[Openmcl-devel] roundings...

[Bill St. Clair]

Joakim,

Please send an example of a .5 case that doesn't round to the even number.

Your example is running into the limit of resolution of single floats.
5.4999999 IS 5.5, so it rounds to 6:

CL-USER> 5.4999999
5.5
CL-USER> (round *)
6
-0.5
CL-USER> (= 5.4999999 (+ 5 1/2))
T
CL-USER> 5.4999999d0
5.4999999D0
CL-USER> (round *)
5
0.4999998999999997D0

Note the rounding error in the final result. Almost, but not quite
0.4999999 . So it is with floats. If you need precise math, use
RATIONALs (INTEGERs and RATIOs).

If your C code works differently, are you sure you're using single
floats there? May I see it?

-Bill

On Fri, Sep 7, 2012 at 4:30 AM, Joakim Sandgren
<joakim at joakimsandgren.com> wrote:
> Hi,
>
> how can I do a more 'c-like' rounding in lisp ?
> that is in the .5 cases not always go to the even number.
> i would like to have it to round up
> and perhaps have better float resolution ? in lisp: (round 5.499999) => 5 (round 5.4999999) => 6
>
>
>
> are there some way around this?
>
> sincerely
> joakim
>
>
>
>
>
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