[R] Avoiding another loop
Prof Brian D Ripley
ripley at stats.ox.ac.uk
Mon Jan 29 19:16:06 CET 2001
On Mon, 29 Jan 2001, Michael Roberts wrote:
> Is there a fancy way I can use a function (like apply)
> to avoid a the follwoing loop, or otherwise make this more
> efficient? It fills a 3-d array, one matrix at a time.
> "nsize" is big, and efficiency is important.
> for (i in 1:nsize) simy[,,i] <- myfunction(p1,p2, ...)
You will need to tell us a lot more. At present it appears that you are
doing the same calculation nsize times, which is of course easy to avoid.
Avoiding loops in R is by no mean always useful or effective. There are
myths about that go back to long-forgotten versions of S, in which for
loops were very much to be avoided. Up to a point *vectorization* is
worthwhile, the point being where handling large vectors becones expensive.
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 272860 (secr)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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