[R] Selecting a random sample

[Scot W McNary]

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?

Thanks in advance,

Scot


--
  Scot W. McNary  email:smcnary at charm.net   

-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-
r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html
Send "info", "help", or "[un]subscribe"
(in the "body", not the subject !)  To: r-help-request at stat.math.ethz.ch
_._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._