Python 3

(use Stack to evaluate the postfix expression as shown in lecture slides)

1.Write a function that accepts a valid postfix expression and evaluates it.

CODE:

# Stack.py

class Stack:

#------------------------------------------------------------

def __init__(self):

'''post: creates an empty LIFO stack'''

self.items = []

#------------------------------------------------------------

def push(self, item):

'''post: places x on top of the stack'''

self.items.append(item)

#------------------------------------------------------------

def pop(self):

'''post: removes and returns the top element of

the stack'''

return self.items.pop()

#------------------------------------------------------------

def top(self):

'''post: returns the top element of the stack without

removing it'''

return self.items[-1]

#------------------------------------------------------------

def size(self):

'''post: returns the number of elements in the stack'''

return len(self.items)