// applies an operator to preEntry and entry
        private void operatorClick(object sender, EventArgs e)
        {
            Button currentButton = (Button)sender;
            string operationText = "";

            // formats the text of the button into a string variable
            if (currentButton.Text == "−")
            {
                operationText = "-";
            }
            else if (currentButton.Text == "×")
            {
                operationText = "*";
            }
            else if (currentButton.Text == "÷")
            {
                operationText = "/";
            }
            else
            {
                operationText = currentButton.Text;
            }

            functions.Operation(operationText);

            display.Text = functions.preEntry;
            buttonEquals.Focus();
        }