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

From: Peter Dalgaard <p.dalgaard_at_biostat.ku.dk>
Date: Tue 02 Aug 2005 - 01:08:03 EST

Chris Wallace <c.wallace@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.

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

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