Importing and Exporting Data (External Interfaces/API)

External Interfaces/API

Examples of MAT-Files

This section includes C and Fortran examples of writing, reading, and diagnosing MAT-files. The examples cover the following topics:

Creating a MAT-File in C

This sample program illustrates how to use the library routines to create a MAT-file that can be loaded into MATLAB. The program also demonstrates how to check the return values of MAT-function calls for read or write failures.

/*
 * MAT-file creation program
 *
 * See the MATLAB API Guide for compiling information.
 *
 * Calling syntax:
 *
 *   matcreat
 *
 * Create a MAT-file which can be loaded into MATLAB.
 *
 * This program demonstrates the use of the following functions:
 *
 *  matClose
 *  matGetVariable
 *  matOpen
 *  matPutVariable
 *  matPutVariableAsGlobal
 *
 * Copyright 1984-2000 The MathWorks, Inc.
 * $Revision: 1.9 $
 */
#include <stdio.h>
#include <string.h> /* For strcmp() */
#include <stdlib.h> /* For EXIT_FAILURE, EXIT_SUCCESS */
#include "mat.h"

#define BUFSIZE 256

int main() {
  MATFile *pmat;
  mxArray *pa1, *pa2, *pa3;
  double data[9] = { 1.0, 4.0, 7.0, 2.0, 5.0, 8.0, 3.0, 6.0, 9.0 };
  const char *file = "mattest.mat";
  char str[BUFSIZE];
  int status; 

  printf("Creating file %s...\n\n", file);
  pmat = matOpen(file, "w");
  if (pmat == NULL) {
    printf("Error creating file %s\n", file);
    printf("(Do you have write permission in this directory?)\n");
    return(EXIT_FAILURE);
  }

  pa1 = mxCreateDoubleMatrix(3,3,mxREAL);
  if (pa1 == NULL) {
      printf("%s : Out of memory on line %d\n", __FILE__,
              __LINE__); 
      printf("Unable to create mxArray.\n");
      return(EXIT_FAILURE);
  }

  pa2 = mxCreateDoubleMatrix(3,3,mxREAL);
  if (pa2 == NULL) {
      printf("%s : Out of memory on line %d\n", __FILE__,
             __LINE__);
      printf("Unable to create mxArray.\n");
      return(EXIT_FAILURE);
  }
  memcpy((void *)(mxGetPr(pa2)), (void *)data, sizeof(data));
  
  pa3 = mxCreateString("MATLAB: the language of technical
                         computing");
  if (pa3 == NULL) {
      printf("%s :  Out of memory on line %d\n", __FILE__,
              __LINE__);
      printf("Unable to create string mxArray.\n");
      return(EXIT_FAILURE);
  }

  status = matPutVariable(pmat, "LocalDouble", pa1);
  if (status != 0) {
      printf("%s :  Error using matPutVariable on line %d\n",
              __FILE__, __LINE__);
      return(EXIT_FAILURE);
  }  
  
  status = matPutVariableAsGlobal(pmat, "GlobalDouble", pa2);
  if (status != 0) {
      printf("Error using matPutVariableAsGlobal\n");
      return(EXIT_FAILURE);
  } 
  
  status = matPutVariable(pmat, "LocalString", pa3);
  if (status != 0) {
      printf("%s :  Error using matPutVariable on line %d\n",
              __FILE__, __LINE__);
      return(EXIT_FAILURE);
  } 
  
  /*
   * Ooops! we need to copy data before writing the array.  (Well,
   * ok, this was really intentional.) This demonstrates that
   * matPutVariable will overwrite an existing array in a 
MAT-file.
   */
  memcpy((void *)(mxGetPr(pa1)), (void *)data, sizeof(data));
  status = matPutVariable(pmat, "LocalDouble", pa1);
  if (status != 0) {
      printf("%s :  Error using matPutVariable on line %d\n",
              __FILE__, __LINE__);
      return(EXIT_FAILURE);
  } 
  
  /* Clean up. */
  mxDestroyArray(pa1);
  mxDestroyArray(pa2);
  mxDestroyArray(pa3);

  if (matClose(pmat) != 0) {
    printf("Error closing file %s\n",file);
    return(EXIT_FAILURE);
  }

  /* Re-open file and verify its contents with matGetVariable. */
  pmat = matOpen(file, "r");
  if (pmat == NULL) {
    printf("Error reopening file %s\n", file);
    return(EXIT_FAILURE);
  }

  /* Read in each array we just wrote. */
  pa1 = matGetVariable(pmat, "LocalDouble");
  if (pa1 == NULL) {
    printf("Error reading existing matrix LocalDouble\n");
    return(EXIT_FAILURE);
  }
  if (mxGetNumberOfDimensions(pa1) != 2) {
    printf("Error saving matrix: result does not have two
            dimensions\n");
    return(EXIT_FAILURE);
  }

  pa2 = matGetVariable(pmat, "GlobalDouble");
  if (pa2 == NULL) {
    printf("Error reading existing matrix GlobalDouble\n");
    return(EXIT_FAILURE);
  }
  if (!(mxIsFromGlobalWS(pa2))) {
    printf("Error saving global matrix: result is not global\n");
    return(EXIT_FAILURE);
  }

  pa3 = matGetVariable(pmat, "LocalString");
  if (pa3 == NULL) {
    printf("Error reading existing matrix LocalString\n");
    return(EXIT_FAILURE);
  }
  
  status = mxGetString(pa3, str, sizeof(str));
  if(status != 0) {
      printf("Not enough space. String is truncated.");
      return(EXIT_FAILURE);
  }
  if (strcmp(str, "MATLAB: the language of technical
             computing")) {
    printf("Error saving string: result has incorrect
            contents\n");
    return(EXIT_FAILURE);
  }

  /* Clean up before exit. */
  mxDestroyArray(pa1);
  mxDestroyArray(pa2);
  mxDestroyArray(pa3);

  if (matClose(pmat) != 0) {
    printf("Error closing file %s\n",file);
    return(EXIT_FAILURE);
  }
  printf("Done\n");
  return(EXIT_SUCCESS);
}

To produce an executable version of this example program, compile the file and link it with the appropriate library. Details on how to compile and link MAT-file programs on the various platforms are discussed in the section, Compiling and Linking MAT-File Programs.

Once you have compiled and linked your MAT-file program, you can run the stand-alone application you have just produced. This program creates a MAT-file, mattest.mat, that can be loaded into MATLAB. To run the application, depending on your platform, either double-click on its icon or enter matcreat at the system prompt.

```
matcreat
Creating file mattest.mat...
```

To verify that the MAT-file has been created, at the MATLAB prompt enter

whos -file mattest.mat
  Name               Size         Bytes  Class

  GlobalDouble       3x3             72  double array (global)
  LocalDouble        3x3             72  double array
  LocalString        1x43            86  char array

Grand total is 61 elements using 230 bytes

Finding Associated Files Reading a MAT-File in C