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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