public void compute_expression(string expr)
    {
        System.Collections.Generic.Stack<char> myStack = new System.Collections.Generic.Stack<char>();
        string output = "";
        // Rewriting the string into Reverse Polish Notation(RPN) format
        foreach(char token in expr)
        {
             //Read a token.
            //If the token is a number, then add it to the output queue.
            if (Char.IsNumber(token) == true)
            {
                output = output + ' ' +token;
            }
            if(token == '+' || token == '-')
            {
                if (myStack.Count() != 0)
                {
                        while(myStack.Peek() == '*' || myStack.Peek() == '/' || myStack.Peek() == '-')
                        {
                            if (myStack.Count() != 0)
                            {
                                output = output + ' ' + myStack.Peek();
                                myStack.Pop();
                            }
                            if (myStack.Count() == 0)
                            {
                                break;
                            }
                        }
                }
                myStack.Push(token);
            }
            if(token == '(')
            {
                myStack.Push(token);
            }

            if (token == ')')
            {
                if (myStack.Count() != 0)
                {
                    while (myStack.Peek() != '(')
                    {
                        if (myStack.Count() != 0)
                        {
                            output = output + ' ' + myStack.Peek();
                            myStack.Pop();
                        }
                    }
                myStack.Pop();
                }
            }
        }
           while(myStack.Count() != 0)
           {
               output = output + " " + myStack.Peek();
               myStack.Pop();
           }
        //Done RPN
           //Now we need to Evaluate the RPN
           evalu(output);
    }