HI, Try this: ds1<- data.frame(id,time) which(with(ds1,ave(time,id,FUN=seq))==1) #[1] 1 4 9 A.K.
----- Original Message ----- From: Gallon Li <gallon...@gmail.com> To: R-help@r-project.org Cc: Sent: Thursday, June 13, 2013 3:56 AM Subject: [R] find the position of first observation for each subject 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>? [[alternative HTML version deleted]] ______________________________________________ R-help@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. ______________________________________________ R-help@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.
