[R] Sample of a subsample

[Eric Berger]

Hi David,
I was about to post a reply when Bert responded. His answer is good
and his comment to use the name 'dat' rather than 'data' is instructive.
I am providing my suggestion as well because I think it may address
what was causing you some confusion (mainly to use "which", but also
the missing !)

idx2 <- sample( which( (!data$var1%%2) & data$sampleNo==0 ), size=10,
replace=F)
data[idx2,]$sampleNo <- 2

Eric



On Mon, Sep 25, 2017 at 9:03 PM, Bert Gunter <bgunter.4567 at gmail.com> wrote:

> For personal aesthetic reasons, I changed the name "data" to "dat".
>
> Your code, with a slight modification:
>
> set.seed (1357)  ## for reproducibility
> dat <- data.frame(var1=seq(1:40), var2=seq(40,1))
> dat$sampleNo <- 0
> idx <- sample(seq(1,nrow(dat)), size=10, replace=F)
> dat[idx,"sampleNo"] <-1
>
> ## yielding
> > dat
>
>    var1 var2 sampleNo
> 1     1   40        0
> 2     2   39        1
> 3     3   38        0
> 4     4   37        0
> 5     5   36        0
> 6     6   35        1
> 7     7   34        0
> 8     8   33        0
> 9     9   32        0
> 10   10   31        0
> 11   11   30        0
> 12   12   29        0
> 13   13   28        0
> 14   14   27        0
> 15   15   26        1
> 16   16   25        1
> 17   17   24        0
> 18   18   23        0
> 19   19   22        0
> 20   20   21        1
> 21   21   20        0
> 22   22   19        1
> 23   23   18        0
> 24   24   17        1
> 25   25   16        0
> 26   26   15        1
> 27   27   14        0
> 28   28   13        0
> 29   29   12        0
> 30   30   11        0
> 31   31   10        0
> 32   32    9        0
> 33   33    8        0
> 34   34    7        0
> 35   35    6        1
> 36   36    5        0
> 37   37    4        1
> 38   38    3        0
> 39   39    2        0
> 40   40    1        0
>
> ## This is basically a transcription of your specification into indexing
> logic
>
> dat <- within(dat,sampleNo[sample(var1[(var1%%2 == 0) &
> sampleNo==0],10,rep=FALSE)] <- 2)
>
> ##yielding
> > dat
>
>    var1 var2 sampleNo
> 1     1   40        0
> 2     2   39        1
> 3     3   38        0
> 4     4   37        2
> 5     5   36        0
> 6     6   35        1
> 7     7   34        0
> 8     8   33        2
> 9     9   32        0
> 10   10   31        2
> 11   11   30        0
> 12   12   29        0
> 13   13   28        0
> 14   14   27        2
> 15   15   26        1
> 16   16   25        1
> 17   17   24        0
> 18   18   23        2
> 19   19   22        0
> 20   20   21        1
> 21   21   20        0
> 22   22   19        1
> 23   23   18        0
> 24   24   17        1
> 25   25   16        0
> 26   26   15        1
> 27   27   14        0
> 28   28   13        2
> 29   29   12        0
> 30   30   11        2
> 31   31   10        0
> 32   32    9        2
> 33   33    8        0
> 34   34    7        2
> 35   35    6        1
> 36   36    5        2
> 37   37    4        1
> 38   38    3        0
> 39   39    2        0
> 40   40    1        0
>
>
>
>
>
> dat <- within(dat,sampleNo[sample(var1[(var1%%2 == 0) &
> sampleNo==0],10,rep=FALSE)] <- 2)
>
>
>
>
> Bert Gunter
>
> "The trouble with having an open mind is that people keep coming along and
> sticking things into it."
> -- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )
>
> On Mon, Sep 25, 2017 at 10:27 AM, David Studer <studerov at gmail.com> wrote:
>
> > Hello everybody!
> >
> > I have the following problem: I'd like to select a sample from a
> subsample
> > in a dataset. Actually, I don't want to select it, but to create a new
> > variable sampleNo that indicates to which sample (one or two) a case
> > belongs to.
> >
> > Lets suppose I have a dataset containing 40 cases:
> >
> > data <- data.frame(var1=seq(1:40), var2=seq(40,1))
> >
> > The first sample (n=10) I drew like this:
> >
> > data$sampleNo <- 0
> > idx <- sample(seq(1,nrow(data)), size=10, replace=F)
> > data[idx,]$sampleNo <- 1
> >
> > Now, (and here my problems start) I'd like to draw a second sample
> (n=10).
> > But this sample should be drawn from the cases that don't belong to the
> > first sample only. *Additionally, "var1" should be an even number.*
> >
> > So sampleNo should be 0 for cases that were not drawn at all, 1 for cases
> > that belong to the first sample and 2 for cases belonging to the second
> > sample (= sampleNo equals 0 and var1 is even).
> >
> > I was trying to solve it like this:
> >
> > idx2<-data$var1%%2 & data$sampleNo==0
> > sample(data[idx2,], size=10, replace=F)
> >
> > But how can I set sampleNo to 2?
> >
> >
> > Thank you very much for your help!
> >
> > David
> >
> >         [[alternative HTML version deleted]]
> >
> > ______________________________________________
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> > 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.
> >
>
>         [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> 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.
>

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