On Thu, Jun 4, 2009 at 9:18 AM, Allan Engelhardt <all...@cybaea.com> wrote:
> I want to apply this code to a few tens of thousands of vectors so speed > does matter. In C or similar I would of course calculate the result with a > single pass through x, and not with three passes as in 'max2'. > So why not code it in C? See the R guide on writing extensions for how to link C code with R it's pretty easy especially if you know the size of the objects being passed around, then there's no messy memory management to deal with. Wikipedia has some algorithms, with an inevitable dose of Knuth: http://en.wikipedia.org/wiki/Selection_algorithm and apparently something like that is in the standard C++ library: "C++ also provides the partial_sort algorithm, which solves the problem of selecting the smallest k elements (sorted), with a time complexity of O(nlog k). No algorithm is provided for selecting the greatest k elements, but this can easily be achieved by inverting the ordering predicate." Barry ______________________________________________ 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.
