r - Aggregate a dataframe on a given column and display another column

Question

I have a dataframe in R of the following form:

> head(data)
  Group Score Info
1     1     1    a
2     1     2    b
3     1     3    c
4     2     4    d
5     2     3    e
6     2     1    f

I would like to aggregate it following the Score column using the max function

> aggregate(data$Score, list(data$Group), max)

  Group.1         x
1       1         3
2       2         4

But I also would like to display the Info column associated to the maximum value of the Score column for each group. I have no idea how to do this. My desired output would be:

  Group.1         x        y
1       1         3        c
2       2         4        d

Any hint?

Answer 1

A base R solution is to combine the output of aggregate() with a merge() step. I find the formula interface to aggregate() a little more useful than the standard interface, partly because the names on the output are nicer, so I'll use that:

The aggregate() step is

maxs <- aggregate(Score ~ Group, data = dat, FUN = max)

and the merge() step is simply

merge(maxs, dat)

This gives us the desired output:

R> maxs <- aggregate(Score ~ Group, data = dat, FUN = max)
R> merge(maxs, dat)
  Group Score Info
1     1     3    c
2     2     4    d

You could, of course, stick this into a one-liner (the intermediary step was more for exposition):

merge(aggregate(Score ~ Group, data = dat, FUN = max), dat)

The main reason I used the formula interface is that it returns a data frame with the correct names for the merge step; these are the names of the columns from the original data set dat. We need to have the output of aggregate() have the correct names so that merge() knows which columns in the original and aggregated data frames match.

The standard interface gives odd names, whichever way you call it:

R> aggregate(dat$Score, list(dat$Group), max)
  Group.1 x
1       1 3
2       2 4
R> with(dat, aggregate(Score, list(Group), max))
  Group.1 x
1       1 3
2       2 4

We can use merge() on those outputs, but we need to do more work telling R which columns match up.