Re: Semantic of the mod operator in Ada 83

["Jeff C," <jcreem@yahoo.com>]

"SPHINX" <sphinx.moro@wanadoo.fr> wrote in message
news:464ad5ee.0403060145.6c03b8d9@posting.google.com...
> I just wander what is the semantic of
> What is the exact semantic
> of the function mod  in that case?
> -- Compiler is gnat 3.15
> -- modular types are not used, because the mode shall be Ada83
>
> with text_io;
> use  text_io;
> procedure counter is
>   Size : constant := 32;
>   type Counter is range 0 .. (2 ** 31) - 1;  -- = 0..2147483647
>   for  Counter'Size use Size;
>
> V1  : Counter := Counter'LAST;
> V2  : Counter := Counter'LAST;
> R   : Counter := 0;
> Z   : Counter := 0;
> function Add (Val1 :in Counter;Val2 :in Counter) return Counter is
> begin
>   return ((Val1+Val2) mod Counter'Last);
> end Add;
> package counter_io is new integer_io(Counter);use counter_io;
> begin
> put("V1="); counter_io.put(V1);new_line;
> put("V2="); counter_io.put(V2);new_line;
> R := Add (V1, V2);
> put("R=");counter_io.put(R);new_line;
> --Result for GNAT 3.15 => R=-2
> put("LAST=");put(Counter'Last);new_line;
> R := V1+V2;
> put("R=");counter_io.put(R);new_line;
> Z := R;
> put("Z=");counter_io.put(Z);new_line;
> end counter;
>
> And why there is no containt error on the direct additions
> Thanks for the help



I am a little confused about what you are asking given the title and the
message contents but I'll take a
shot.

One thing you are asking is why is there no constraint error on the
addition. As it turns
out there is. This code is not really valid and does not do what it thinks
it does. The reason
you are not getting a contraint_error is likely that you are compiling
without the -gnato flag.

For various reasons, the people that implemented GNAT decided to leave
integer overflow
checking OFF with the default set of compiler switches. Delete your .exe and
.o files and try
gnatmake -gnato counter

And see what you get (I don't have 3.15 installed but I suspect you will
indeed get your constraint error).

As for the semantic of the mod operator in Ada 83 (and 95).

Look  here

http://archive.adaic.com/standards/83rat/html/ratl-05-02.html

In general though it is not clear to me that using rem and mod with a right
hand side equal to
the type'last (especially when used as this code is doing) is going to do
anything useful at all.