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From: John McCormick <mccormick@cs.uni.edu>
Newsgroups: comp.lang.ada
Subject: Data_Error and Enumeration_IO
Date: Thu, 12 Jan 2012 14:41:07 -0800 (PST)
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Date: 2012-01-12T14:41:07-08:00
List-Id: <comp.lang.ada>

I need some assistance in understanding the consumption of input
characters when a Data_Error is raised during the input of an
enumeration value.  My experiments show that there is a different
behavior depending on whether the illegal input consists of alphabetic
characters or digits.

Here is a short example to illustrate my ingorance.  The following
code fragment reads in an enumeration value using a Data_Error
exception to catch invalid entries.


type    Choice_Type is (Encode, Decode);
package Choice_IO   is new Ada.Text_IO.Enumeration_IO (Enum =3D>
Choice_Type);

loop
   Ada.Text_IO.Put_Line ("Would you like to encode or decode a
message");
   begin
      Choice_IO.Get (Choice);
      exit; -- the data validation loop
   exception
      when Ada.IO_Exceptions.Data_Error =3D>
         Ada.Text_IO.Put_Line ("Invalid entry. Please enter Encode or
Decode.");
         Ada.Text_IO.Skip_Line;  -- Skip over the offending data
   end;
end loop;


This code is my usual pattern for data validation and works for
whatever invalid input I enter.

However, when my students leave out the call to Skip_Line in the
Data_Error exception handler the behavior depends on the actual value
entered.  It still works as desired when the input is alphabetic
characters such as =93abc=94.  But when the input consists of digits such
as =93123=94 the loop becomes infinite, repeating the prompt and error
message.  It appears to me that the call to Choice_IO.Get consumes the
alphabetic input but does not consume the digit input leaving it
available for the next call to Get.  I could not find anything in the
language reference manual about how Enumeration_IO consumes input
characters when Data_Error is raised.  Can anyone give me an
explanation?  Is it implementation defined?  I am running GNAT GPL
2011.