[R] Apply functions along "layers" of a data matrix

[saschaview at gmail.com]

Hello

How can I apply functions along "layers" of a data matrix?

Example:

daf <- data.frame(
   'id' = rep(1:5, 3),
   matrix(1:60, nrow=15, dimnames=list( NULL, paste('v', 1:4, sep='') )),
   rep = rep(1:3, each=5)
)

The data frame "daf" contains 3 repetitions/layers (rep) of 4 variables 
of 5 persons (id). For some reason, I want to calculate various 
statistics (e.g., mean, median) *along* the repetitions. The "mean" 
calculation, for example, would produce the means of daf[1, 'v1'] 
*along* the 3 repetition:

(daf[1, 'v1'] + daf[6, 'v1'] + daf[11, 'v1']) / 3

That is to say, each of the calculations would result in a data frame 
with 4 variables (and the id) of the 5 persons:

   id v1 v2 v3 v4
1  1  6 21 36 51
2  2  7 22 37 52
3  3  8 23 38 53
4  4  9 24 39 54
5  5 10 25 40 55

Currently, I do this in a loop, but I was wondering about a quick and 
ressource-friendly way to achieve this?

Thanks
*S*

-- 
Sascha Vieweg, saschaview at gmail.com