how to set a vector element in a mex structure - c

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));