[R] Looping over a matrix passed to .C

[Nathan S. Watson-Haigh]

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jim holtman wrote:
> Why don't you generate all the possible combinations using 'combn' and
> then use that matrix to address your data:

I had thought of using this, however the size of data I'm using will more likely
be orders of magnitude larger, such as:

> dim(m)
[1] 10000 10000

Which takes looooong time to compute using:
indx <- combn(10000, 3)



> 
>> m <- matrix(1:25,5)
>> indx <- combn(5, 3)
>> indx
>      [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
> [1,]    1    1    1    1    1    1    2    2    2     3
> [2,]    2    2    2    3    3    4    3    3    4     4
> [3,]    3    4    5    4    5    5    4    5    5     5
> 
>> mxy <- m[cbind(indx[1,], indx[2,])]
>>
>>
>> mxy
>  [1]  6  6  6 11 11 16 12 12 17 18
> 
> You can use each of the columns as x,y,z values and compute everything at once.
> 
> On Tue, Feb 10, 2009 at 9:49 PM, Nathan S. Watson-Haigh
> <nathan.watson-haigh at csiro.au> wrote:
> 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
> 
> 
>>
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>>

- --
- --------------------------------------------------------
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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