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From: mfeldman@seas.gwu.edu (Michael Feldman)
Subject: Re: Exception problem
Date: 1997/02/18
Message-ID: <5eds57$iog@felix.seas.gwu.edu>#1/1
X-Deja-AN: 219751675
References: <330A0D25.313@fs2.assist.uci.edu>
Organization: George Washington University
Newsgroups: comp.lang.ada
Date: 1997-02-18T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <330A0D25.313@fs2.assist.uci.edu>,
Larry Coon  <larry@fs2.assist.uci.edu> wrote:

>   loop
>      begin
>         Put ("Enter a positive number: ");
>         Get (X);
>         exit; -- No exception, so input was valid.
>      exception
>         when Constraint_Error =>
>            Put_Line ("Entry must be positive.  Try again.");
>         when Data_Error =>
>            Put_Line ("Entry must be a number.  Try again.");
You need 
             Skip_line;
here, which advances the input beyond the line terminator.
The problem is that if your first character is non-numeric,
the exception is read and the character is left in the buffer.
This is correct RM behavior; numeric input ceases when a character
is encountered that can't be part of a numeric token. It stays in
the input buffer because it might have been intended to be picked
up on the next read.

>      end;
>   end loop;

>The idea is that if input is not valid an exception will be thrown, the 

in Ada jargon, exceptions are "raised" and "handled".

>appropriate message will be displayed, and the code will loop back to 
>the prompt.  If no exception is thrown the input is valid, and the exit 
>statement take the program out of the loop.

Right - a standard Ada "robust input loop".

>This code handles non-positive numeric input (eg: 0 or -3) correctly.  
>But when I give it non-numeric input (eg: C), it displays the "Entry 
>must be a number.  Try again" message repeatedly and never stops for 
>input again.

Well, because that character is left in the buffer and never read,
your program loops and keeps trying to read it again and again.

This is a very common situation for beginners. I think it's discussed
in the programmer FAQ accessible from www.adahome.com. It's also
covered in some detail in that textbook by Feldman/Koffman, in Chap. 6.

Since it's quite proper RM behavior, you'll find this to be compiler-
independent.

I hope this helps.

Mike Feldman