Assuming your matrix is:

    mm <- matrix(runif(6*6),6,6)

And your blocks are defined by integers or factors:

   cfact <- c(1,1,1,2,3,3)
   rfact <- c(1,1,1,2,2,3)

Then the following should do the trick:

   matrix(tapply(mm, outer(rfact,cfact,paste), mean),
              length(unique(rfact)))

The 'outer' calculates a joint factor for each element of the matrix; the
'tapply' treats the matrix as a vector, grouping by factor and calculating
means; the 'matrix' rearranges them as a matrix corresponding to the
original block structure.

Is that what you had in mind?

              -s


On Mon, Feb 16, 2009 at 12:43 PM, Titus von der Malsburg <malsburg@gmail.com
> wrote:

>
> Hi list!  I have a large matrix which I'd like to partition into blocks
> and for each block I'd like to compute the mean.  Following a example
> where each letter marks a block of the partition:
>
>     a a a d g g
>     a a a d g g
>     a a a d g g
>     b b b e h h
>     b b b e h h
>     c c c f i i
>
> I'm only interested in the resulting matrix of means.  How can this be
> done efficiently?
>
> Thanks!  Titus
>
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