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Nov 7th, 2009

Need Help on Reverse Polish Notation program code

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

Python Syntax (Toggle Plain Text)
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
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

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ljvasil

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Nov 7th, 2009

Re: Need Help on Reverse Polish Notation program code

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

Python Syntax (Toggle Plain Text)
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()
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:

Python Syntax (Toggle Plain Text)
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()
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()

Last edited by sneekula; Nov 7th, 2009 at 2:38 pm. Reason: code

sneekula

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Nearly a Posting Maven

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since Oct 2006

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Nov 7th, 2009

Re: Need Help on Reverse Polish Notation program code

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

ljvasil

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Nov 8th, 2009

Re: Need Help on Reverse Polish Notation program code

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.

Last edited by vegaseat; Nov 8th, 2009 at 5:51 pm.

vegaseat

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