how to access multidimensional array input in C++ mex file

Question

I have written my program in c++ and needed to write a c++mex function to pass input parameter and retrieve some output values in matlab. but i have been facing some issues accessing my multidimensional array. here follows a piece of what i have written and the error message. w was declare as follows:

double **W;

W[i][j];

for  (int i=0;i < 6; i++)
{
    for (int j=0;j<6;j++)
    {
        //B[j + 6*i] = W[i][j];
        B[j + 6*i] = W[i + 6*j];     
    }
}

and when compiling i get the following error message:

cannot convert ‘double*’ to ‘double’ in assignment

Answer 1

Your 2-D array represented as double** W needs to have space allocated. First you need to allocate each row. The code below illustrates this:

#include <iostream>

using namespace std;

int main() {

int rows = 6, cols = 6;
double **W; 

W = new double* [rows];

// allocate each row
for (int i = 0; i < rows; i++) {
    W[i] = new double [cols];
}

for (int i = 0; i < 6; i++) {
    for (int j = 0; j<6; j++) { 
        // do work here
    }
}

// clean up in the end:
if (W != nullptr) {
    for (int i = 0; i < rows; i++) {
        delete[] W[i];
    }
    delete[] W;
    W = nullptr;
}

return 0;
}

Answer 2

The following code is wrong:

double **W;

W[i][j];

Either declare W as:

double W[i][j];

Or read: How do I declare a 2d array in C++ using new.

Your 2D array accessing syntax

B[j + 6*i] = W[i + 6*j]; 

is wrong too.

Are you trying to do this?

B[j][6*i] = W[i][6*j];// if B is 2D array

Or

B[j + 6*i] = W[i][6*j];// if B is 1D array