From: Roy Pardee <rpardee@...>
Date: 2009-03-17T11:57:35+09:00
Subject: Re: upping numerical precision

On Mar 16, 2:06 pm, Siep Korteling <s.kortel...@gmail.com> wrote:
> rpar...@gmail.com wrote:
> > Hey All,
> (...)
> >   chi = -2.0 * Math.log(p)
>
> > I'm finding that p is often going to 0.0 b/c the numbers returned by
> > calc_prob are sometimes outlandishly small
> (...)
> > That works, but makes me wonder if there's a smarter thing to do w/
> > those rational and mathn libs to really get the effect I hoped for
> > just from including them in my script.
>
> > Is there?
>
> > Many thanks!
>
> > -Roy
>
> Are you sure you required 'mathn' before defining your calc_prob method?
>
> big = 10**100
>
> small = 1/big
> p small.zero? # true
>
> require 'mathn'
> small = 1/big
> p small.zero? # false
> p small.class # Rational
>
> p -2.0*Math.log(small)
>
> hth,
>
> Siep

Thanks for the response!  I think the issue may be that I'm not doing
any division--just multiplication.  Check it out:

  irb(main):001:0> require 'mathn'
  => true
  irb(main):002:0> x = 0.5
  => 0.5
  irb(main):003:0> 1000.times do
  irb(main):004:1* x *= x
  irb(main):005:1> end
  => 1000
  irb(main):006:0> x
  => 0.0
  irb(main):007:0> x.class
  => Float
  irb(main):008:0>

But the more I think about it, the more I think I'm fussing over
nothing (ha ha!).  I think if my p var goes to zero, I should just set
it = Float::MIN & break out of that loop.  My calc_prob method will
only ever return values <= 1, so there's no sense in letting it
continue to spin down the value of p (if you can tell what I'm trying
to say).

Thanks!

-Roy