[R] Ratio of huge products
David Winsemius
dwinsemius at comcast.net
Thu Dec 15 20:07:51 CET 2011
On Dec 15, 2011, at 1:35 PM, Alberto Magni wrote:
> Hello everybody,
>
> I have to compute something in this form:
>
> x = prod(a:b) / prod(c:d), where: a < c and b < d and obviously: a
> < b and c < d
>
> I cannot make assumptions on the relative position of c,b and a,d.
>
> The problem is that a,b,c,d are large and the products are huge (R
> return Inf).
Well, R does have some limitations.
> Their ratio is less than 1 but significantly higher than 0: it is a
> non-tiny probability.
>
> I need to find a way to simplify this ratio.
x <- exp( sum(log(a:b)) -sum(log(c:d)) )
> The only way to solve this that I see is to decompose into prime
> factors all the
> numbers in the numerator and the denominator and to remove the ones
> in common
Ewww. That does sound painful.
--
David Winsemius, MD
West Hartford, CT
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