Or: DT[,lapply(.SD,sum),by=...,.SDcols=myvars] > with = FALSE will let you use literal column names > > > On Wed, Apr 24, 2013 at 3:29 PM, Sam Steingold <[email protected]> wrote: > >> What do I do if I want to operate on several columns? >> E.g., >> --8<---------------cut here---------------start------------->8--- >> length(names(dt)) = 25 >> myvars = c("col1","col4","col7") >> myid = "user" >> setkeyv(dt,myid) >> --8<---------------cut here---------------end--------------->8--- >> and I want to summarize columns in myvars by myid. >> the point is that nowhere in the code the literal "col4" or "user" may >> appear. >> >> Thanks. >> >> -- >> Sam Steingold (http://sds.podval.org/) on Ubuntu 12.10 (quantal) X >> 11.0.11300000 >> http://www.childpsy.net/ http://ffii.org http://palestinefacts.org >> http://openvotingconsortium.org http://camera.org http://jihadwatch.org >> The difference between genius and stupidity is that genius has its >> limits. >> _______________________________________________ >> 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
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