From: Sander Land <sander.land@...>
Date: 2009-03-18T07:39:20+09:00
Subject: Re: upping numerical precision

I don't think you need more precision. Basic math can help you here:
log(a*b) = log(a) + log(b)

so

logp=0
words.each do |w|
   logp += Math.log( calc_prob(w) )
end
chi = -2.0 * logp

On Mon, Mar 16, 2009 at 6:57 PM, rpardee@gmail.com <rpardee@gmail.com> wrote:
> Hey All,
>
> I got me this fancy method for classifying documents that basically
> does this at one point:
>
>  p = 1
>  words.each do |w|
>    p *= calc_prob(w)
>  end
>  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 (or there are just so many
> words in the doc that the loop runs long enough to zero out the
> variable p).  This causes problems for the call to Math.log of course
> (e.g., Errno::EDOM).
>
> I have tried two things.  First, after some desperate flailing on
> google I added:
>
>  require 'rational'
>  require 'mathn'
>
> to my script and hoped that ruby would read my mind WRT using
> rationals where possible & that rationals would extend the reach of
> ruby's arithmetic into the too-outlandishly-small-for-floats range.
>
> When that did not seem to avail me, I put this in my words.do loop:
>
>  if p == 0.0 then
>    p = Float::MIN
>  end
>
> 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
>
>