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From: eachus@spectre.mitre.org (Robert I. Eachus)
Subject: Re: floating point comparison
Date: 1997/09/05
Message-ID: <EACHUS.97Sep5165959@spectre.mitre.org>#1/1
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References: <33EA46CC.226@pseserv3.fw.hac.com>
 <871377938snz@nezumi.demon.co.uk> <33FE4D83.4DCD@pseserv3.fw.hac.com>
Organization: The Mitre Corp., Bedford, MA.
Newsgroups: 
 comp.lang.ada,sci.math.num-analysis,comp.software-eng,comp.theory,sci.math
Date: 1997-09-05T00:00:00+00:00
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In article <33FE4D83.4DCD@pseserv3.fw.hac.com> "W. Wesley Groleau x4923" <wwgrol@pseserv3.fw.hac.com> writes:

  > Let me put my original question another way.  Can we (as Sam 
  > suggested above) come up with something _at_the_level_of_ Ada 
  > Quality and Style that tells the "analysis-challenged" when to 
  > call for help--rather than telling them to never use floating point?

  For many calculations floating point offers convenience while
preserving more than enough numerical accuracy.  On other types of
calculations, fixed point accuracy is required.  (For example,
checking your bank statement can only be done correctly using fixed
point, and knowing the rules for computing interest on that account.)
If you can't tell which you should be using, get help fast.

   In general it is much more work to write fixed point than floating
point code, but especially in Ada, the error analysis is easy.  Errors
are only introduced by explicit conversions never by arithmetic
operations.  The much more work comes from defining the various types
and putting in the conversions.  It used to be difficult to use Ada
fixed point with some compilers, but in Ada 95, most vendors are
supporting the decimal types and 64-bit binary fixed point types.

   (If that sounds like I recommend using fixed point everywhere, far
from it.  If you are building flight control system, for example, and
your control laws are stable over the envelope, then go ahead and use
floating point.  But if you are inverting large matrices or solving
linear programming problems, then the real work needs to be done not
only in fixed point, but with appropriate scaling, rebuilding of
basises etc.)

--

					Robert I. Eachus

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