Hi Dennis, Welcome to the list.
To reply to a particular part of a thread you can use any of the mirrors listed on the data.table homepage. I like Nabble (it's search is good too). You can also post via your newsreader or from Google groups. As long as you are subscribed to the Mailman list, as you must be already, then you can find a part of a thread on that mirror and reply to it from there. Just like r-help. You can bcc from Nabble too (but not cc afaik). ave() uses the lapply(split(...)) paradigm, though. That's inefficient. == is a 'vector scan' so we try to avoid that in the i wherever possible. Matthew "Dennis Murphy" <[email protected]> wrote in message news:[email protected]... Hi: It seems to me that another way to handle this problem easily would be to add a variable indicating the 2009 school before converting it to a data table. The function is simple enough: find the max SCHOOL_NUMBER per STUDENT_ID and use ave() to map it to all rows with the same STUDENT_ID. library(data.table) txt$SCHOOL2009 <- with(txt, ave(SCHOOL_NUMBER, STUDENT_ID, FUN = max)) dtxt <- data.table(txt) dtxt[YEAR == 2008, mean(SCORE), by = 'SCHOOL2009'] SCHOOL2009 V1 [1,] 200 48.2 [2,] 400 53.0 As this is my first posting on this list, how do I follow-up to an existing thread? Dennis _______________________________________________ datatable-help mailing list [email protected] https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help _______________________________________________ datatable-help mailing list [email protected] https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help
