[R] Merging levels of matrix dimensions

[BXC (Bendix Carstensen)]

If I have a matrix I can sum some of the rows by hand:
> M
     [,1] [,2] [,3]
[1,]    1    6   11
[2,]    2    7   12
[3,]    3    8   13
[4,]    4    9   14
[5,]    5   10   15
> rbind(M[1,],M[2,]+M[3,],M[4,]+M[5,])
     [,1] [,2] [,3]
[1,]    1    6   11
[2,]    5   15   25
[3,]    9   19   29

This is a trivial special case of the following problem that I
wuold like to have a solution to:

Consider a matix m, and suppose I have factors f.row and f.rcol 
of lengths dim(m), respectively.
These factors induce a division of m into submatrices.
How do I get a nlevels(f.row) by nlevels(f.col) matrix where each
entry is the sum (or any old function) of the elements in 
each submatrix?

Of course there is an array-generalization of this problem as well.

Any help would be appreaciated.

Bendix

----------------------
Bendix Carstensen
Senior Statistician
Steno Diabetes Centre
Niels Steensens Vej 2
DK-2820 Gentofte
Denmark
tel: +45 44 43 87 38
mob: +45 28 25 87 38
fax: +45 44 43 73 13
bxc at novo.dk
www.biostat.ku.dk/~bxc
----------------------

-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-
r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html
Send "info", "help", or "[un]subscribe"
(in the "body", not the subject !)  To: r-help-request at stat.math.ethz.ch
_._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._