[R] get level combinations from "by" list

[bogdan romocea]

Dear useRs,

Given this code I end up with a list of class "by":

a <- sample(1:5,200,replace=TRUE)
b <- sample(c("v1","v2","v3"),200,replace=TRUE)
c <- sample(c(11,22,33),200,replace=TRUE)
data <- runif(200)
grouped <- by(data,list(a,b,c),function(x) {c(min=min(x),max=max(x),
	median=round(median(x),digits=2),mean=round(mean(x),digits=2))})
dfr <- do.call("rbind",grouped)    #the levels are missing
#----------
grouped
typeof(grouped)
class(grouped)
dimnames(grouped)
 
How do I get at the levels of the 'group by' variables for each
subset? For example, from this part of the "by" list I want 4, v2 and
33:
: 4
: v2
: 33
      min       max    median      mean
0.3897450 0.9215315 0.7300000 0.6700000
---------------------------------------

Thank you,
b.