r - 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(1,2,3,-4,-5,6))
> a
  id value
1  1     1
2  1     2
3  2     3
4  2    -4
5  3    -5
6  4     6

I want to remove duplicates, keeping only one entry per ID - the one having the largest absolute value of the "value" column. I.e., this is what I want:

> a[c(2,4,5,6), ]
  id value
2  1     2
4  2    -4
5  3    -5
6  4     6

How might I do this in R?

Answer 1

First. Sort in the order putting the less desired items last within I’d groups

 aa <- a[order(a$id, -abs(a$value) ), ] #sort by id and reverse of abs(value)

Then: Remove items after the first within I’d groups

 aa[ !duplicated(aa$id), ]              # take the first row within each id
  id value
2  1     2
4  2    -4
5  3    -5
6  4     6