Thank you all for your helpful comments and suggestions. Both proper indexing and subsetting a random sample of 300 work well.
Best wishes, Cesar On 2011-03-07, at 5:31 PM, <rex.dw...@syngenta.com> <rex.dw...@syngenta.com> wrote: Cesar, I think your basic misconception is that you believe 'sample' returns a list of indices into the original vector. It does not; it returns actual elements of the vector: > sample(runif(100),3) [1] 0.4492988 0.0336069 0.6948440 I'm not sure why you keep resetting the seed, but if it's important, replace d2<-d1[-i] with d2<- setdiff(d1,i) Otherwise Duncan's suggestion is must nicer: s = sample(d1,300,replace=FALSE) s1 = sort(s[1:100]) s2 = sort(s[101:200]) s3 = sort(s[201:300]) If what you actually need are indices into the original vector, replace d1 with length(d1). (When you say 'distinct', I'm assuming you mean 'disjoint'.) -----Original Message----- From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] On Behalf Of Duncan Murdoch Sent: Monday, March 07, 2011 3:52 PM To: Cesar Hincapié Cc: r-help@r-project.org Subject: Re: [R] generate 3 distinct random samples without replacement On 07/03/2011 2:17 PM, Cesar Hincapié wrote: > Hello: > > I wonder if I could get a little help with random sampling in R. > > I have a vector of length 7375. I would like to draw 3 distinct random > samples, each of length 100 without replacement. I have tried the following: > > d1<- 1:7375 > > set.seed(7) > i<- sample(d1, 100, replace=F) > s1<- sort(d1[i]) > s1 > > d2<- d1[-i] > set.seed(77) > j<- sample(d2, 100, replace=F) > s2<- sort(d2[j]) > s2 > > d3<- d2[-j] > set.seed(777) > k<- sample(d3, 100, replace=F) > s3<- sort(d3[k]) > s3 > > D<- data.frame(a=s1,b=s2,c=s3) > > > However, s2 is only 97 elements long, and s3, only 96 long. > > I would appreciate any suggestions on a better approach. > I'm also curious to know why my second and third samples are less than 100 > elements in length. If you want 3 non-overlapping, non-repeating samples of 100, why not draw one sample of 300, and take 3 subsets of it? The reason you were finding shorter samples is because you were using j and k as indices into vectors d2 and d3 that didn't have enough elements, and then you sorted the result, losing the NAs. For example, d2 <- 1:10 d2[10:12] sort(d2[10:12]) See ?sort for an explanation of how to keep NA values when you sort. Duncan Murdoch > Thanks for your time and consideration, > > Cesar A. Hincapié, DC, MHSc > > Research Fellow, Division of Health Care and Outcomes Research, Toronto > Western Research Institute > PhD Candidate in Epidemiology, Dalla Lana School of Public Health, University > of Toronto > e. cesar.hinca...@utoronto.ca > > > > > > [[alternative HTML version deleted]] > > > > ______________________________________________ > R-help@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. message may contain confidential information. If you are not the designated recipient, please notify the sender immediately, and delete the original and any copies. Any use of the message by you is prohibited. ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.