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From: m5d@bobkat.UUCP
Newsgroups: comp.lang.c
Subject: Re: C and Floating Point
Message-ID: <843@bobkat.UUCP>
Date: Tue, 7-Apr-87 09:23:28 EST
Article-I.D.: bobkat.843
Posted: Tue Apr  7 09:23:28 1987
Date-Received: Sat, 11-Apr-87 00:31:56 EST
References: <15958@sun.uucp> <5716@brl-smoke.ARPA> <14680@cca.CCA.COM>
Reply-To: m5d@bobkat.UUCP (Mike McNally (Man Insane))
Organization: Digital Lynx, Inc; Dallas, TX
Lines: 30
Keywords: C Fortran Floating Point

In article <14680@cca.CCA.COM> g-rh@CCA.CCA.COM.UUCP (Richard Harter) writes:
>
>Doug, much as we all respect your knowledge of C and Unix, this
>just doesn't wash.  The essence of the problem is that floating point
>arithmetic is commutative but not associative.  From a mathematicians
>viewpoint 'x + y + z', where x, y, and z are floating point numbers
>and '+' is floating point addition, is not well defined.  I repeat,
>NOT WELL DEFINED.  ...
>
>Richard Harter, SMDS Inc. [Disclaimers not permitted by company policy.]

Not well defined for floating point (machine) arithmetic, but perfectly
well defined for abstract real numbers.  When a math guy writes down an
equation on the blackboard, he thinks in terms of real numbers, not
floating point.  When he decides to incorporate the equation into a 
computer program, he is then faced with the problem of the discrete
nature of all machine arithmetic.  If he is using FORTRAN, he will know
that parentheses have a certain meaning.  He writes code accordingly.
If he is using, C, he writes code accordingly, perhaps by breaking 
expressions into lists of simple expressions.  I think this is directly
analogous to the problem of developing a sorting algorithm on paper,
then implementing it: if it is done in LISP, it will be very different
from an implementation in C.


-- 
Mike McNally, mercifully employed at Digital Lynx ---
    Where Plano Road the Mighty Flood of Forest Lane doth meet,
    And Garland fair, whose perfumed air flows soft about my feet...
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