Receiving stack errors when trying to evaluate a postfix expression

[Joemeister]

Hello all,

I am getting 3 errors when I try to use stacks when I want to evaluate a postfix expression. I am not very experienced with the usage of stacks so please be patient with me.

Here is my code:

//The postfix parameter is a postfix string that was done through a shunting yard algorithm and is now passed through to the evaluate function
int Expression::evaluate(string postfix)
{
    // ERROR 1 //
    stack<int> resultStack = new stack<int>();

    int length = postfix.length();

    for (int i = 0; i < length; i++)
    {
        if ((postfix[i] == '+') || (postfix[i] == '-') || (postfix[i] == '*') || (postfix[i] == '/') || (postfix[i] == '^') || (postfix[i] == 'sqrt') || (postfix[i] == 'log') || (postfix[i] == 'abs') || (postfix[i] == '~'))
        {
            // ERROR 2 //
            int result = doTheOperation(resultStack.pop(), resultStack.pop(), postfix[i]);

            resultStack.push(result);
        }
        else if ((postfix[i] >= '0') || (postfix[i] <= '9'))
        {
            resultStack.push((int)(postfix[i] - '0'));
        }
        else
        {
        }
    }

    // ERROR 3 //
    return resultStack;
}

//The operations that must be done if a specific operator is found in the string
int Expression::doTheOperation(int left, int right, char op)
{
    switch (op)
    {
        case '+':
            return left + right;
        case '-':
            return left - right;
        case '*':
            return left * right;
        case '/':
            return left / right;
        case '^':
            return pow(left,right);
        case 'sqrt':
            if(right < 0)
            {
                string temp = "Square root of a negative number.";
                throw temp;
            }
            else
            {
                return (sqrt(right)) ;
            }
        case 'log':
            if (right < 0)
            {
                string temp = "Error. Not able to get the log of zero.";
                throw temp;
            }
            else
            {
                int temp = log10(right);
                return ceil(temp);
            }
        case 'abs':
            if (right < 0)
            {
                return (right*-1);
            }
            else
            {
                return right;
            }
        case '~':
            return (right*-1);

        default:
            return -1;
    }
    return -1;
}

Then it gives me the following errors:

error 1: conversion from 'std::stack<int>*' to non-scalar type 'std::stack<int>' requested
error 2: invalid use of void expression
error 3: cannot convert 'std::stack<int>' to 'int' in return

I will mark in the code where exactly these errors are occuring. I have absolutely no idea why I am getting these errors.

[Moschops]

conversion from 'std::stack<int>*' to non-scalar type 'std::stack<int>' requested

stack<int> resultStack = new stack<int>();

The bit on the right gives you a pointer to a stack<int>. You're trying to store that pointer value as a stack<int> object. This makes no sense. A stack<int>* is not the same as a stack<int>

invalid use of void expression

What kind of object does resultStack.pop() return? Does it return void (i.e. no returned object)?

cannot convert 'std::stack<int>' to 'int' in return

The function claims it returns an int( int Expression::evaluate(string postfix) ). You're trying to return a stack<int>. A stack<int> is not an int.

[rubberman]

Also, you are popping a value off of the result stack BEFORE you pushed anything onto it. This is the cause of the error there. There is nothing to return to you!

[iamthwee]

The other thing is postfix[i] will call a string of only one character long, whereas sqrt is four characters long. So you need to rethink that part at least.

[iamthwee]

Just to add to this thread I did a bit of digging, and I think a handy way to handle sqrt and abs functions is to use an arbitary single digit character instead.

Otherwise you would have to tokenise your expression first.

http://programmingnotes.freeweq.com/?p=1436

^^In the above snippet the OP uses special characters to denote sqrt and modulus functions, it's definitely something you can take a look at to at least get some more ideas to move forward with your own code.