Lecture Notes on 01 Mar 2017

class Link (object):
  def __init__ (self, data, next = None):
    self.data = data
    self.next = next

class LinkedList (object):
  def __init__ (self):
    self.first = None

  def insertFirst (self, data):
    newLink = Link (data)

    newLink.next = self.first
    self.first = newLink

  def insertLast (self, data):
    newLink = Link (data)

    current = self.first
    if (current == None):
      self.first = newLink
      return

    while (current.next != None):
      current = current.next

    current.next = newLink

  def findLink (self, data):
    current = self.first

    if (current == None):
      return None

    while (current.data != data):
      if (current.next == None):
        return None
      else:
        current = current.next
    
    return current

  def deleteLink (self, data):
    current = self.first
    previous = self.first

    if (current == None):
      return None

    while (current.data != data):
      if (current.next == None):
        return None
      else:
        previous = current
	current = current.next
	
    if (current == self.first):
      self.first = self.first.next
    else:
      previous.next = current.next

    return current

# Read a valid RPN string and calculate its value

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 what an item is on the top of the stack
  def peek (self):
    return self.stack[-1]

  # check if stack is empty
  def isEmpty (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):
  if (token == "+"):
    return oper1 + oper2
  elif (token == "-"):
    return oper1 - oper2
  elif (token == "*"):
    return oper1 * oper2
  elif (token == "/"):
    return oper1 / oper2

def rpn (s):
  theStack = Stack ()

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

  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():
  in_file = open ("rpn.txt", "r")
  for line in in_file:
    line = line.strip ()
    value = rpn (line)
    print (line, " = ", value)
  in_file.close()

main()