find the position of first observation for each subject

I would typically use rle() for this kind of thing:
tmp <- cumsum(rle(id)$lengths)
c(1, tmp[-length(tmp)]+1)
[1] 1 4 9

It does assume that all rows for each unique value of id are grouped
together, but does not require that the rows be sorted by id.

-Don
Don MacQueen

Lawrence Livermore National Laboratory
7000 East Ave., L-627
Livermore, CA 94550
925-423-1062

On 6/13/13 12:56 AM, "Gallon Li" <gallon.li at gmail.com> wrote:

>suppose I have the following data
>
>id=c(rep(1,3),rep(2,5),rep(3,4))
>time=c(seq(1,3),seq(2,6),seq(1,4))
>
>ds=cbind(id,time)
>
>> ds
>      id time
> [1,]  1    1
> [2,]  1    2
> [3,]  1    3
> [4,]  2    2
> [5,]  2    3
> [6,]  2    4
> [7,]  2    5
> [8,]  2    6
> [9,]  3    1
>[10,]  3    2
>[11,]  3    3
>[12,]  3    4
>
>i want to return a vector that indicates the position of the first
>observation for each id. for the above data, i wish to get (1,4,9).
>
>is it possible to get this quickly>?
>
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>
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find the position of first observation for each subject

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