Re: Exception Handling

[randhol+nospam@pvv.org (Preben Randhol)]

On Mon, 29 May 2000 23:53:18 +0100, NANCY HEHIR wrote:
>The Ada language reference manual (chapter 11) indicates that when an
>exception is raised the main body of code is abandoned and that if code
>exists for handling the exception then this is executed.
>
>Is it possible to use the exception handler to return to the main body at a
>point before the point where the exception is raised?
>
>For example: If I prompt for a keyboard input of ,say an eight character
>string, and a seven char string is actually inputted, a Constraint_Error is
>raised. Can I use a handling device to return the execution to the prompt
>for input ?

Yes by using a loop around a begin exception end block. 

This is from the top of my head so it might not work without
modifications:

   loop
      begin    -- This is the start of the block
         read_from_keyboard;
         exit; -- The code won't reach here if Constraint_Error is
               -- raised by the previous function.
      exception
         when E : Constraint_Error =>
         Put_Line("Wrong input, try again!");
      end;
   end loop;

Note it might increase readability a bit if you did something like this
(I'm not sure):

   correct_input : boolean := false;
   
   [...]

   loop
      begin    -- This is the start of the block
         read_from_keyboard;
         correct_input := true;
      exception
         when E : Constraint_Error =>
         Put_Line("Wrong input, try again!");
      end;
      exit when correct_input;
   end loop;


Also note that you can use :

   Input_String : String(1..30);
   Length       : Natural := 0;

   [...]
   
      Ada.Text_IO.Get_Line(Item => Input_String, Last => Length);

   then you can use:

      Input_String(1..Length)

I would recommend that you read the "Ada 95 Problem Solving and Program
Design", 3rd ed. by Feldman and Koffman. For a nice introduction to Ada.
More info can be found here:
http://www.seas.gwu.edu/faculty/mfeldman/cs1book

-- 
Preben Randhol -- [randhol@pvv.org] -- <http://www.pvv.org/~randhol/>
         "Det eneste trygge stedet i verden er inne i en fortelling."
                                                      -- Athol Fugard