On Mon, 24 Jul 2006 11:18:10 -0400, "Wade Wall" <[EMAIL PROTECTED]> wrote:
> Hi all, I have a matrix of 474 rows (samples) with 565 columns > (variables). each of the 474 samples belong to one of 120 groups, with > the groupings as a column in the above matrix. For example, the group > column would be: > 1 1 1 2 2 2 . . . 120 120 > I want to randomly select one from each group. Not all the groups have > the same number of samples, some have 4, some 3 etc. Is there a > function to do this, or would I need to write a looping statement to > look at each successive group? I use the following for that (some of it hacked from help("sample")): ".resample" <- function(x, size, ...) { if(length(x) <= 1) { if(!missing(size) && size == 0) x[FALSE] else x } else sample(x, size, ...) } "randpick" <- function(x, by, size = 1, ...) { nx <- seq(nrow(x)) ind <- unlist(tapply(nx, by, .resample, size, ...)) x[nx %in% ind, ] } So, for instance: R> randpick(Indometh, Indometh$Subject, 3) Subject time conc 2 1 0.50 0.94 7 1 3.00 0.12 11 1 8.00 0.05 15 2 1.00 0.70 16 2 1.25 0.64 19 2 4.00 0.20 25 3 0.75 1.16 29 3 3.00 0.22 32 3 6.00 0.08 34 4 0.25 1.85 43 4 6.00 0.07 44 4 8.00 0.07 48 5 1.00 0.39 54 5 6.00 0.10 55 5 8.00 0.06 58 6 0.75 1.03 64 6 5.00 0.13 65 6 6.00 0.10 R> randpick(Indometh, Indometh$Subject, 2) Subject time conc 8 1 4.00 0.11 10 1 6.00 0.07 14 2 0.75 0.71 20 2 5.00 0.25 23 3 0.25 2.72 28 3 2.00 0.39 39 4 2.00 0.40 43 4 6.00 0.07 48 5 1.00 0.39 52 5 4.00 0.11 57 6 0.50 1.44 66 6 8.00 0.09 The 'by' argument allows to sample within any combination of factors desired. Cheers, -- Seb ______________________________________________ R-help@stat.math.ethz.ch 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.