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

A data.table approach might be in order if your data set is very large:

library(data.table)

aDT <- as.data.table(a)
setkey(aDT,"id")

aDT[J(unique(id)), list(value = value[which.max(abs(value))])]

Or a not as fast, but still fast, alternative :

library(data.table)
as.data.table(a)[, .SD[which.max(abs(value))], by=id]

This version returns all the columns of a, in case there are more in the real dataset.