Question

How do I compute order statistics by group in R. I want to aggregate results according to a column and then return only 1 row per group. That row should be the n-th element of the group according to some ordering. Ideally I would like to use base functions only.

x <- data.frame(Group=c("A","A", "A", "C", "C"), 
                Name=c("v", "u", "w", "x", "y"), 
                Quantity=c(3,3,4,2,0))
> x
  Group Name Quantity
1     A    v        3
2     A    u        3
3     A    w        4
4     C    x        2
5     C    y        0

I want to take nth highest based on an ordering on Quantity and then Name. For N=2 this is

  Group Name Quantity
1     A    u        3
5     C    y        0

For N=1
  Group Name Quantity
3     A    w        4
4     C    x        2

I tried the following but I get a uninformative error message.

 aggregate.data.frame(x, list(x$Group), function(y){ max(y[,'Quantity'])})
 Error in `[.default`(y, , "Quantity") (from #1) : incorrect number of dimensions"

Answer 1

I went with

do.call(rbind, by(x, x$Group, function(x)
      x[order(-x$Quantity, x$Name),][1,]))

per someone else’s suggestion. I found it suited my thought process a bit better than the other posted solutions (which I appreciate).