[R] Compute rank within factor groups

[Greg Snow]

Look at ?ave and try something like:

> wc$rank <- ave( wc$score, wc$report, FUN=rank )

This works even if the dataframe is not pre sorted.

Hope this helps,

-- 
Gregory (Greg) L. Snow Ph.D.
Statistical Data Center
Intermountain Healthcare
greg.snow at intermountainmail.org
(801) 408-8111
 
 

> -----Original Message-----
> From: r-help-bounces at stat.math.ethz.ch 
> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Ken Williams
> Sent: Thursday, July 12, 2007 12:09 PM
> To: R-help at stat.math.ethz.ch
> Subject: [R] Compute rank within factor groups
> 
> 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
> 
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