Question

I’m trying to populate a structure in a mex function. I have the basics of the structure created in the mexFunction, but its getting a little confusing when a field in the structure is an array and i want to populate each element in this array as the ‘for’ loop iterates.

....
mxArray *value;
mwSize dims[2] = {16,8};
int numFields = 2;  
const char *fieldNames[] = {"array1", "array2"}; 
plhs[2] = mxCreateStructArray(2, dims, numFields, fieldNames);

for (int i = 0; i < 16; i++)
{
    for (int j = 0; j < 8; j ++)
    {
        value1 = (some calculation);
        value = mxCreateDoubleMatrix(1,18,mxREAL); 
        *mxGetPr(value[sampleIndex]) = value1;
        mxSetField(plhs[2], i, "array1", value);
        // i want to set the array1[j] element

        value2 = (some other calculation);
        value = mxCreateDoubleMatrix(1,8,mxREAL); 
        *mxGetPr(value) = value2;
        mxSetField(plhs[2], i, "array2", value);


    }
}
....

The fields “array1” and “array2” are both vectors of the same size (lets say 8 elements) i am not sure how to populate each individual element. How do i set the field name by array index ‘j’.

To aid in the explanation, this is how the structure should look:
structure is an array of 16 elements, each element has 2 fields, each field has 8 elements each

structure(0).array1 = [1 2 3 4 5 6 7 8];
structure(0).array2 = [11 12 13 14 15 16 17 18];

structure(1).array1 = [21 22 23 24 25 26 27 28];
structure(1).array2 = [211 212 213 214 215 216 217 218];

structure(2).array1 = [31 32 33 34 35 36 37 38];
structure(2).array2 = [311 312 313 314 315 316 317 318];

structure(3).array1 = [41 42 43 44 45 46 47 48];
structure(3).array2 = [411 412 413 414 415 416 417 418];
...

The values are just arbitrary.

Answer 1

Consider the following C code:

structArrayMEX.c

#include "mex.h"

void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])
{   
    mxArray *x, *y;
    double *xData, *yData;
    mwIndex idx, i;
    int counter = 1;

    // create a 16x1 structure array, each with two fields x and y
    const char *fieldNames[] = {"x", "y"}; 
    mxArray *s = mxCreateStructMatrix(16, 1, 2, fieldNames);

    // fill structure array
    for (idx=0; idx<16; idx++) {
        // create x and y matrices each of size 1x8
        x = mxCreateDoubleMatrix(1, 8, mxREAL);
        y = mxCreateDoubleMatrix(1, 8, mxREAL);

        // fill x and y matrices: x[i], y[i]
        xData = mxGetPr(x);
        yData = mxGetPr(y);
        for (i=0; i<8; i++) {
            xData[i] = counter;
            yData[i] = counter + 100;
            counter++;
        }

        // set fields: s(idx).x = x and s(idx).y = y
        mxSetField(s, idx, "x", x);
        mxSetField(s, idx, "y", y);
    }

    // return structure array
    plhs[0] = s;
}

The returned structure array:

>> s = structArrayMEX();
>> whos s
  Name       Size            Bytes  Class     Attributes

  s         16x1              4096  struct         

>> s(1)
ans = 
    x: [1 2 3 4 5 6 7 8]
    y: [101 102 103 104 105 106 107 108]
>> s(16)
ans = 
    x: [121 122 123 124 125 126 127 128]
    y: [221 222 223 224 225 226 227 228]

The above MEX-function should be equivalent to the following MATLAB code

vals = reshape(1:16*8, [8 16])';
s = struct('x',num2cell(vals,2), 'y',num2cell(vals+100,2));