Writing MAT file from c++. I have fully compiled code but values come out odd.

So I have a regular old c++ array of size 1 by 2919111 and I write this to a mat file using the matimport.c example. Where n(1) = 7.734261 in memory when written to the mat file it's value is 9.624101589057462e+04 and all other array entries are equal to zero though the mat file has the correct array size. Anyone have any ideas?

#include "creader.h"
#include "mat.h"
#include "matrix.h"

using namespace std;

int main(int argc, char *argv[]) {

    /* MAT-file */
    char * pch;
    char fname[50];

    strcpy(fname,argv[1]);
    strcat(fname,".mat");
    pch = strstr (fname,"CER");
    strncpy (pch,"cct",3);

    MATFile *pmat;
    const char *myFile = fname;

    /* Variables for mxArrays  */
    mxArray *pn;

    /* MATLAB variable names */
    const char *myn = "n";

    int status1; 

    /* Create and open MAT-file */
    printf("Creating file %s...\n\n", myFile);
    pmat = matOpen(myFile, "w");
    if (pmat == NULL) {
      printf("Error creating file");
    return(EXIT_FAILURE);
    }

   float * n = new float [k];
_   STUFF TO FILL IN n_

/* Create mxArrays and copy external data */
  pn = mxCreateDoubleMatrix(1,k,mxREAL);

   if ((pn == NULL) {
        printf("Unable to create mxArray with mxCreateDoubleMatrix\n");
        return(EXIT_FAILURE);
    }

memcpy((void *)(mxGetPr(pn)), (void *)n, sizeof(n));

/* Write data to MAT-file */
  status1 = matPutVariable(pmat, myn, pn);
 if ((status1) != 0) {
      printf("Error writing.\n");
      return(EXIT_FAILURE);
  } 

/* Clean up */
  mxDestroyArray(pn);
 return 0;
}

Answer 1

[James Tursa]

Walter has noted your problem already, but I would point out that this is not the best way to go about things. You can avoid the new, delete (which you forgot), and memcpy simply by changing this

float * n = new float [k];
_   STUFF TO FILL IN n_

/* Create mxArrays and copy external data */
  pn = mxCreateDoubleMatrix(1,k,mxREAL);

     if (pn == NULL) { /* you had a typo here */
          printf("Unable to create mxArray with mxCreateDoubleMatrix\n");
          return(EXIT_FAILURE);
      }

memcpy((void *)(mxGetPr(pn)), (void *)n, sizeof(n));

to this:

pn = mxCreateDoubleMatrix(1,k,mxREAL);
   if (pn == NULL) {
        printf("Unable to create mxArray with mxCreateDoubleMatrix\n");
        return(EXIT_FAILURE);
    }
double *n = mxGetPr(pn);
_   STUFF TO FILL IN n_

[Jon]

This is wonderful. Thanks! (The typo was from deleting a lot of other variables)

Answer 2

[Walter Roberson]

You have mxCreateDoubleMatrix but you write "float" (single precision) into its data pointer.

[Jon]

Holy crap, this is what I get for not reading the API carefully enough. Thank you!