Need Help on Reverse Polish Notation program code

[ljvasil]

I am having problems with my reverse polish calculator code. The code is as follows:

import operator

OPERATORS = {
        '+': operator.add,
        '-': operator.sub,
        '*': operator.mul,
        '/': operator.div,
        }

class Stack(object):
    "Creates a stack to enter numbers or operators into"

    def __init__(self):
        self.storage = [0,0,0,0]
        self.sptn = -1        

    def push(self, item):
        self.sptn = self.sptn + 1
        self.storage[self.sptn] = item
        
    def pop(self):
        self.sptn = self.sptn -1
        
    def top(self):        
        return self.storage[self.sptn]

    def __rep__(self):
        return self.storage[:self.sptn + 1]
    

def get_input():

    equation = raw_input("Enter the equation in reverse polish notation: ")
    return equation

def evaluate_rpn(equation):

    s = Stack()

    try:
        for item in equation.split():
            try:
                result = float(item)
                s.push(result)
            except ValueError:
                x = s.pop()
                y = s.pop()
                result = OPERATORS[item](x, y)
                s.push(result)
        result = s.top()
            
    except IndexError:
        print "Please input at least two numbers before entering an operator."

def main():
    equation = get_input()
    answer = evaluate_rpn(raw_input("Enter the equation in reverse polish notation: "))
    print answer

The code trips up when it hits an operator for some reason. The line is causing me trouble is the "results = OPERATOR[item] (x, y)"

I would appreaciate any help!!! Please!!!

Thanks

[sneekula]

It's always good to use a few test prints:

import operator

OPERATORS = {
        '+': operator.add,
        '-': operator.sub,
        '*': operator.mul,
        '/': operator.div,
        }

class Stack(object):
    "Creates a stack to enter numbers or operators into"

    def __init__(self):
        self.storage = [0,0,0,0]
        self.sptn = -1

    def push(self, item):
        self.sptn = self.sptn + 1
        self.storage[self.sptn] = item
        
    def pop(self):
        self.sptn = self.sptn -1
        
    def top(self):
        return self.storage[self.sptn]

    def __rep__(self):
        return self.storage[:self.sptn + 1]
    

def get_input():

    equation = raw_input("Enter the equation in reverse polish notation: ")
    return equation

def evaluate_rpn(equation):
    
    s = Stack()

    try:
        for item in equation.split():
            try:
                result = float(item)
                s.push(result)
            except ValueError:
                x = s.pop()
                y = s.pop()
                print x, y   # test --> None None  problem is here!!!
                result = OPERATORS[item](x, y)
                s.push(result)
        result = s.top()
        print result  # test
        return result
            
    except IndexError:
        print "Please input at least two numbers before entering an operator."

def main():
    # use for testing
    equation = '2 3 +'
    #equation = get_input()
    answer = evaluate_rpn(equation)
    print answer

main()

The problem is in your class Stack. If you use a Python Queue as a stack it works fine:

import operator
import Queue

OPERATORS = {
        '+': operator.add,
        '-': operator.sub,
        '*': operator.mul,
        '/': operator.div,
        }

def get_input():

    equation = raw_input("Enter the equation in reverse polish notation: ")
    return equation

def evaluate_rpn(equation):
    
    # use a Python queue for a stack
    s = Queue.Queue(-1)

    try:
        for item in equation.split():
            try:
                result = float(item)
                s.put(result)
            except ValueError:
                x = s.get()
                y = s.get()
                #print x, y   # test
                result = OPERATORS[item](x, y)
                s.put(result)
        result = s.get()
        #print result  # test
        return result
            
    except IndexError:
        print "Please input at least two numbers before entering an operator."

def main():
    # use for testing
    equation = '2 3 +'
    # otherwise use ...
    #equation = get_input()
    answer = evaluate_rpn(equation)
    print answer  # '2 3 +' --> 5.0

main()

[ljvasil]

Ok, thank you for the corrections. The problem is that I have to use a class for this part of the course. I thought I started the positions by s.sptn (start point) = -1 in the __init__(self) function. Any more help with trying to figure out why the class Stack is incorrect would be appreciated!

Thanks,
Linda

[vegaseat]

Why don't you simply separate your class Stack out and send it through some simple tests to figure out the errors?

Looks like pop() should return something.