R help - Mar 2011 - generate 3 distinct random samples without replacement

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.

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.hincapie@utoronto.ca





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would this work?

s <- sample(d1, 300, F)
D <- data.frame(a = s[1:100], b = s[101:200], c = s[201:300])
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r-help-bounces at r-project.org wrote on 03/07/2011 02:17:19 PM:
> [image removed] 
> 
> [R] generate 3 distinct random samples without replacement
> 
> Cesar Hincapi? 
> 
> to:
> 
> r-help
> 
> 03/07/2011 03:06 PM
> 
> Sent by:
> 
> r-help-bounces at r-project.org
> 
> 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.
> 
> Thanks for your time and consideration,
> 
> Cesar A. Hincapi?, 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.hincapie at utoronto.ca
> 
> 
> 
> 
> 
>    [[alternative HTML version deleted]]
> 
> ______________________________________________
> R-help at 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.

Cesar, your indexing is wrong:

On Mon, Mar 7, 2011 at 2:17 PM, Cesar Hincapi?
<cesar.hincapie at utoronto.ca> 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
d1 is a continuous vector of integers, 1 thru 7375 and of length 7375
> d2 <- d1[-i]
but you've taken out 100 of those numbers, so d2 is now of length
7275 and has gaps in the sequence.
> set.seed(77)
> j <- sample(d2, 100, replace=F)
> s2 <- sort(d2[j])
> s2
j is a sample *of the values* and those values are no longer the
indices of the vector d2

You need instead
j <- sample(1:length(d2), 100, replace=FALSE)
s2 <- sort(d2[j])

Some of the value in j no longer exist in d2 as indices. 7375 could be
selected, but since d2 only has 7275 elements d2[7375] doesn't
return anything (actually NA).

Same for your third sample, only the indices are even less like the
elements of the vector because you've removed another random
set of values.

Sarah


-- 
Sarah Goslee
http://www.functionaldiversity.org

On 07/03/2011 2:17 PM, Cesar Hincapi? 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. Hincapi?, 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.hincapie at utoronto.ca
>
>
>
>
>
> 	[[alternative HTML version deleted]]
>
>
>
> ______________________________________________
> R-help at 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.