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§

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

[email protected] wrote:

Hey All,

(…)

chi = -2.0 * Math.log§

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

On Mar 16, 2:06 pm, Siep K. [email protected] wrote:

just from including them in my script.

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

On Mar 16, 10:56 pm, Roy P. [email protected] wrote:

irb(main):005:1> end

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

Roy,

It all depends on how much range of data you want. If you need more

granularity at the tiny end, you can always re-normalize… just

initialize p to be 1e6 or something, rather than 1. Then after the log

you can just subtract the constant exponent to get back to your

original range.

-t3ch.dude

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