On 17/09/2014 3:46 PM, Giovanni Petris wrote:
Hi Duncan,You are right. The idea of the derivation consists in 'throwing' k placeholders ("*" in the example below) in the list of the individuals of the population. For example, if the population is letters[1:6], and the sample size is 4, the following code generates uniformly a 'sample'. > n <- 6; k <- 4 > set.seed(2) > xxx <- rep("*", n + k) > ind <- sort(sample(2 : (n+k), k)) > xxx[setdiff(1 : (n+k), ind)] <- letters[seq.int(n)] > noquote(xxx) [1] a b * c d * * e f * This represents the sample (b, d, d, f). I am still missing the "all" I need to do that you mention, that is how I can transform the vector xxx into something more readily usable, like c(b, d, d, f), or even a summary of counts. I guess I am looking for a bit of R trickery here...
I think this works, but you'd better check! Sample the placeholders: ind <- sort( sample(n + k -1, n-1) ) # I don't think sort() is necessary... Add placeholders at the start and end: ind <- c(0, ind, n+k) Take the diffs, and subtract one: diff(ind) - 1 I think this gives the counts you want. Duncan Murdoch ______________________________________________ [email protected] 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.
