Ramiro, `dt[, lapply(.SD, mean), by=name]` is the idiomatic way.
I suggest reading through the new HTML vignettes at https://github.com/Rdatatable/data.table/wiki/Getting-started Ista, thanks for linking to the new vignette. On Wed, Jun 10, 2015 at 2:17 AM, Ista Zahn <istaz...@gmail.com> wrote: > Hi Ramiro, > > There is a demonstration of this on the data.table wiki at > https://rawgit.com/wiki/Rdatatable/data.table/vignettes/datatable-intro-vignette.html. > You can do > > dt[, lapply(.SD, mean), by=name] > > or > > dt[, as.list(colMeans(.SD)), by=name] > > BTW, there are pretty straightforward ways to do this in base R as well, e.g, > > data.frame(t(sapply(split(df[-1], df$name), colMeans))) > > Best, > Ista > > On Tue, Jun 9, 2015 at 4:22 PM, Ramiro Barrantes > <ram...@precisionbioassay.com> wrote: >> Hello, >> >> I am trying to do something that I am able to do with the "by" function >> within data.frame but can't figure out how to achieve with data.table. >> >> Consider >> >> dt<-data.table(name=c(rep("a",5),rep("b",6)),var1=0:10,var2=20:30,var3=40:50) >> myFunction <- function(x) { mean(x) } >> >> I am aware that I can do something like: >> >> dt[, .(meanVar1=myFunction(var1)) ,by=.(name)] >> >> but how could I do the equivalent of: >> >> df<-data.frame(name=c(rep("a",5),rep("b",6)),var1=0:10,var2=20:30,var3=40:50) >> myFunction <- function(x) { mean(x) } >> >> columnNames <- c("var1","var2","var3") >> result <- by(df, df$name, function(x) { >> output <- c() >> for(col in columnNames) { >> output[col] <- myFunction(x[,col]) >> } >> output >> }) >> do.call(rbind,result) >> >> Thanks in advance, >> Ramiro >> >> [[alternative HTML version deleted]] >> >> ______________________________________________ >> R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see >> https://stat.ethz.ch/mailman/listinfo/r-help >> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html >> and provide commented, minimal, self-contained, reproducible code. > > ______________________________________________ > R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. ______________________________________________ R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
