Tech Talk 2 Takeaways

In the Tech Talk 2 challenge I was able to use Stacks practically in the form of a calculator:

Sort of tokenized items with reverse polish notation

Stack<String> tokenStack = new Stack<String>();
for (String token : tokens) {
    switch (token) {
        // If left bracket push token on to stack
        case "(":
            tokenStack.add(token);
            break;
        case ")":
            while (tokenStack.peak() != null && !tokenStack.peak().equals("("))
            {
                reverse_polish.add( (String)tokenStack.pop() );
            }
            tokenStack.pop();
            break;
        case "+":
        case "-":
        case "*":
        case "/":
        case "%":
            // While stack
            // not empty AND stack top element
            // and is an operator
            while (tokenStack.peak() != null && isOperator(tokenStack.peak()))
            {
                if ( isPrecedent(token, (String) tokenStack.peak() )) {
                    reverse_polish.add((String)tokenStack.pop());
                    continue;
                }
                break;
            }
            // Push the new operator on the stack
            tokenStack.add(token);
            break;
        default:    // Default should be a number, there could be test here
            this.reverse_polish.add(token);
    }
}

I had to write my own small method to verify numbers for result calculation algorithm

    public boolean checkIfNum(String item) {
        try {
            int intValue = Integer.parseInt(item);
            return true;
        } catch (NumberFormatException e) {
            return false;
        }
    }

Actual written out rpnToResult method

private void rpnToResult()
{
    int a;
    int b;
    double c;
    double res;
    // Stack used to hold calculation while process RPN
    Stack calculation = new Stack();

    for ( int i = 0; i < reverse_polish.size(); i++)
    {
        if (checkIfNum(reverse_polish.get(i)))
        {
            calculation.add(reverse_polish.get(i));
        }
        else
        {
            // Pop the two top entries
            a = (Integer) calculation.pop();
            b = (Integer) calculation.pop();

            // Based off of Token operator calculate result

            switch ( (String) reverse_polish.get(i)) {
                // If left bracket push token on to stack
                case "(":
                case ")":
                    calculation.add(a);
                    calculation.add(b);
                    break;
                case "+":
                    c = a + b;
                    calculation.add(r);
                case "-":
                    c = a - b;
                    calculation.add(r);
                case "*":
                    c = a * b;
                    calculation.add(r);
                case "/":
                    c = a/b;
                    calculation.add(r);
                default:
                    calculation.add(a);
                    calculation.add(b);
            }

        }
    }
    res = (Double) calculation.pop();
}