[R] Select exactly n elements of a vector
Kort, Eric
Eric.Kort at vai.org
Mon Aug 12 19:39:06 CEST 2002
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
-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-
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
_._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._
More information about the R-help
mailing list