How can I rank observations in-group faster?

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The answer from Marek in this question has proven very useful in the past. I wrote it down and use it almost daily since it was fast and efficient. We’ll use ave() and seq_along().

foo <-data.frame(person=c(rep("pers1",3),rep("pers2",2)),year=c(1999,2000,2003,1998,2011))

foo <- transform(foo, obs = ave(rep(NA, nrow(foo)), person, FUN = seq_along))
foo

  person year obs
1  pers1 1999   1
2  pers1 2000   2
3  pers1 2003   3
4  pers2 1998   1
5  pers2 2011   2

Another option using plyr

library(plyr)
ddply(foo, "person", transform, obs2 = seq_along(person))

  person year obs obs2
1  pers1 1999   1    1
2  pers1 2000   2    2
3  pers1 2003   3    3
4  pers2 1998   1    1
5  pers2 2011   2    2

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