MATLAB: Read and access MAT file data in C

.mat filec

Hi everyone,
I'm new to the C programmation with the API library in Matlab and I'm trying to read and access a mat file data which is a 3d array. In my C code "p" is a pointer to the 3d array which values I'm trying to print, but when I compile (mex MyCode.c) I have a errors, here's my C code:
#include "mex.h"
#include "matrix.h"
#include "mat.h"
   void mexFunction( int nlhs, mxArray *plhs[],int nrhs, const mxArray *prhs[] )
  {
    MATFile *pmat;
    double x;
    mxArray *pa;
    int n;
    const char *name;
    const char **dir;
    int    ndir;
    int i,c;
    int num_of_dim, num_of_fields, rows, col, bands,r,cc,b;
   const int *dim_array;
   mxArray ***p;
    /*
     * Open file to get directory
     */
    pmat = matOpen("data3.mat", "r");
    if (pmat == NULL)
    {
      printf("Error creating file \n");
      printf("(Do you have write permission in this directory?)\n");
      return(EXIT_FAILURE);
    }
    /*
     * get directory of MAT-file
     */
    dir = (const char **)matGetDir(pmat, &ndir);
    if (dir == NULL) {
        printf("Error reading directory of file ");
      return(EXIT_FAILURE);
    } 
    mxFree(dir);
    if (matClose(pmat) != 0) 
    {
      printf("Error closing file \n");
      return(EXIT_FAILURE);
    }
    pmat = matOpen("data3.mat", "r");
    if (pmat == NULL) {
      printf("Error reopening file \n");
      return(EXIT_FAILURE);
    }
     /* Read in each array. */
    printf("\nReading in the actual array contents:\n");
    for (i=0; i<ndir; i++) 
    {
        pa = matGetNextVariable(pmat, name);
        if (pa == NULL) 
        {
      printf("Error reading in file \n");
      return(EXIT_FAILURE);
        }
    } 
     /* Get the number of dimensions in array */
      num_of_dim=mxGetNumberOfDimensions(pa);
      dim_array = mxGetDimensions(pa);
      printf("the number of dimensions are %d \n",num_of_dim);
      for (c=0; c<num_of_dim; c++)
      {
         mexPrintf("%d\n", *(dim_array+c)); 
      }
     rows = *(dim_array);
     col = *(dim_array+1);
     bands = *(dim_array+2);
      p=mxGetData(pmat);
      /* Display the values of the mat file */
     for (r=0;r<rows;r++,printf("\n"))
     {
         for(cc=0;cc<col;cc++,printf("\n"))
         {
             for(b=0;b,<bands;b++,printf("\n"))
             {
                mexPrintf("%d\n",*(p + col*bands*r + bands*cc + b));
             }
         }
     }
   }
I will be really grateful to know what's wrong with my C code.
Thanks.
-J

Best Answer

This:
const int *dim_array;
should probably be this
const mwSize *dim_array;
Also, this
mxArray ***p;
should probably be this, assuming the array is double class (you should check this in your code)
double *p;
And if the values really are double, then you should be using a format other than %d for printing, since that is for integer types. E.g., this
              mexPrintf("%d\n",*(p + col*bands*r + bands*cc + b));
should be something like this instead
              mexPrintf("%g\n",*(p + col*bands*r + bands*cc + b));
And this
    p=mxGetData(pmat);
should probably be this
    p = mxGetData(pa);
And you should have this at the end of your outer loop
    mxDestroyArray(pa);
This might not be totally correct either ... seems like you would need to free the name strings as well:
    mxFree(dir);
Finally, it is not clear to me which variable you are trying to display here. You read them all in a loop, and then display only the last one? Is that intended?
And then in the following call, I think "name" needs to have memory allocated behind it prior to the call ... I will have to check on this later.
    pa = matGetNextVariable(pmat, name);
EDIT:
I checked, and that last line should be this:
    pa = matGetNextVariable(pmat, &name);
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