 5 = 25

Jan Lucasiewicz (1924) invented the Polish notation or Prefix notation
where the operator precedes the operands:
+ 2 3 = 5
- 9 7 = 2
* 4 6 = 24
/ 45 15 = 3

Charles Hamblin (1950) extended this notation to the Reverse Polish
Notation (RPN) or Postfix notation. RPN leads to faster computation. 

Postfix Notation (Reverse Polish Notation)

2 3 + = 5

2 3 5 * + = 17

2 3 + 5 * = 25