Re: Floating point is like that... (was Re: Floating point error)

• Date: Wed, 13 Mar 1996 14:55:38 PST
• From:
  "Robert J. Brown" <rj@eli.wariat.org>
• CC: moo-cows@parc.xerox.com
• In-reply-to: <9603132208.AA06055@tom.eng.mvision.com>

>>>>> "Tom" == Tom Ritchford <tom@mvision.com> writes:

    Tom>     If programmers are assuming that they are going to get
    Tom> "exact" numbers with their floating point calculations, I
    Tom> fear that they will be sadly disappointed.  It's a really Bad
    Tom> Idea to test floating point numbers for equality -- you
    Tom> should always build in a little shrinkage into the tests, or
    Tom> else you will get a nasty surprise one day.  (ie, write abs(
    Tom> a-b ) < epsilon, rather than a == b, where epsilon is a small
    Tom> number.  If a and b can vary greatly in magnitude, try abs(
    Tom> a-b )/b < epsilon ((when b isn't zero of course!)))

    Tom>     Donald Knuth discusses this at great length in the Art of
    Tom> Computer Programming, vol 1.

    Tom>     /t

    Tom> Tom Ritchford tom@mvision.com, tom@weirdos.com

    Tom> Verge's "Little Idiot" -- Music for the mentally peculiar.
    Tom> 1-800-WEIRDOS http://www.fly.net/~tom/verge.html

CORRECTION: Knuth vol 1 is "Fundamental Algorithms"; vol 2 is the
            desired reference -- "Seminumerical Algorithms".

-- 
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• Floating point is like that... (was Re: Floating point error)
• From: tom@mvision.com (Tom Ritchford)

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