[R] Looping over a matrix passed to .C

[Nathan S. Watson-Haigh]

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I've written a function in R which takes a symmetrical matrix as input and
processes all triplicate combinations of values from the matrix. The function
looks something like:

my_fun <- function(m) {
  if( nrow(mat) != ncol(mat) ) {
    stop("'m' must be a square matrix")
  }

  size <- nrow(m)

  for(x in 1:(size -2)) {
    for(y in (x+1):(size -1)) {
      xy <- m[x,y]

      for(z in (y+1):size ) {
        xz <- m[x,z]
        yz <- m[y,z]

        # do something with xy, xz, yz
      }
    }
  }
}

I'd like to speed this up since when size gets > a few thousand, I estimate it
would take 3yrs to complete the "do something" task. I could implement the "do
something" in C and have it called from within the nested for loops in R, but I
think I should get better performance if I implement the for loops in C as well.
As such, I'm trying to get my head around looping through matrix values when
passed to .C()

As I understand it, once a matrix (n x m in size) is passed to .C() it is seen
as an unwrapped array of length n*m. Could someone help/guide me in implementing
this?

Cheers,
Nathan


- --
- --------------------------------------------------------
Dr. Nathan S. Watson-Haigh
OCE Post Doctoral Fellow
CSIRO Livestock Industries
Queensland Bioscience Precinct
St Lucia, QLD 4067
Australia

Tel: +61 (0)7 3214 2922
Fax: +61 (0)7 3214 2900
Web: http://www.csiro.au/people/Nathan.Watson-Haigh.html
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