[R] efficient submatrix extraction

[Rajarshi Guha]

Hi,
  I have a matrix of say 1024x1024 and I want to look at it in chunks.
That is I'd like to divide into a series of submatrices of order 2x2.

| 1 2 3 4 5 6 7 8 ... |
| 1 2 3 4 5 6 7 8 ... |
| 1 2 3 4 5 6 7 8 ... |
| 1 2 3 4 5 6 7 8 ... |
...

So the first submatrix would be

| 1 2 |
| 1 2 |

the second one would be

| 3 4 |
| 3 4 |

and so on. That is I want the matrix to be evenly divided into 2x2
submatrices. Now I'm also doing this subdivision into 4x4, 8x8 ...
256x256 submatrices.

Currently I'm using loops and I'm sure there is a mroe efficient way to
do it:

    m <- matrix(runif(1024*1024), nrow=1024)
    boxsize <- 2^(1:8)

    for (b in boxsize) {
        bcount <- 0
        bstart <- seq(1,1024, by=b)
        for (x in bstart) {
            for (y in bstart) {
                xend <- x + b - 1
                yend <- y + b - 1
                if (length(which( m[ x:xend, y:yend ] > 0.7)) > 0) {
                    bcount <- bcount + 1
                }
            }
        }
    }

Is there any way to vectorize the two inner loops?

Thanks,

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Rajarshi Guha <rxg218 at psu.edu> <http://jijo.cjb.net>
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