HOWTO: Obtain consecutive index numbers out of triple loop indices

1) Given three index loop over {i,j,k}

int idx = 0;

for (int i = 0, i < ni, ++i) {
  for (int j = 0, j < nj, ++j) {
    for (int k = 0, k < nk, ++k) {

      idx = k + j*nk + i*nj*ni;

      ...

    }
  }
}