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 <mardones.p at 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]]
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