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

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Qandeel 2012-6-26

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/42101-problem-extracting-data-out-of-a-variable-of-mat-file-in-c

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!!

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James Tursa 2012-6-26

Please post you code so we can point out the errors and suggest corrections.

Qandeel 2012-6-27

编辑：Walter Roberson 2012-6-28

在 MATLAB Online 中打开

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);
    return(1);
  }
    /*
     * get directory of MAT-file
     */
    dir = (const char **)matGetDir(pmat, &ndir);
    if (dir == NULL) {
      printf("Error reading directory of file %s\n", file);
      return(1);
    } else {
      printf("Directory of %s:\n", file);
      for (i=0; i < ndir; i++)
        printf("%s\n",dir[i]);
    }
    mxFree(dir);
   /* 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]);
    else{
      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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Answer 1

James Tursa 2012-6-28

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/42101-problem-extracting-data-out-of-a-variable-of-mat-file-in-c#answer_51988

在 MATLAB Online 中打开

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.

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Qandeel 2012-6-28

编辑：Walter Roberson 2012-6-28

在 MATLAB Online 中打开

/* Read in each array. */
   for (i=0; i<ndir; i++) {
      printf("\nReading in the actual array contents:\n");
            pa = matGetVariable(pmat, dir[i]);
         pr  =mxGetPr(pa);
            printf("element data = %d\n",*pr);
            int total_num_of_elements = mxGetNumberOfElements(pa);
            printf("size is %d\n",total_num_of_elements );
              if(mxIsNumeric(pa))
    { printf("Data is numeric\n");}
     m= mxGetM (pa);
        printf("rows is %d\n",m);
       l= mxGetN(pa);
         printf("columns is %d\n",l);
      mxFree(dir);
      }
     }
int main(int argc, char **argv)
{
    int result;
    if (argc > 1)
      result = diagnose(argv[1]);
    else{
      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;
}

but now the output is:

Reading file file.mat...
Directory of file.mat:
s
Reading in the actual array contents:
element data=0
size is 2
data is numeric
rows is 1
columns is 2

I cannot find the problem in this code-Thanking you in advance.

Ketan 2012-7-1

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:

...%d\n",(int)(*pr));

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:

http://stackoverflow.com/questions/2398791/how-is-conversion-of-float-double-to-int-handled-in-printf

Qandeel 2012-7-1

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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