Rick Taube <[EMAIL PROTECTED]> writes:
> this behavior of inexact->exact in guile 1.7.1 causes my program to
> become confused:
>
> guile> (inexact->exact 48.0)
> 48
> guile> (inexact->exact (* .1 480.0))
> 48
> guile> (inexact->exact (* (- 1.1 1) 480.0))
> 3377699720527875/70368744177664
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
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