Re: Non-deterministic numerical inaccuracies in Mathematica
- To: mathgroup at smc.vnet.net
- Subject: [mg94932] Re: [mg94908] Non-deterministic numerical inaccuracies in Mathematica
- From: DrMajorBob <btreat1 at austin.rr.com>
- Date: Wed, 31 Dec 2008 06:07:01 -0500 (EST)
- References: <200812301053.FAA13049@smc.vnet.net>
- Reply-to: drmajorbob at longhorns.com
I had no failures on my iMac Leopard system:
$Version
"7.0 for Mac OS X x86 (64-bit) (November 11, 2008)"
myMod := Module[{p},
Dot[{0.5018839897852984567457648, 0.544293499215831476457645674576,
0.67220312964800834576456745674567}, {-0.5359693986690249`, \
-0.41257352636613975`, -0.736559494563862`}]*{-0.5359693986690249`, \
-0.41257352636613975`, -0.736559494563862`} - {0.5579131132707782`,
0.2715072333003815`, 0.7842300557273234`}]
For[ii = 1, ii <= 250000, ii++,
If[myMod =!= myMod,
Print["Iteration " <> ToString[ii] <> " fails comparison "]];]
(no prints)
Bobby
On Tue, 30 Dec 2008 04:53:22 -0600, Sjoerd C. de Vries
<sjoerd.c.devries at gmail.com> wrote:
> Hi all,
>
> Last week Budgemano posted an issue which involved numerical
> inaccuracies in Mathematica 7. Due to the xmas season it hasn't
> received much attention, or so it seems.
>
> Meanwhile, I've been able to drill the problem down to the following
> code:
>
> myMod := Module[
> {p},
> Dot[{0.5018839897852984567457648, 0.544293499215831476457645674576,
> 0.67220312964800834576456745674567}, {-0.5359693986690249`, \
> -0.41257352636613975`, -0.736559494563862`}]*{-0.5359693986690249`, \
> -0.41257352636613975`, -0.736559494563862`} - {0.5579131132707782`,
> 0.2715072333003815`, 0.7842300557273234`}]
>
> For[ii = 1, ii <= 250, ii++,
> If[myMod =!= myMod,
> Print["Iteration " <> ToString[ii] <> " fails comparison "]];]
>
> As you can see the module myMod is called twice every iteration. Its
> calculations only involve constants, so the results should always be
> the same. However, on my system (Mathematica 7.WinXP) the results differ
> from one
> call to the other at seemingly random times.
>
> This phenomenon crucially depends on the presence of the local
> variable p (that isn't used!). With this variable it's there, without
> it isn't. In M6 everything is OK, so this is a bug specific to
> Mathematica 7.
>
> Could you check on the various platforms whether you can reproduce
> this?
>
> Cheers -- Sjoerd
>
--
DrMajorBob at longhorns.com