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From: fyodor@sally.nas.nasa.gov (Chris L. Kuszmaul)
Subject: Re: floating point comparison
Date: 1997/08/06
Message-ID: <5sar4r$t7m$1@cnn.nas.nasa.gov>#1/1
X-Deja-AN: 262657730
References: <33E61497.33E2@pseserv3.fw.hac.com>
 <33E6D359.3EF4@imbi.uni-freiburg.de> <33E74A62.53E2@pseserv3.fw.hac.com>
Organization: NAS - NASA Ames Research Center, Moffett Field, CA
Newsgroups: 
 comp.lang.ada,sci.math.num-analysis,comp.software-eng,comp.theory,sci.math
Date: 1997-08-06T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <33E74A62.53E2@pseserv3.fw.hac.com> "W. Wesley Groleau x4923" <wwgrol@pseserv3.fw.hac.com> writes:
> 
>What I'm trying to say is that it should be possible to come up with a 
>few simple guidelines that cover most cases, and a guideline for 
>identifying the cases that require a mathematician--excuse me, a 
>numerical analyst.
>


  Fair enough. There are two basic guidlines I follow:

1: Do not test for equality between floating point values. If you do, then
you better get a numerical analysit, or figure out how to avoid using floating
point, or see if you can use a 'close to equal' check.

2: Watch out when you sum or difference floating point numbers of similar
value. You can introduce arbitrarily large errors relative to the resulting
value. ((a-b) - a) + b can turn up as equal to b if a is so much larger than b
that subtracting it has no effect.

Also

3: If you just do multiplication and division, then it is hard to mess up.


CLK