[R] Select exactly n elements of a vector

ripley@stats.ox.ac.uk ripley at stats.ox.ac.uk
Mon Aug 12 19:49:08 CEST 2002

On Mon, 12 Aug 2002, Kort, Eric wrote:

> 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.

That's part of sample().  sample(10,4, TRUE) selects 4 elements from 1:10.
(and the negation will extract the other sample).

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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