Larissa,
  So is the problem "in the matrix reference is mat[col][row] whereas in R
it is mar[row, col]?"
  The solution is just recognizing the difference in references.
Dave


On Fri, Dec 6, 2013 at 7:21 AM, Larissa Hauer
<larissahauer@googlemail.com>wrote:

>
> Hi everybody,
>
> I'm trying to pass a matrix from R to C, where some computation is done
> for performance reasons, and back to R for evaluation. But I've run into
> the problem that R and C seem to have different ways of representing the
> matrix in main memory. The C representation of a 2D matrix in linear memory
> is concatenation of the rows whereas in R, it's a concatenation of the
> columns.  That leads to the problem. that an R-matrix, for example
> 123
> 456
> 789
> is seen by C as
> 147
> 258
> 369
> and vice versa.
>
> Here's an example of C code that simply prints the matrix it gets from R:
>
> #include <stdlib.h>
> #include "R.h"
>
> void printMatrix(int *mPtr, int *m, int *n) {
>   int (*matrix)[*n] = mPtr;
>
>   int j,k;
>
>   for(j = 0; j < *m; j++){
>     for(k = 0; k < *n; k++) {
>       printf("%d", matrix[j][k]);
>     }
>   printf("\n");
>   }
> }
>
> And here's what happens when I call the function in R:
>
>  m <- 3; n <- 3
>> mat <- matrix(c(1:9), nrow=m, ncol=n, byrow=TRUE)
>> mat
>>
>      [,1] [,2] [,3]
> [1,]    1    2    3
> [2,]    4    5    6
> [3,]    7    8    9
>
>> mat <- .C("printMatrix", mat, as.integer(m), as.integer(n))[[1]]
>>
> 147
> 258
> 369
>
>
> No matter if you create the matrix with byrow=TRUE or FALSE, C always
> interprets it the other way round. Is there a way to avoid this? I've read
> previous posts on passing a matrix from R to C, but the essence of the
> answers was that "a matrix in R is just a vector with attributes", but I
> don't see how this helps. Maybe someone can clarify.
>
> Thanks a lot in advance!
>
> Cheers
> Larissa
>
> Here's the C main function showing that the C code itself is correct:
>
> #include <stdlib.h>
>
> void printMatrix(int *mPtr, int *m, int *n);
>
> int main(void) {
>   int m, n, i;
>   int *mPtr, *nPtr;
>   m = 3;
>   n = 3;
>   mPtr = &m;
>   nPtr = &n;
>
>   int *M = malloc(m * n * sizeof(int));
>
>   for (i = 0; i < m * n; i++){
>   M[i] = i + 1;
>   }
>
>   printMatrix(M, mPtr, nPtr);
>
>   return EXIT_SUCCESS;
>
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>

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