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According to the official documentations, they suggest that the most efficient way is to get the pointer to the row first, and then just use the plain C operator []. It also saves a multiplication for each iteration.

// compute sum of positive matrix elements
// (assuming that M isa double-precision matrix)
double sum=0;
for(int i = 0; i < M.rows; i++)
{
    const double* Mi = M.ptr<double>(i);
    for(int j = 0; j < M.cols; j++)
        sum += std::max(Mi[j], 0.);
}