[R] Code is too slow: mean-centering variables in a data frame by subgroup

[Dimitri Liakhovitski]

Dear R-ers,

I have  a large data frame (several thousands of rows and about 2.5
thousand columns). One variable ("group") is a grouping variable with
over 30 levels. And I have a lot of NAs.
For each variable, I need to divide each value by variable mean - by
subgroup. I have the code but it's way too slow - takes me about 1.5
hours.
Below is a data example and my code that is too slow. Is there a
different, faster way of doing the same thing?
Thanks a lot for your advice!

Dimitri


# Building an example frame - with groups and a lot of NAs:
set.seed(1234)
frame<-data.frame(group=rep(paste("group",1:10),10),a=rnorm(1:100),b=rnorm(1:100),c=rnorm(1:100),d=rnorm(1:100),e=rnorm(1:100),f=rnorm(1:100),g=rnorm(1:100))
frame<-frame[order(frame$group),]
names.used<-names(frame)[2:length(frame)]
set.seed(1234)
for(i in names.used){
       i.for.NA<-sample(1:100,60)
       frame[[i]][i.for.NA]<-NA
}
frame

### Code that does what's needed but is too slow:
Start<-Sys.time()
frame <- do.call(cbind, lapply(names.used, function(x){
  unlist(by(frame, frame$group, function(y) y[,x] / mean(y[,x],na.rm=T)))
}))
Finish<-Sys.time()
print(Finish-Start) # Takes too long

-- 
Dimitri Liakhovitski
Ninah.com
Dimitri.Liakhovitski at ninah.com