[R] converting stata's by syntax to R

[Peter Dalgaard]

Chris Wallace <c.wallace at qmul.ac.uk> writes:

> 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.

How about

unsplit(lapply(split(dat,dat$fam), 
               function(x) seq(length=nrow(x)) == which.max(x$wt)),
        dat$fam)

or 

do.call("rbind", lapply(split(dat,dat$fam),
                        function(x) x[which.max(x$wt),]))

or (same thing, basically)

do.call("rbind", by(dat,dat$fam,function(x) x[which.max(x$wt),]))

-- 
   O__  ---- Peter Dalgaard             Øster Farimagsgade 5, Entr.B
  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
 (*) \(*) -- University of Copenhagen   Denmark          Ph:  (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)                  FAX: (+45) 35327907