Re: Fixed Point number mul, is it a bug?

[Adam Beneschan <adam@irvine.com>]

On Thursday, October 18, 2012 12:35:38 AM UTC-7, Vinzent Höfler wrote:
> kylix wrote:
> 
> 
> 
> > -- GNAT GPL 2012 
> 
> > with Ada.Text_IO;
> 
> > procedure FixPoint is
> >    type FP is delta 0.01 range 0.00 .. 99.99;
> >    -- type FP is delta 0.01 digits 4;
> >    x : FP := 0.01;
> > begin
> >    for i in 1 .. 5 loop
> >       x := x * 2;
> >       Ada.Text_IO.Put_Line("x =>" &
> >                            FP'Image(x)); 
> >    end loop;
> > end Fixpoint;
> 
> 
> 
> > In my machine, it yield results:
> 
> > x => 0.02
> > x => 0.03
> > x => 0.06
> > x => 0.13
> > x => 0.25
> 
> This is expected. The definition requests a fixed point type with a 
> delta of at least 0.01, most compilers would choose 2**-7 then, which 
> is 0.0078125 in decimal notation. Hence the results you see.

To elaborate a bit, the RM says that the 'Small of the type *must* be a power of two if not specified (3.5.9(8)).  The 'Small has to be no greater than the specified delta, and most compilers would choose the largest power of 2 that is <= the delta.  (This applies only to ordinary fixed-point types, not those with "digits" in the declaration.)  Shark8 showed how you can specify the 'Small, although that isn't completely portable because it's allowable for compilers not to let you specify a 'Small that isn't a power of 2 (3.5.9(21)).  

                              -- Adam