Hi! You didn't provide an reproducable example (repex), so I created one for you including the solution.
Consider you want to add the median values for each column of the iris dataset by species. iris <- as.data.table(iris) iris[iris[, lapply(.SD, median), keyby = "Species"], paste0(names(iris), ".med") := get(paste0("i.", names(iris))), on = "Species"] Cheers, Gerhard 2018-03-07 11:04 GMT+01:00 Matthew Forrest <matthew.forr...@senckenberg.de>: > Hi all, > > A real data.table question here ;-) > > I am currently doing this: > > output.dt <- input.dt[,lapply(.SD, method.function), by=by.dims] > > where method.function is an arbitrary function and by.dims is a list of > columns (although perhaps I could remove this is the data.table was keyed by > those columns?) > > That doesn't look very data.table-y to me, potentially it would be great to > do it with := in the proper data.table way. Is that possible? I have tried > a couple of things but haven't managed. The problem is that I need to be > flexible to deal with arbitrary columns in my data.table. > > Any suggestions? > > Many thanks, > > Matt > > > -- > Dr Matthew Forrest > Senckenberg Biodiversity and Climate Research Centre (BiK-F) > Visiting address: Georg-Voigt-Straße 14-16, room 3.04, Frankfurt am Main > Postal address: Senckenberganlage 25 > D-60325 Frankfurt am Main > Tel.: +49-69-7542-1867 > Fax: +49-69-7542-7904 > E-mail: matthew.forr...@senckenberg.de > Homepage: www.bik-f.de > > Senckenberg Gesellschaft für Naturforschung > Rechtsfähiger Verein gemäß § 22 BGB > Senckenberganlage 25 > 60325 Frankfurt > > Direktorium: Prof. Dr. Dr. h.c. Volker Mosbrugger, Prof. Dr. Andreas Mulch, > Stephanie Schwedhelm, Prof. Dr. Katrin Böhning-Gaese, Prof. Dr. Uwe Fritz, > PD Dr. Ingrid Kröncke > > Präsidentin: Dr. h.c. Beate Heraeus > > Aufsichtsbehörde: Magistrat der Stadt Frankfurt am Main (Ordnungsamt) > > _______________________________________________ > datatable-help mailing list > datatable-help@lists.r-forge.r-project.org > https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help _______________________________________________ datatable-help mailing list datatable-help@lists.r-forge.r-project.org https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help