Re: Restarting Tread: Why isn't this program working with Unchecked_Converstion

[Martin <martin.dowie@btopenworld.com>]

On Jan 16, 3:11 pm, Ludovic Brenta <ludo...@ludovic-brenta.org> wrote:
> On Jan 16, 3:11 pm, ChristopherL <clusard...@aol.com> wrote:
>
>
>
> > Thanks Adam, yours is a working solution, see below.
>
> > Thanks everyone,
> > Chris L.
>
> > On Jan 15, 6:27 pm, Adam Beneschan <a...@irvine.com> wrote:
>
> > > I think you'll have to force the compiler to work with 8-bit types
> > > instead of 10-bit types.  Since you can't put a 'Size clause on a
> > > subtype of Integer, you'll need to declare new types:
>
> > >    type Short_Integer1 is range 0 .. (2**8)-1;   --not (2**10)-1
> > >    for Short_Integer1'Size use 8;
>
> > >    type Short_Integer2 is range -(2**7) .. (2**7)-1;
> > >    for Short_Integer2'Size use 8;
>
> > > Or, if Short_Integer2 *has* to be a subtype, then declare a new type
> > > with a different name, force its size to be 8, and do a type
> > > conversion from that new type to Short_Integer2 afterwards.
>
> > with Unchecked_Conversion;
>
> > with Integer_Text_IO ;
>
> > procedure test is
> >   type Short_integer1 is range 0.. ((2 ** 8) - 1);
> >   for Short_Integer1'Size use 8;
>
> >   subtype Short_integer2 is Integer range -(2 ** 7)..((2 ** 7) - 1);
>
> >   type Short_Integer3 is range -(2**7) .. (2**7)-1;
> >   for Short_Integer3'Size use 8;
>
> >   subtype A_Float is Float;
>
> >   subtype A_Natural is Natural range 0..((2 ** 8) - 1);
> >   -- The next line of code (if used) gives compilation error message:
> >   -- A_NATURAL does not denote a first subtype
> >   -- for A_Natural'Size use Short_integer1'Size;
>
> >   Size_Of_Float : integer := Float'Size;      --32 bits long
>
> >   Size_Of_short_integer: integer := Short_integer1'Size;--10 bits
> > long
> >   Size_Of_short_int: integer := Short_integer2'Size;--10 bits long
>
> >   Size_Of_Natural: integer := A_Natural'Size;       --8 bits long
>
> >   Arg     : A_Float;
> >   Result3 : Short_integer2;
> >   Result2 : Short_integer3;
> >   Result1 : Short_integer1;
>
> >   function Get_Bits2 is new Unchecked_Conversion (Source =>
> >     Short_integer1, Target => Short_integer3);
>
> > Begin
>
> > Arg := 200.0;
>
> > Result1 := Short_integer1(Arg);  -- Result1 becomes 200
> > Result2 := Get_Bits2 (Result1);
> > Result3 := Short_integer2(Result2);
> > Integer_Text_IO.Put ( Result3 ) ;
> > End test;
>
> OK, you've managed to change the floating-point value 200.0 into an
> integer value of -56. The bit representations of these two values are
> different. (That's because the explicit type conversion of Arg from
> type Float to type Short_integer1 changes the representation to
> preserve the value).
>
> I'm curious to understand: why did you do that?
>
> PS. I think you can simplify your code into:
>
> declare
>    type Unsigned_8 is mod 2 ** 8;
>    for Unsigned_8'Size use 8;
>    type Signed_8 is range - 2**7 .. 2**7 - 1;
>    for Signed_8'Size use 8;
>    function To_Signed_8 is new Ada.Unchecked_Conversion (Unsigned_8,
> Signed_8);
>
>    Arg : Float := 200.0;
>    Result : Signed_8 := To_Signed_8 (Unsigned_8 (Arg));
> begin
>    Ada.Text_IO.Put (Signed_8'Image (Result));
> end;
>
> --
> Ludovic Brenta.

Or using nothing but pre-defined standard types:

with Ada.Text_IO;
with Ada.Unchecked_Conversion;
with Interfaces;
procedure Test is
   function To_Integer_8 is
     new Ada.Unchecked_Conversion (Source => Interfaces.Unsigned_8,
                                   Target => Interfaces.Integer_8);

   Arg : Float := 200.0;
   Result : Interfaces.Integer_8 := To_Integer_8
(Interfaces.Unsigned_8 (Arg));
begin
   Ada.Text_IO.Put (Interfaces.Integer_8'Image (Result));
end Test;