[R] How sample without replacement on more than one variables?
Erik Iverson
eriki at ccbr.umn.edu
Sun May 23 09:33:23 CEST 2010
thmsfuller066 at gmail.com wrote:
> Hello All,
>
> sample() only sample on one variable x. But I'm interested in sampling
> more than one variable without replacement.
>
> Suppose I have 3 vectors x, y, z. I want to draw samples from all
> three vectors such that the combination of the three elements in each
> draw is not the same as any previous draws. I could use expand.grid to
> generate a vector out of the three vectors. But when the number of
> vectors are large and the number of elements in some vectors are
> large, it will be infeasible to do so.
>
> If you know there is a method on sampling on more than one variables,
> would you please let me know? Thank you!
>
Can you give a reproducible example? Since you suggested the method that is
most reasonable, but it will not work in large cases, I suppose you'll have to
draw independently from each vector one at a time, then somehow concatenate the
results, perhaps as a character vector, even if the vectors are, say, integers.
Then repeat this process checking each time if your new vector is %in% the
vector.
There may be a much better way, too, see if anyone else responds.
Also, you'll have to think about what a unique sample is.
If
x <- 1:3
y <- 2:4 ,
is x = 2, y = 3 the same as x = 3, y = 2?
Good luck,
Erik
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