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Need Help on Reverse Polish Notation program code

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Join Date: Sep 2009

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ljvasil

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Need Help on Reverse Polish Notation program code

19 Days Ago

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

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 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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Join Date: Oct 2006

Posts: 2,273

Reputation: sneekula has a spectacular aura about

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sneekula

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Nearly a Posting Maven

19 Days Ago

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

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 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:

 Help with Code Tags 
 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; 19 Days Ago at 2:38 pm. Reason: code

No one died when Clinton lied.

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Join Date: Sep 2009

Posts: 2

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ljvasil

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Newbie Poster

19 Days Ago

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

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Join Date: Oct 2004

Posts: 3,983

Reputation: vegaseat is just really nice

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vegaseat

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DaniWeb's Hypocrite

18 Days Ago

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; 18 Days Ago at 5:51 pm.

May 'the Google' be with you!

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