[TAG] Paul Sephton's article "A Question Of Rounding"

[Ben Okopnik]

On Wed, Oct 03, 2007 at 10:59:43PM +0200, Paul Sephton wrote:
> On Wed, 2007-10-03 at 15:47 -0500, Ben Okopnik wrote:
> > On Wed, Oct 03, 2007 at 10:04:15PM +0200, Ren???? Pfeiffer wrote:
> > > 
> > > Floating point numbers aren't exact.
> > 
> > [blink] Pardon my ignorance, but... what's the use of them, then?
> > Particularly since the rounding can happen (essentially arbitrarily) in
> > any direction?
> 
> FP numbers are extremely useful for performing calcs very accurately.
> Yes, there's a loss in accuracy with calcs, but they are still very
> convenient.  Rounding is always consistent according to the currently
> selected IEEE rounding mode which averages out error.

Ah. That makes sense. I was visualizing long calculations where the
inaccuracy just kept building up. 

>  Effectively, as
> long as you need less than 16 significant decimals you are cooking.

No trouble for me - I usually get along with three.

> > Also, given the above, _is_ there a way of producing a meaningful
> > fixed-point part of a number with precision?
> 
> Yes, there is a way to produce a meaningful fixed point representation
> from the IEEE binary as per the code listing in the article.

As long as you use C, that is. :) Thanks, Paul!


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* Ben Okopnik * Editor-in-Chief, Linux Gazette * http://LinuxGazette.NET *