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 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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