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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