Grouping variables are length 1. Background in FAQ 2.10. .N can be used instead of length().
On Sat, 2011-11-12 at 12:08 -0800, Muhammad Waliji wrote: > I sometimes want to check the number of rows for each value of a > variable. I use the following method. The first syntax doesn't work, > but the second does. I think that they ought to give the same result. > > > > DT <- data.table(x=rep(c(1, 2), each=10), y=1:20) > > DT[, length(x), list(x)] > x V1 > [1,] 1 1 > [2,] 2 1 > > DT[, length(y), list(x)] > x V1 > [1,] 1 10 > [2,] 2 10 > > > _______________________________________________ > 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
