How to get a pointer to 2D array when writing a C source Mex file?

Hi, I'm trying to write a highly simple C source .mex file (function) that takes a 1 x n vector and n x n matrix, and yields their matrix product, again a 1 x n vector (all the arrays are of binary and I'm doing computations for binary case). My concern is how to get a pointer to 2D array,(e.g the line arr = mxGetDoubles(prhs[1]);) I tried mxGetDoubles as it works for 1xn arrays, but I came across with the warning ": assignment from incompatible pointer type [-Wincompatible-pointer-types]". I then tried mxGetData; it did not give any warnings yet when I called the function from the command window, MATLAB crashed and was forced to close.

I would be highly appreciated if anyone can help me out. Thanks and here is the whole .c code:

/*==========================================================
 * arrayMultiplier.c 
 *
 * Multiplies a 1xN matrix with a N X N matrix
 * and outputs a 1xN matrix 
 *
 * The calling syntax is:
 *    outMatrix = arrayProduct(vector, array)
 *========================================================*/

    #include "mex.h"

    /* The computational routine */
    void arrayMultiplier( double *vec, double **arr, mwSize n ,double *z){
        for (mwSize i=0; i<n; i++) {
            for(mwSize j=0; j<n; j++) {
                z[i] += vec[j]*arr[j][i]; 
            }
            z[i] = (double)((int)z[i] % 2); 
        }   
    }

    /* The gateway function */
    void mexFunction( int nlhs, mxArray *plhs[],
                      int nrhs, const mxArray *prhs[]) {
        double *vec;              
        double **arr;               
        mwSize nr;
        double *outMatrix;              

        nr = mxGetN(prhs[0]);   /*The "n" parameter of the 1 x n output matrix*/

        vec = mxGetDoubles(prhs[0]);

        arr = mxGetData(prhs[1]);       /* This is where the warning appears */

        /* creating the output matrix */
        plhs[0] = mxCreateDoubleMatrix(1,nr,mxREAL);

        /* get a pointer to the real data in the output matrix */
        outMatrix = mxGetDoubles(plhs[0]);

        /* call the computational routine */
        arrayMultiplier(vec,arr,nr,outMatrix);
  }

Answer 1

[James Tursa]

You can't use the [j][i] indexing syntax with the pointer returned by mxGetDoubles. You would need to set up a separate pointer array in order to use this syntax (never worth it IMO), or just do the indexing calculations manually (my preference).

E.g., make these changes in mexFunction:

        double *arr;  // **arr changed to *arr
             :
        arr = mxGetDoubles(prhs[1]);

And make these changes in arrayMultiplier:

    void arrayMultiplier( double *vec, double *arr, mwSize n ,double *z){  // **arr changed to *arr
                      :
                z[i] += vec[j] * (*arr++);

Since the 2D matrix is stored column-wise and the vector is on the left of the multiply, you can access the 2D matrix in a simple linear fashion with the *arr++ syntax. A functionally equivalent version using explicit indexing would have been this (but with extra unneeded indexing calculations for this particular case):

                z[i] += vec[j] * arr[j+i*n];  // Manually calculate the equivalent linear index

[Mustafa Aydin]

Thank you!

[KayLee Trickey]

I know you say this isn't worth it, but how would you set up a separate pointer array to use the [i][j] syntax? My code uses that syntax way more then the original poster, and I'm trying to figure out how to get it to work correctly. I'm getting the same incompatible error.

[James Tursa]

I don't have MATLAB or C handy at the moment, but an outline for a 2d matrix is something like this:

#define ARR(i,j) arr[j][i]
    double **arr;
    mwSize i, j, m, n;
        :
    m = mxGetM(prhs[1]);
    n = mxGetN(prhs[1]);
    arr = (double **) mxMalloc( n * sizeof(*arr) );
    arr[0] = mxGetDoubles(prhs[1]);
    for( j=1; j<n; j++ ) {
        arr[j] = arr[j-1] + m;  // The column pointers
    }
        :
    // Then use arr[j][i] in your code, i is row and j is column, 0-based
    // Or use ARR(i,j) in your code, i is row and j is column, 0-based
        :
    mxFree(arr);

Note that when using arr[j][i] syntax the j comes first, since MATLAB array memory is column ordered. But when using the macro ARR(i,j) the i comes first since that is how I have defined the macro to work.

If you really want to use the syntax arr[i][j] with i (row) first and j (column) second, you would have to copy all of the 2D matrix values to new memory (essentially transposing it). This, IMO, is too high a price to pay and I don't include that code here.

Answer 2

[Walter Roberson]

mxGetDoubles will always be pointer to double, with the pointer being to the column-major linear storage of the data. To use this as a 2D array at the C level, you need to cast it to the appropriate double vec[m,n] declaration.

[Mustafa Aydin]

Thank you!

[KayLee Trickey]

I am trying to do something similar to original poster, how do you cast it as the appropriate double declaration? I am knew to C and 2D arrays and pointers all all new to me.

In my c code, I use a lot of arr[i][j] indexing syntax.