public void EvaluateTestOperateWithSubtract()
        {
            string input = "2 - 1";
            ProcessInput PI = new ProcessInput(input);
            Evaluate ev = new Evaluate(PI.GetFirstInputNumber(), PI.GetOperator(), PI.GetSecondInputNumber());
            int expected = 1;

            Assert.AreEqual(expected, ev.Operate());
        }
 static void Main(string[] args)
 {
     Console.WriteLine("Calculator");
     int count = 0;
     string input;
     Constant constant = new Constant();
     while (true)
     {
         Console.Write("[{0}]> ", count);
         input = Console.ReadLine();
         if(input == "exit" || input == "quit")
         {
             Console.WriteLine("loggin out");
             break;
         }
         ProcessInput PI = new ProcessInput(input);
         try
         {
             Evaluate ev = new Evaluate(PI.GetFirstInputNumber(), PI.GetOperator(), PI.GetSecondInputNumber());
             Console.WriteLine("   = " + ev.Operate());
         }
         catch
         {
             constant.ChangeExpression(input);
             try
             {
                 Console.WriteLine(constant.GetExpression());
             }
             catch
             {
                 Console.WriteLine("Error! Try again . . .");
             }
         }
         count++;
     }
 }
 public int GetExpression()
 {
     if(Input.Trim().Length == 1)
     {
         if (Char.IsLetter(char.Parse(Input.Trim()))) {
             if (IsInList(char.Parse(Input.Trim())))
             {
                 return GetValueOf(char.Parse(Input.Trim()));
             }
             else
             {
                 throw new FormatException();
             }
         }
         else
         {
             throw new FormatException();
         }
     }
     else
     {
         var leftside = Input.Split('=')[0].Trim();
         if(leftside.Length != 1 || Input.Split('=').Length != 2)
         {
             throw new FormatException();
         }
         else
         {
             if (IsInList(char.Parse(leftside)))
             {
                 throw new FormatException();
             }
             var rightside = Input.Split('=')[1].Trim();
             if(rightside.Length == 0)
             {
                 throw new FormatException();
             }
             try
             {
                 for (int i = 0; i < rightside.Length; i++)
                 {
                     if (Char.IsLetter(rightside[i]))
                     {
                         if (IsInList(rightside[i]))
                         {
                             //this line has the error
                             //string newRightside = rightside.Insert(i, GetValueOf(rightside[i]).ToString());
                             //rightside.Replace(i, 1);
                             //s.Remove(3, 2).Insert(3, "ZX");
                             rightside = rightside.Remove(i, 1).Insert(i, GetValueOf(rightside[i]).ToString());
                         }
                         else
                         {
                             throw new FormatException();
                         }
                     }
                 }
                 ProcessInput PI = new ProcessInput(rightside);
                 try
                 {
                     PI.GetOperator();
                     Evaluate ev = new Evaluate(PI.GetFirstInputNumber(), PI.GetOperator(), PI.GetSecondInputNumber());
                     int rightsideResult = ev.Operate();
                     AddToConstants(char.Parse(leftside), rightsideResult);
                     return rightsideResult;
                 }
                 catch
                 {
                     AddToConstants(char.Parse(leftside), int.Parse(rightside));
                     return int.Parse(rightside);
                 }
             }
             catch
             {
                 throw new FormatException();
             }
         }
     }
 }