efficient way?

gistfile1.cpp

vex::vector<double> h(ctx, n1 * n2);
vex::vector<uint32_t> N(ctx, n1 * n3);
vex::vector<double> V(ctx, n4 * n2);
vex::vector<double> L(ctx, n1 * n3);
for (int i = 0; i < h.rows(); ++i) {
  for (int j = 0; j < N.cols(); ++j) {
    row(V, N(i,j)) -= L(i, j) * row(h, i);
  }
}