error C2109: subscript requires array or pointer type (MEX, Windows 10)

I am using mex to implement C++ codes in MATLAB on Windows 10. I want to modify the elements of a 2D array that I created but I keep getting this error. Below are the codes and the error message.

#include <omp.h>
#include "mex.h"
#include "matrix.h"

// net.idx_phiz{m}(:,1:num_v) => get_index(net.idx_phiz{m}, num_v)

extern "C" void mexFunction(int nlhs, mxArray* plhs[], int nrhs, const mxArray* prhs[])
{
    //receive value and create output variable
    auto& matrix = prhs[0];
    auto& size = prhs[1];
    auto& out = plhs[0];

    //get size
    auto m = mxGetM(matrix);
    auto n = mxGetPr(size)[0];

    //get value
    auto val = mxGetPr(matrix);

    //create output pointer
    out = mxCreateNumericMatrix(m, n, mxDOUBLE_CLASS, mxREAL);
    auto B = (double*)mxGetPr(out);

#pragma omp parallel for schedule(static)
    for (int i = 0; i < m; i++) {
        for (int j = 0; j < n; j++) {
            B[i][j] = val[i][j];
        }
    }
}

From the last line, I get the error as below:

error C2109: subscript requires array or pointer type

I looked up past discussions but they are usually the error C2109 not pertaining to mex. Thank you in Advance.

Solution

This line is incorrect, B and val are pointers to double, and therefore cannot be indexed twice, only once.

B[i][j] = val[i][j];

Think of the data in a MATLAB array as a 1D array, where the columns of the logical array are stacked. Given m rows and n columns, one can obtain the location in memory for the matrix element (i,j) with i + m*j. Consequently, your copy operation should read:

B[i + m*j] = val[i + m*j];

However, the above is likely to lead to problems since n is given by a second argument, and there is no guarantee that the input matrix has than many columns. You need to add an explicit test to verify that matrix has at least n columns, and that it is a double-valued matrix (if it is, for example, of type single you'll be reading out of bounds as well, since mxGetPr always returns a pointer to double, not matter what the data in the array actually is.