Re: Numeric problem in BlackBox 1.4

From: [at]} <robert.d.campbell{>
Date: Tue, 18 Feb 2003 11:35:00 +0000 (GMT)

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> Robert and Dominik:
>
> testing REALs for equality is in fact advised against
> in every programming textbook, precisely because of the
> small differences that can get lost along the way.
> The textbooks would rather suggest the test of the form
> IF (ABS (real1 - real2) < epsilon) THEN
> where the epsilon is a constant considered insignificant
> in a given application.


The 'rule' above, like all programming rules (except this one),
should be regarded as a guideline. Testing REALs for equality
should only be done when you have deliberately controlled the
state of the LSBit of the binary in your application.

The problem above is what value of epsilon should I use.

A good solution (in its appropriate context) is the following
line of code

"if (abs(e(m))+dd.eq.dd) goto 2"

taken from Numerical Recipes in FORTRAN 77 2'nd edition
page 474 line 9.

If fact this is the exception that 'proves' several rules!

Cheers

Robert

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Received on Tue Feb 18 2003 - 12:35:00 UTC