Hi Matthew, Thanks for your response.
I'm following things right up to: > "Advanced: In the X[Y,j] form of grouping, the j expression sees > variables in X first, then Y. We call this join inherited scope. If the > variable is not in X or Y then the calling frame is searched, its > calling frame, and so on in the usual way up to and including the global > environment." I'm mainly confused about X[Y] In the example Y does not have key. Y's column names are also different. So how did data.table know to use column "V1" to match with X's key. Also, if the sentence above applies how does X[Y j] relate to this example in which I have used X[Y]. And yes, without data.table, I would have been in trouble for my current project. I had to repeat the aggregation 20 or so times. It would have been ugly. Thanks, Juliet _______________________________________________ datatable-help mailing list [email protected] https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help
