# [R] converting stata's by syntax to R

From: Chris Wallace <c.wallace_at_qmul.ac.uk>
Date: Tue 02 Aug 2005 - 00:24:27 EST

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