On Thu, Dec 15, 2011 at 10:35 AM, Alberto Magni <alberto.magn...@gmail.com> 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). > 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. > 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 > > Do you know a better way to do this ?
Yes, exp( sum(log(a:c))-sum(log(b:d)) ), which is mathematically exactly equivalent, unless I made a typo. Remeber that log of a product is a sum of the logs of the arguments. Peter. ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
