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Re: Troubling Bug involving RandomReal, NestList, and Table

  • To: mathgroup at smc.vnet.net
  • Subject: [mg86261] Re: Troubling Bug involving RandomReal, NestList, and Table
  • From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
  • Date: Fri, 7 Mar 2008 02:27:21 -0500 (EST)
  • References: <fqo8tn$t2o$1@smc.vnet.net>

Hi,

this is a bug when Mathematica "Compile" the expression
in Table[] try

opts = "CompileOptions" /. SystemOptions[];
opts = opts /. ( "TableCompileLength" -> _) :> (
      "TableCompileLength" -> Infinity);
SetSystemOptions["CompileOptions" -> opts];

and the bug disappear.

Regards
   Jens


jwmerrill at gmail.com wrote:
> When making tables of 250 or more directions, Mathematica seems to
> forget how to do arithmetic.  I realize these "random" directions are
> not properly uniform, but that's beside the point.
> 
> $Version
>> "6.0 for Mac OS X x86 (32-bit) (April 20, 2007)"
> 
> randomDirection3[] :=
>  NestList[Sqrt[1 - #^2] &, RandomReal[], 1]
> 
> ListPlot[Table[randomDirection3[], {249}], AspectRatio -> Automatic]
> ListPlot[Table[randomDirection3[], {250}], AspectRatio -> Automatic]
> 
>> [Graphs you'll need to generate for yourself]
> 
> Variance[Table[Norm[randomDirection3[]], {249}]]
> Variance[Table[Norm[randomDirection3[]], {250}]]
> 
>> 1.49104*10^-33
>> 0.0513843
> 
> What!?!
> 
> Jason Merrill
> 


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