[R] Generating unordered, with replacement, samples
Duncan Murdoch
murdoch.duncan at gmail.com
Wed Sep 17 22:02:24 CEST 2014
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
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