public string calc(string s)
 {
     Calculator calculator = new Calculator();
     Num answer = calculator.calculate(s);
     if (answer == null)
         return "Sorry. The order is valid. Please check your order.";
     return answer.toString();
 }
 public void generateCorrectList(string questionFile)
 {
     Calculator calc = new Calculator();
     StreamReader readerQ = new StreamReader(questionFile);
     string line = readerQ.ReadLine();
     while (line != null)
     {
         int p = line.IndexOf(" ");
         string ques = line.Substring(p + 1);
         Num re = calc.calculate(ques);
         correctList.Add(re.toString());
         line = readerQ.ReadLine();
     }
     readerQ.Close();
 }