Re: inexact->exact in 1.7.1

[Rick Taube]

Thank you, i was porting my code from 1.6.7 to the new version where it 
broke and I forgot about this. time for a m-x tags-search!
--rick


> This happens because Guile now has exact rationals.  This NEWS entry
> explains the behavior:
>
>     ** inexact->exact no longer returns only integers.
>
>     Without exact rationals, the closest exact number was always an
>     integer, but now inexact->exact returns the fraction that is 
> exactly
>     equal to a floating point number.  For example:
>
>         (inexact->exact 1.234)
>         => 694680242521899/562949953421312
>
>     When you want the old behavior, use 'round' explicitely:
>
>         (inexact->exact (round 1.234))
>         => 1
>
>
> In your case, "1.1" is not exactly 11/10 since IEEE double can not
> represent that number exactly.  You can write "#e1.1" instead, if you
> want an exact 11/10:
>
>     guile> #e1.1
>     11/10
>     guile> (inexact->exact (* (- #e1.1 1) #e480.0))
>     48
>
> A IEEE double can not represent 0.1 exactly either, and you just got
> lucky that (* 0.1 480.0) is exactly 48:
>
> guile> (- (* 0.1 480.0)
>           (* (- 1.1 1) 480.0))
> -4.2632564145606e-14