you can just do: > DT[J(letters[4:6]), list(v1=v1, 1),by=v3]
If you use `by` only the columns in it will be included in the output. But it seems like what you request is a nicer feature and I am for it. Because, when you just do, DT[J(letters[4:6]), sum(v1)] it gives you "v1". But when using `by`, it disappears. I find with your suggestion this would be more consistent. Arun On Friday, June 21, 2013 at 10:26 PM, Frank Erickson wrote: > Hi, > > I thought that when joining with J(x) and doing by=y, all the columns > involved were put into .BY. However, I see that they are not: > > DT <- data.table(v1=letters[1:10],v2=1:10,v3=c(TRUE,FALSE),key="v1") > DT[J(letters[4:6]),1,by=v3] > > I think I've just forgotten how to do this correctly (so that both v1 and v3 > show up in the output). Any help would be appreciated. > > Thanks, > > Frank > _______________________________________________ > datatable-help mailing list > [email protected] > (mailto:[email protected]) > https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help > >
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