[R] Select exactly n elements of a vector

[Kort, Eric]

In discussing permutation tests, _Modern Applied Statistics With S Plus_ suggests this (IMHO) rather elegant approach to generating a distribution of t-tests:

d<- ...some data...
ttest <- function(x) mean(x)/sqrt(var(x)/length(x))
n<-1000
res<-numeric(n)
for(i in 1:n) res[i] <- ttest(x<-d*sign(runif(10)-0.5)

The problem is that the sign(runif) results in random permutation of the observations into two groups of random size.  For my analysis, however, I want to fix the size of the two groups.  So I wonder if someone can point me toward an elegant way to randomly split a vector into two groups of specified sizes.  Or, similarly, randomly select a specific number of elements from a vector.

Thanks,
Eric
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