Re: LOGO-L> floating point errors in MSWLogo

[George Mills <mills@softronix.com>]

I hope you received the many replies.

In addition you can try working in integer mode to minimize
floating point errors. This of course may not always be applicable
to your porblem. It is an "art" learning how to manage floating point
errors.

to  atest
(local "times "number)
make "number 0
for [j 1 6 1 ]~
  [ make "times power 10 :j
    type [times -> ]
    print :times
    for[i 1 :times 1]~
        [make "number sum :number 1]
    type [result -> ]
    print :number / 100
    make "number 0
  ]
end


jstclair@omsd.cerf.fred.org wrote:
> 
> Please reply to agauvin@umcs.maine.edu or logo-l@gsn.org NOT me.  I'm
> just re-posting Tony Gauvin's bounced message.
> 
> -John
> --Message below re-posted by <jstclair@omsd.cerf.fred.org>
> Date - Fri, 21 Nov 1997 16:15:34 -0800
> >From - owner-logo-l@gsn.org
> To - logo-l-mod
> Subject - BOUNCE logo-l@gsn.org:  Admin request: /^subject:\s*help\b/i
> >From jstclair@cyberg8t.com  Fri Nov 21 16:15:33 1997
> Received: from gandalf.umcs.maine.edu (GANDALF.umcs.maine.edu [130.111.112.21])
>         by gsn.org (8.8.8/8.8.8) with SMTP id QAA07681
>         for <logo-l@gsn.org>; Fri, 21 Nov 1997 16:15:33 -0800
> Received: from agauvin.umcs.maine.edu (AGAUVIN.umcs.maine.edu [130.111.112.59]) by gandalf.umcs.maine.edu (8.6.12/8.6.12) with SMTP id TAA24058 for <logo-l@gsn.org>; Fri, 21 Nov 1997 19:15:52 -0500
> Message-Id: <199711220015.TAA24058@gandalf.umcs.maine.edu>
> Comments: Authenticated sender is <agauvin@[130.111.112.21]>
> From: "Anthony Gauvin" <agauvin@umcs.maine.edu>
> To: logo-l@gsn.org
> Date: Fri, 21 Nov 1997 19:15:54 -0500
> MIME-Version: 1.0
> Content-type: text/plain; charset=US-ASCII
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> Subject: Help - floating point errors in MSWlogo
> Reply-to: agauvin@umcs.maine.edu
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> 
>    I recently wrote a Molecular dynamics simulation in MSW LOGO and
> ran into a problem in how LOGO handles floating points numbers. One
> of the calculations in the program was
> 
>     make "time sum :delta_t :time
> 
> Adding a delta_t of  0.02 to a time of  0  1640 times gave me a
> value of time that was 32.7799999999995 , that is a small error but
> an error none the less. The problem is that with more iterations of the
> program  the error grows exponential and after  a hour or so
> of running the simulations the values walk straight into the
> ether.
>  This poses a severe limitations on the type
> and scale of models that can be constructed in LOGO. I realize that
> I'm probably pushing LOGO beyond it intended domain but you except
> the correct answer from a computer
> 
>   As a test of this error, I wrote the following program
> 
> to  atest
> (local "times "number)
> make "number 0
> for [j 1 6 1 ]~
>   [ make "times power 10 :j
>     type [times -> ]
>     print :times
>     for[i 1 :times 1]~
>         [make "number sum :number 0.01]
>     type [result -> ]
>     print :number
>     make "number 0
>   ]
> end
> 
>   Running the program produces the same wrong results on two different
> machines (one WIN95, the other WINNT).
> ********************************************************************
> 
>   Tony Gauvin                       (207) 581-1367
>   Graduate Assistant                 agauvin@umcs.maine.edu
>   109 Neville Hall
>   Dept of Computer Science
>   University of Maine
>   Orono Me 04469
> 
>           HTTP://www.umfk.maine.edu/hpage/tony/menu.htm
> 
> ******************************************************************************
> 
> 
> 
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-- 
===============================================================
George Mills
email: mills@softronix.com
http://www.softronix.com
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