[R] determine dimension on which by() applies

[Werner Wernersen]

Hi,

having solved one problem with your kindest help I
directly ran into the next one:
I now have a 4-dimensional array and I want to
aggregate subsets (sum up over) for some of the
dimensions. This sort of aggregating works beautifully
for matrices with by(). Is there a similar function
where I additionally can choose the dimension to do
the group calculations over?

Thanks again!
  Werner