[R] Compute rank within factor groups

[Ken Williams]

Hi,

I have a data.frame which is ordered by score, and has a factor column:

  Browse[1]> wc[c("report","score")]
          report score
  9         ADEA  0.96
  8         ADEA  0.90
  11 Asylum_FED9  0.86
  3         ADEA  0.75
  14 Asylum_FED9  0.60
  5         ADEA  0.56
  13 Asylum_FED9  0.51
  16 Asylum_FED9  0.51
  2         ADEA  0.42
  7         ADEA  0.31
  17 Asylum_FED9  0.27
  1         ADEA  0.17
  4         ADEA  0.17
  6         ADEA  0.12
  10        ADEA  0.11
  12 Asylum_FED9  0.10
  15 Asylum_FED9  0.09
  18 Asylum_FED9  0.07
  Browse[1]> 

I need to add a column indicating rank within each factor group, which I
currently accomplish like so:

  wc$rank <- 0
  for(report in as.character(unique(wc$report))) {
    wc[wc$report==report,]$rank <- 1:sum(wc$report==report)
  }

I have to wonder whether there's a better way, something that gets rid of
the for() loop using tapply() or by() or similar.  But I haven't come up
with anything.

I've tried these:

  by(wc, wc$report, FUN=function(pr){pr$rank <- 1:nrow(pr)})

  by(wc, wc$report, FUN=function(pr){wc[wc$report %in% pr$report,]$rank <-
1:nrow(pr)})

But in both cases the effect of the assignment is lost, there's no $rank
column generated for wc.

Any suggestions?

 -Ken