MATLAB Application Program Interface Reference

Create an unpopulated N-dimensional structure mxArray

#include "matrix.h"
mxArray *mxCreateStructArray(int ndim, const int *dims, int nfields,
         const char **field_names);

Arguments

ndim

Number of dimensions. If you set ndims to be less than 2, mxCreateNumericArray creates a two-dimensional mxArray.

dims

The dimensions array. Each element in the dimensions array contains the size of the array in that dimension. For example, setting dims[0] to 5 and dims[1] to 7 establishes a 5-by-7 mxArray. Typically, the dims array should have ndim elements.

nfields

The desired number of fields in each element.

field_names

The desired list of field names.

Returns

A pointer to the created structure mxArray, if successful; otherwise, returns NULL. The most likely cause of failure is insufficient heap space to hold the returned mxArray.

Description

Call mxCreateStructArray to create an unpopulated structure mxArray. Each element of a structure mxArray contains the same number of fields (specified in nfields). Each field has a name; the list of names is specified in field_names. A structure mxArray in MATLAB is conceptually identical to an array of structs in the C language.

Each field holds one mxArray pointer. mxCreateStructArray initializes each field to NULL. Call mxSetField or mxSetFieldByNumber to place a non-NULL mxArray pointer in a field.

When you finish using the returned structure mxArray, call mxDestroyArray to reclaim its space.

Examples

Create a 2-by-3 structure mxArray, in which each element contains two fields (the "name" field and the "grade" field). Then, populate the field with data. Each "name" field holds a string mxArray and each "grade" field holds a numeric mxArray.

#define  rows 3
#define  cols 2
#define  TOTAL_ELEMENTS (rows * cols)
 int      ndim = 2, dims[2] = {rows, cols};
 int      number_of_fields=2; 
 const char *field_names[] = {"name", "grade"};
 const char *names_values[] = {"Rachel","Aysha","Maria",
                               "Per","Martin","Bob"};
 double    grades_values[] = {100, 95, 95, 97, 98, 96};
 double    *pr;
 mxArray  *field_value, *struct_array_ptr; 
 int       index;
 unsigned char  *start_of_pr, *start_of_pi;
 mxArray *array_ptr;
 size_t   bytes_to_copy;
 /* Create a 2-by-3 array of structs. */ 
   struct_array_ptr = mxCreateStructArray(ndim, dims, 
                                     number_of_fields, field_names);

 /* Populate the 6 name fields. */ 
   for (index=0; index<TOTAL_ELEMENTS; index++) {
      field_value = mxCreateString(names_values[index]);
      mxSetField(struct_array_ptr, index, "name", field_value); 
      /* mxDestroyArray(field_value); */
   }

 /* Populate the 6 grade fields. */ 
   for (index=0; index<TOTAL_ELEMENTS; index++) {
      field_value = mxCreateDoubleMatrix(1, 1, mxREAL);
      pr = mxGetPr(field_value);
      pr[0] = grades_values[index];
mxSetField(struct_array_ptr, index, "grade", field_value); 
      /* mxDestroyArray(field_value); */
   }

For an additional example, see mxCreateStructArray.c in the mx subdirectory of the examples directory.