I am struggling with migrating some stata code to R. I have a data frame containing, sometimes, repeat observations (rows) of the same family. I want to keep only one observation per family, selecting that observation according to some other variable. An example data frame is:
# construct example data fam <- c(1,2,3,3,4,4,4) wt <- c(1,1,0.6,0.4,0.4,0.4,0.2) keep <- c(1,1,1,0,1,0,0) dat <- as.data.frame(cbind(fam,wt,keep)) dat I want to keep the observation for which wt is a maximum, and where this doesn't identify a unique observation, to keep just one anyway, not caring which. Those observations are indicated above by keep==1. (Note, keep <- c(1,1,1,0,0,1,0) would be fine too, but not c(1,1,1,0,0,0,1)). The stata code I would use is bys fam (wt): keep if _n==_N This is my (long-winded) attempt in R: # first keep those rows where wt=max_fam(wt) maxwt <- by(dat,dat$fam,function(x) max(x[,2])) maxwt <- sapply(maxwt,"[[",1) maxwt.dat <- data.frame("maxwt"=maxwt,"fam"=as.integer(names(maxwt))) dat <- merge(dat,maxwt.dat) dat <- dat[dat$wt==dat$maxwt,] dat Now I am stuck - I want to keep either row with fam==4, and have tried playing around with combinations of sample and apply or by, but with no success. I can only find an inefficient for-loop solution: # identify those rows with >1 observation more <- by(dat,dat$fam,function(x) dim(x)[1]) more <- sapply(more,"[[",1) more.dat <- data.frame("more"=more,"fam"=as.integer(names(more))) dat <- merge(dat,more.dat) # sample from those for whom more>1 result<-dat[dat$more==1,] for(f in unique(dat$fam[dat$more>1])) { rows <- rownames(dat[dat$fam==f,]) result <- rbind(result,dat[sample(rows,1),]) } result I am sure that for something so simple in stata to be so complicated in R must indicate ignorance of R on my part, but searches of help files and RSiteSearch hasn't led to any better solution. Any suggestions would be most helpful! Thanks, C. ______________________________________________ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html