Remove duplicates keeping entry with largest absolute value

Question

Let\'s say I have four samples: id=1, 2, 3, and 4, with one or more measurements on each of those samples:

> a <- data.frame(id=c(1,1,2,2,3,4), value=c         


        

Answer 1

Check out ?aggregate:

aggregate(value~id,a,function(x) x[which.max(abs(x))])

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I like the answer by @DWin, but I would like show how this could also work with metadata:

aa<-merge(aggregate(value~id,a,function(x) x[which.max(abs(x))]),a)
# Fails if the max value is duplicated for a single id without next line.
aa[!duplicated(aa),]

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I couldn't help myself and created one last answer:

do.call(rbind,lapply(split(a,a$id),function(x) x[which.max(abs(x$value)),]))