[R] get mean of several rows

[Richard A. O'Keefe]

"Jan Wantia" <wantia at ifi.unizh.ch> asked:
	I have a 2-dimensional array, and I know how to split it into its rows 
	and how to get the mean for every row using 'sapply'.
	But what I want is to calculate the mean over the first n rows, and then 
	the second n rows, etc., so that I get a vector like:
	
	v == mean1(row 1:5), mean2(row6:10),...
	
	(trivial, you might say.

There are lots of ways to do it.
Let Arr be a p=nrow(Arr) by q=ncol(Arr) array.
Let p %% n = 0, for simplicity.

Let 1 <= k <= p %/% n.
Then the rows which should contribute to the kth mean are
(k-1)*n+1 .. k*n.
mean(Arr[((k-1)*n+1):(k*n)])
will therefore give you the kth mean.

So

    sapply(1:(nrow(Arr)%/%n), function (k) mean(Arr[((k-1)*n+1):(k*n),]))

should do the trick.  I've tested it on a small example where I knew the
answers, and it worked.  This assumes that I've understood the question...

I'm a bit annoyed, because I thought of several really cute ways to do
this, one involving cumsum(t(Arr)) and diff(), but this is so direct that
the others might only be confusing.