MATLAB: How to fill an mxArray in a MEX-file for MATLAB

createdatafillinitializeMATLABmexmxarray

I would like to know all the ways I can fill an mxArray.

Best Answer

This change has been incorporated into the documentation in Release 2010a (R2010a). For previous releases, read below for any additional information:
Below is code that shows four valid methods for filling an mxArray with data:
 
#include"mex.h"
#define ONE plhs[0]
#define TWO plhs[1]
#define THREE plhs[2]
#define FOUR plhs[3]
void mexFunction( int nlhs, mxArray *plhs[],
                  int nrhs, const mxArray *prhs[] )
{
    const double data[]={2.1, 3.4, 2.3, 2.45};
    double *dynamicData1, *dynamicData2, *dynamicData4;
    double nonDynamicData[4];
    const int dims[]={0,0};
    int i;
/**********************************************************************************    
   METHOD 1, mxSetData with dynamic data
 *********************************************************************************/
      dynamicData1 = (double*)mxMalloc(4 * sizeof( double ));
      // Create the matrix as 0 by 0, since you are about to allocate the memory manually
      ONE = mxCreateNumericMatrix(0, 0, mxDOUBLE_CLASS, mxREAL);
      for( i = 0; i < 4; i++ )
      {
          // Here, you could be loading data from somewhere.
          // Instead, just copy from data[i]
          dynamicData1[i]=data[i];  
      }
      // Put the C array into the mxArray. You can do this because
      // the mxArray is empty (0x0) at this time, so there will be no
      // memory leak.
      mxSetData(ONE, dynamicData1);
      mxSetM( ONE, 2 );  
      mxSetN( ONE, 2 );
      // It is not necessary to call mxFree( dynamicData1 ) because 
      // that memory is pointed to inside the mxArray you are returning.
/**********************************************************************************    
   METHOD 2, mxSetPr with dynamic data, mxSetPr is only for double data 
 *********************************************************************************/
      dynamicData2 = (double*)mxMalloc(4 * sizeof( double ));
      // Create the matrix as 0 by 0, since you are about to allocate the 
      // memory manually.
      TWO = mxCreateNumericMatrix(0, 0, mxDOUBLE_CLASS, mxREAL);
      for( i = 0; i < 4; i++ )
      {
          // Here, you could be loading data from somewhere.
          // Instead, just copy from data[i]
          dynamicData2[i]=data[i];  
      }
      // Put the C array into the mxArray. You can do this because
      // the mxArray is empty (0x0) at this time so there will be no
      // memory leak
      mxSetPr( TWO, dynamicData2 );
      mxSetM( TWO, 2 );  
      mxSetN( TWO, 2 );
      // It is not necessary to call mxFree( dynamicData2 ) because 
      // that memory is pointed to inside the mxArray you are returning.
/**********************************************************************************    
   METHOD 3, mxGetPr with non-dynamic data
 *********************************************************************************/
      // Create the matrix as 2-by-2, since you are just copying existing data into it
      THREE = mxCreateNumericMatrix(2, 2, mxDOUBLE_CLASS, mxREAL);
      for( i = 0; i < 4; i++ )
      {
          // Here, you could be loading data from somewhere.
          // Instead, just copy from data[i]
          ((double*)mxGetPr(THREE))[i]=data[i];  
      }
/**********************************************************************************    
   METHOD 4, mxGetPr with dynamic data, using a copy of the data
 *********************************************************************************/
      dynamicData4 = (double*)mxMalloc(4 * sizeof( double ));
      // Create the matrix as 2-by-2, since you are just copying existing data into it
      FOUR = mxCreateNumericMatrix(2, 2, mxDOUBLE_CLASS, mxREAL);
      for( i = 0; i < 4; i++ )
      {
          // Here, you could be loading data from somewhere.
          // Instead, just copy from data[i]
          dynamicData4[i]=data[i];  
      }
      for( i = 0; i < 4; i++ )
      {
          // Copy the data in.
          ((double*)mxGetPr(FOUR))[i]=dynamicData4[i];  
      }
      // You must call mxFree( dynamicData4 ) at this time because 
      // that memory is not pointed to inside the mxArray you are 
      // returning. This is because you copied it into the array with mxGetPr.
      mxFree( dynamicData4 );
      return;
  }
The above example only uses the double data type, but you can also create mxArrays of other data type. The documentation has an example using unsigned 16-bit integers:
http://www.mathworks.com/help/releases/R2010a/techdoc/matlab_external/f12977.html#f19620
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