Hi Pedro, This may not be much of an improvement, but it was a challenge. selvec<-as.vector(matrix(c(nsel,unlist(by(toy$diam,toy$group,length))-nsel), ncol=2,byrow=TRUE)) TFvec<-rep(c(TRUE,FALSE),length.out=length(selvec)) toynsel<-rep(TFvec,selvec) by(toy[toynsel,]$diam,toy[toynsel,]$group,mean)
Jim On 4/3/16, Pedro Mardones <mardone...@gmail.com> wrote: > Dear all; > > This must have a rather simple answer but haven't been able to figure it > out: I have a data frame with say 2 groups (group 1 & 2). I want to select > from group 1 say "n" rows and calculate the mean; then select "m" rows from > group 2 and calculate the mean as well. So far I've been using a for loop > for doing it but when it comes to a large data set is rather inefficient. > Any hint to vectorize this would be appreciated. > > toy = data.frame(group = c(rep(1,10),rep(2,8)), diam = > c(rnorm(10),rnorm(8))) > nsel = c(6,4) > smean <- c(0,0) > for (i in 1:2) smean[i] <- mean(toy$diam[1:nsel[i]]) > > Thanks > > Pedro > > [[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.