Numeric problem with Ada

[CNAM.CNAM.FR!bortz@ucbvax.Berkeley.EDU]

(It seems that my first attempt to post this was a failure. I repost.
Sorry if you receive it twice.)

Hello, numeric addicts,

I've a strange problem with numeric expressions in Ada. The test
program (the original
 one was a calendar) is:

procedure TEST is
	Mz : integer := 7;
	Z2 : float;
	package FLOAT_TEXT_IO is new TEXT_IO.FLOAT_IO (float);
begin
	Z2 := 2.6 * float (Mz) - 0.2;
	FLOAT_TEXT_IO.PUT (Z2, Aft => 6, Exp => 0);
end TEST;        

The program displays 17.999998 (the mathematical result is 18). I
understand that, since
 float'DIGITS is 6 on all the machines I used (Vax/VMS and Sun3), I
cannot expect the
 rightmost digit (the 8th) to be correct. If I declare my own
floating-point type with 15
 digits, I can have my result (18) fine. But my problem comes from the
fact that the C
 compilers don't behave this way. The program:

main ()
{
	int mz = 7;
	float z2;
	z2 = 2.6 * mz - 0.2;
	printf ("%2.6f\n", z2);
}

displays 18.000000.
This is true also on both the Vax and the Sun3.
Now the problem is: where does the difference comes from? I've checked
the assembly code
 produced and the two programs use the same types (I was thinking that
C was using the
 double-precision multiplication, but it's not the case). I'm not sure the code
s are
 equivalent (it's too difficult for me to read assembly language), but I think 
they
 should.
Is there something in the Ada floating-point model that prevents the
Ada compiler to give
 better results?



Stephane Bortzmeyer           Conservatoire National des Arts et Metiers
	
bortzmeyer@vcnam.cnam.fr      Laboratoire d'Informatique
bortz@cnam.cnam.fr            292, rue Saint-Martin			
                              75141 Paris Cedex 03
                              France	

"C'est la nuit qu'il est beau de croire a la lumiere." E. Rostand