# [R] Looping over a matrix passed to .C

jim holtman jholtman at gmail.com
Wed Feb 11 17:56:31 CET 2009

```Why don't you generate all the possible combinations using 'combn' and

> 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:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> 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
> - --------------------------------------------------------
>
> -----BEGIN PGP SIGNATURE-----
> Version: GnuPG v1.4.9 (MingW32)
> Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org
>
> sLwAn39h1JxNF6MjaF+AgxCq3XVe0X7T
> =KEDP
> -----END PGP SIGNATURE-----
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> and provide commented, minimal, self-contained, reproducible code.
>

--
Jim Holtman
Cincinnati, OH
+1 513 646 9390

What is the problem that you are trying to solve?

```