How to access a specific (row,col) index in an C++ Eigen sparse matrix?

This Code work for me

for (int i=0; i<matrix.rows(); ++i){
     for(int j=0; i<matrix.cols(); ++j)
        cout << " i,j=" << i << "," 
             << j << " value=" 
             << matrix.coeff(i,j) 
             << std::endl;
}

Try coeff:

double value = matrix.coeff(iRow, iCol);

If you want a non-const version use coeffRef instead. Note that when using coeffRef if the element doesn't exist, it will be inserted.