On Mon, 22 May 2000, Scot W McNary wrote:
>
> Hi,
>
> I''m trying to learn R and get an errand done at the same time. I
have a
> list of cases with ids numbered sequentially 1 to 403. I want to select
> at random exactly 20 without replacement. Here''s what
I''ve tried so far:
>
> > ids<-(1:403)
> > pick<-runif(403)
> > picklist<-as.data.frame(cbind(ids,pick))
> > who<-subset(picklist, pick<20/403)
> > who
>
> This works, but I can have anywhere from 16 to 32 cases in the subset
> ''who'', due to random variability in the runif generator.
What''s a more
> efficient (and general) way to extract a subset of exactly n from a total
> sample N?
sample(N, n, replace=F)
Easy? See ?sample for variations on the theme.
By the way, seq(N)[runif(N) < n/N] would have done it your way in
simpler code, and
seq(N)[rank(runif(N)) <= n]
is another general solution.
--
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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