Handling very large integers with factorial and combinat (nCm)
On Mon, 4 Apr 2005, Marco Chiarandini wrote:
Dear list, perhpas this question is more suitable for R-dev but since I am not really a developer I post it here first. Apparently the following lines do not create any problem in R: library(combinat) r <- 20; b <- 2; sum( sapply(0:r,function(x) nCm(r,x)^(2*b)) ) > 2^64 while in C I obtain an overflow of data even using unsigned long long and with long double I incurr in precision problems. Where can I find information about how R (or the combinat package) handles very large integer numbers?
In this case, as doubles. R numeric variables are doubles, and 'r' and 'b' are numeric, not integer. However,
r <- as.integer(20); b <- as.integer(2) sum( sapply(0:r,function(x) nCm(r,x)^(2*b)) )
gives the same result (and internally nCm computes in doubles: Note that factorials are computed via lgamma).
Brian D. Ripley, ripley at stats.ox.ac.uk Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/ University of Oxford, Tel: +44 1865 272861 (self) 1 South Parks Road, +44 1865 272866 (PA) Oxford OX1 3TG, UK Fax: +44 1865 272595