Re: Swarm Futures re-cap

[Andy Cleary]

At 06:04 PM 10/15/2002 +0100, you wrote:

> > I'd be interested to hear how other people deal with floating points.

This is an entire academic discipline, basically. But for this kind of 
purpose, a little common sense is probably going to net you more bang for 
buck than learning an entire field... The comments below (about rounding 
etc) are good places to start: if you are doing a stochastic simulation 
with lots of error in its formulation, it doesn't make sense that your 
simulation should depend on the last digit of a floating point number. 
Round them and then do your logic. Also avoid things like testing for 
equality of floating point numbers; as you've seen, floating point 
arithmetic does not necessarily commute. Despite a background in these 
issues, I tend to think that only a very small community of people should 
actually know all that much about floating point arithmetic and that 
everyone else should be able to program largely without consideration of 
the details (don't tell my advisor that though).


> I've never needed more than 5 or 6 decimal places of precision, and so I 
> just floor, round, or ceiling as required.  That's probably not what you 
> wanted to know, though.
>
> IIRC, there's a GNU library that provides arbitrary precision arithmetic.

Do NOT go this route if you care about performance at all. These kinds of 
packages generally achieve extra precision by performing arithmetic 
operations in software rather than hardware.

My 2c,
Andy Cleary

============================================
Andrew J. Cleary, Software Engineer/Computational Scientist
Lawrence Livermore National Labs, 925-424-5890.
============================================




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