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Problem extracting data out of a variable of mat file in C++

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I have to extract the data out of a variable(double array) of a mat file for use in my C++ code.I have tried the mxGetPr, mxGetVariable and mxGetData functions but they do not seem to work.I am able to read the mat file and even get the variable's name and no. of rows and columns but I haven't been able to extract data.Please help me out ,need urgent help!!
James Tursa
James Tursa on 26 Jun 2012
Please post you code so we can point out the errors and suggest corrections.
Qandeel on 27 Jun 2012
Edited: Walter Roberson on 28 Jun 2012
Here is my code -I have actually modified the matlab diagnose mat file code and added the getvariable command to extract the data.
#include <stdio.h>
#include <stdlib.h>
#include "mat.h"
#include "matrix.h"
#include "mex.h"
#include <iostream>
int diagnose(const char *file) {
MATFile *pmat;
const char **dir;
const char *a;
int ndir;
int i;
mxArray *pa;
double *pr;
printf("Reading file %s...\n\n", file);
* Open file to get directory
pmat = matOpen(file, "r");
if (pmat == NULL) {
printf("Error opening file %s\n", file);
* get directory of MAT-file
dir = (const char **)matGetDir(pmat, &ndir);
if (dir == NULL) {
printf("Error reading directory of file %s\n", file);
} else {
printf("Directory of %s:\n", file);
for (i=0; i < ndir; i++)
/* Read in each array. */
printf("\nReading in the actual array contents:\n");
/*Get Variable*/
pa = matGetVariable(pmat, file);
//I donot know how to get the variable pointed to by pointer 'pa'
and use it in my C++ code.
int main(int argc, char **argv)
int result;
if (argc > 1)
result = diagnose(argv[1]);
result = 0;
printf("Usage: matdgns <matfile>");
printf(" where <matfile> is the name of the MAT-file");
printf(" to be diagnosed\n");
return (result==0)?EXIT_SUCCESS:EXIT_FAILURE;

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Answers (1)

James Tursa
James Tursa on 28 Jun 2012
Your incorrect code:
/*Get Variable*/
pa = matGetVariable(pmat, file);
The 2nd argument is supposed to be the variable name, not the file name. E.g.,
/*Get Variable*/
pa = matGetVariable(pmat, dir[i]);
Then you can get at the data using mxGetPr(pa) etc. Just be sure not to free dir before you are done with it. And when you are done with it, free each dir[i] first since all of these strings are dynamically allocated also.
Ketan on 1 Jul 2012
The %d format specifier indicates an integer argument, but I do not think the compiler converts the double *pr to int.
Try casting *pr from a double to an int datatype. For example:
You only see one element because you are not iterating over the #elements in the array (the variable l).
This stack overflow article goes into some detail regarding the printf issue:
Qandeel on 1 Jul 2012
Thank you Ketan for your timely help, the casting solved my issue. I am really grateful to your help on my question .

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