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From: Amun-Ra <Amun_Ra72@gmx.net>
Subject: Exception handling problem
Date: 2000/03/06
Message-ID: <38C34C24.3955@gmx.net>
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Organization: Regionales Rechenzentrum Erlangen, Germany
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Newsgroups: comp.lang.ada
Date: 2000-03-06T00:00:00+00:00
List-Id: <comp.lang.ada>

Hi all!

I have a big problem handling exceptions with Ada95 using GNAT.
I need to enter a number and, if something else is entered, the raised
exception
Ada.IO_Exceptions.Data_Error has to be handled and the asking for a
number 
should continue (I guess the problem is probably simple, but I don't see
a 
solution yet). The code I am worrying about is the following:

    -- Procedure "RAS_AskBreakpoint":
    -- Asks if a breakpoint should be set or unset. It returns 0 if the
    -- breakpoint should be unset, 1 if it should be set and also
returns the 
    -- entered breakpoint value as out-argument.
    --
    procedure RAS_AskBreakpoint (Input: out Long_Long_Integer; Mode: out
Integer) is
        Keystroke:      Character;
    begin
        RAS_Set_Position (Position => (Col => 1, Line => 10)); 
        Put (EmptyLine);
        RAS_Set_Position (Position => (Col => 1, Line => 10)); 
        Put ("Do you want to set <S> or unset <U> a breakpoint?");
        RAS_Set_Position (Position => (Col => 1, Line => 10));
        loop
            Get_Immediate (Keystroke);
            if Keystroke = 'S' or Keystroke = 's' then
                Mode := 1;
                Put (EmptyLine);
                RAS_Set_Position (Position => (Col => 1, Line => 10));
                Put ("Set breakpoint at which instruction (<0 to abort):
");
                Get (Input);
                exit;
            elsif Keystroke = 'U' or Keystroke = 'u' then
                Mode := 0;
                Put (EmptyLine);
                RAS_Set_Position (Position => (Col => 1, Line => 10));
                Put ("Unset which breakpoint (<0 to abort): ");
                Get (Input);
                exit;
            end if;
        end loop;
        RAS_Set_Position (Position => (Col => 1, Line => 10));
        Put (EmptyLine);
        RAS_Set_Position (Position => (Col => 1, Line => 11));
    end RAS_AskBreakpoint;

This procedure can be called many times and of course the only inputs
that need
to be done are a 'S'|'s' or 'U'|'u' and subsequently a number of type 
Long_Long_Integer. The only lines where the Data_Error can occur, are
the 
"Get (Input);"-lines. However, if a Data_Error is raised, the asking for
a 
number should continue until the user enters a valid number.

I already tried handling the exception by declaring another block inside
the
loop...end loop-section with own exception handler:

        loop
            begin
                Get_Immediate (Keystroke);
                if Keystroke = 'S' or Keystroke = 's' then
                    Mode := 1;
                    Put (EmptyLine);
                    RAS_Set_Position (Position => (Col => 1, Line =>
10));
                    Put ("Set breakpoint at which instruction (<0 to
abort): ");
                    Get (Input);
                    exit;
                elsif Keystroke = 'U' or Keystroke = 'u' then
                    Mode := 0;
                    Put (EmptyLine);
                    RAS_Set_Position (Position => (Col => 1, Line =>
10));
                    Put ("Unset which breakpoint (<0 to abort): ");
                    Get (Input);
                    exit;
                end if;
            exception
                when others => Put_Line ("Enter a number!");
            end;
        end loop;

as well as trying to localize the exception handling down to the
Get-Instruction:

                    loop
                        begin
                            Get (Input);
                            exit;
                        exception
                            when others => Put_Line ("Enter a number!");
                        end;
                    end loop;

The behaviour of the procedure then (regardless of where in the loop I
put the
handler) is always the same: 
If I enter a number as wanted, I can call the procedure as often as I
want and 
it works perfectly. However, if I enter something else, like a letter (I
want to
trap typing errors), upon subsequent calls of the procedure, the "Get
(Input)"-
Statements are no longer executed (the Put-Statements immediately in
front of 
them are executed), so I cannot enter a number anymore. And strange
enough, the
Put_Line-statement in the exception handler is not executed in the first
code
snippet, but leads to an infinite loop in the second one.
I already thought of doing a "Flush;" in the exception handler, but that
did not
work either.

Does anyone know why that strange behaviour occurs, why after giving
wrong Input
the "Get (Input);"s are no longer executed upon calling the procedure
again and
why the exception is not handled as wanted???
Can anyone tell me, where I made a mistake and how the code must be
written so
that I can keep asking for a number if I mistyped???

I would appreciate any hints!!!

Thanks in advance!!!

Andreas

P.S. Could you please also send answers via e-mail to: Amun_Ra72@gmx.net
Thanks!