Thanks David It works perfectly Pedro Sent from my iPhone
> On Apr 3, 2016, at 17:44, David L Carlson <dcarl...@tamu.edu> wrote: > > Here are several ways to get there, but your original loop is fine once it is > corrected: > >> for (i in 1:2) smean[i] <- mean(toy$diam[toy$group==i][1:nsel[i]]) >> smean > [1] 0.271489 1.117015 > > Using sapply() to hide the loop: >> smean <- sapply(1:2, function(x) mean((toy$diam[toy$group==x])[1:nsel[x]])) >> smean > [1] 0.271489 1.117015 > > Or use head() >> smean <- sapply(1:2, function(x) mean(head(toy$diam[toy$group==x], nsel[x]))) >> smean > [1] 0.271489 1.117015 > > Or mapply() instead of sapply >> smean <- mapply(function(x, y) mean(head(x, y)) , x=split(toy$diam, >> toy$group), y=nsel) >> smean > 1 2 > 0.271489 1.117015 > > ------------------------------ > David L. Carlson > Department of Anthropology > Texas A&M University > > -----Original Message----- > From: R-help [mailto:r-help-boun...@r-project.org] On Behalf Of Jim Lemon > Sent: Saturday, April 2, 2016 6:14 PM > To: Pedro Mardones <mardone...@gmail.com> > Cc: r-help mailing list <r-help@r-project.org> > Subject: Re: [R] apply mean function to a subset of data > > 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. ______________________________________________ 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.
