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Zero does not equal zero et al.
*To*: mathgroup at smc.vnet.net
*Subject*: [mg31586] Zero does not equal zero et al.
*From*: "Alan Mason" <swt at austin.rr.com>
*Date*: Wed, 14 Nov 2001 03:42:08 -0500 (EST)
*Organization*: Road Runner - Texas
*Sender*: owner-wri-mathgroup at wolfram.com
Hello,
I would like to comment on some of the posts in the thread "zero does not
equal zero". I agree completely with Richard Fateman and am disappointed
that Dan Lichtblau "passed" on addressing the question of whether
Mathematica's fuzzy handling of floating point makes such programming
mainstays as x==y and x===y untenable. This is not a minor point but a
fundamental design issue. I continue to think that 1.0 = = 1 returning True
is an outright error, because it trashes the pattern matcher and offers no
defense against propagation of errors -- how fuzzy does something have to
get before it becomes intolerable? At the very least, why not introduce a
new symbol, such as =N=, to test for floating point "equality". The meaning
of == is sacrosanct and should not be sullied by contact with the dirty
linen of floating point arithmetic, vagaries of hardware representations,
etc. I also think Mathematica's adoption of a "least common denominator"
approach to comparing floats with other floats or even with what are
supposed to be exact numbers (like 1 as opposed to 1.0) is objectionable.
Thinking probabilistically, if 1.0 is the result of an experimental
measurement, there is zero chance that 1.0 can exactly equal 1, so why
confound them?
Concerning Fateman's comment (in Re Re Zero does not equal Zero):
> Another choice is to test, and reset the
> accuracy of numbers at critical points.
> Realize, for example, that certain convergent
> iterations will not produce more accurate
> answers (as is normal), but will produce
> vaguer answers at each step because of Mathematica's
> arithmetic. They will terminate not when answers
> are close, but when they are essentially unknown.
>
This is a major hazard, but at least a way to defeat this behavior-- by
setting $MinPrecision=16 (say)-- is discussed in the Mathematica Book. This
device obviates the need to reset accuracy at various critical points (how
tedious!) and is crucial, as otherwise with root-finding algorithms (e.g.)
Mathematica will return a result with zero precision. This is because those
algorithms rely on the fact that although approximate roots of F[x] = 0 are
accurate only to the square root of the working precision, when you feed the
approximation back to F in the iterative loop you regain the lost precision;
there is no way for Mathematica to know this, so it assumes the worst-case
scenario of progressive degradation of accuracy. I have found Mathematica's
high-precision arithmetic, fuzzy or not, to be useful for empirical error
analysis.
But even leaving aside the question of whether Mathematica's treatment of
arbitrary precision floats is sound numerically, and serious doubts have
been raised, it's far from obvious (to me) that floating point arithmetic as
currently implemented in Mathematica is compatible with Mathematica's
essential purpose as a symbolics package.
Alan
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