return value of mxGetPr() -- equivalent looping - c++

Question

I am trying to implement a mexFunction() into "pure" C++ (OpenCV), but the returned value by mxGetPr() is not clear at all for me.
The following code is aimed to be implemented:
void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])
{
int *D = new int[N*L];
// where N and L are dimensions (cols and rows) of matrix prhs[3]
// prhs[3] is a NxL matrix containing floating point value
for (int i=0; i<N*L; i++)
D[i] = mxGetPr(prhs[3])[i];
}
My question is, what kind of value is given by mxGetPr(prhs[3])[i] and mxGetPr(prhs[4])[i]? And how is it looping through matrix?
I tried to do something like this:
for (int i=0; i<l; i++)
{
for(int j=0; j<n; j++)
{
D[iCounter] = (int)d.at<uchar>(i,j);
iCounter++;
}
}
Looping through d matrix which is the same as input value prhs[3], but apparently it is not correct.
I guess the order/type of the returned value is not the same as in the original mexFunction.
EDIT
Now I have cv::Mat d; instead of prhs[3] and try to do the same as in mexfunction.
int *D = new int[N*L];
int iCounter = 0;
for (int i=0; i<L; i++)
{
for(int j=0; j<N; j++)
{
D[iCounter] = (int)d.at<uchar>(i,j);
iCounter++;
}
}
But here (int)d.at(i,j) returns value of the "d" matrix...where in the roiginal code a pointer was returned by mxGetPr().

Answer 1

mxGetPr returns a pointer of type double so you can access your data using pointer arithmetic. Also, you must remember that the pointer returned to you has the data in column-major order. This means that you must traverse your data row-wise instead of column-wise like in traditional C order.
In column-major order, you access location (i, j) with the following linear index:
j * rows + i
rows is the number of rows in your matrix, with i and j being the row and column you want to access. In row-major or C order, the way you access data is:
i * cols + j
Here cols is the number of columns in your matrix. I'm assuming you want to lay out your data in row-major format rather than column major. Therefore if you want to loop through the data using two for loops, do something like this:
double *ptr = mxGetPr(prhs[3]);
// A L x N matrix - L rows, N columns
for (int i = 0; i < L; i++)
{
for (int j = 0; j < N; j++)
{
D[i * N + j] = (int) ptr[j * L + i];
}
}
Here D is a pointer pointing to integer data. You have to cast the data in order to do this as the pointer to the data from MATLAB is already double. It's nasty but that's what you have to do. You can use D in row-major order so it's compatible with the rest of your code. I'm assuming that you are using MATLAB MEX as way of making pre-written C++ code to be interfaced with MATLAB.