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

library(plyr)
ddply(a, .(id), function(x) return(x[which(abs(x$value)==max(abs(x$value))),]))