iterator for sparse matrix

Hi, 
I need some help to use iteratiors for sparse matrix.
I define my matrix as:
typedef mtl::matrix<std::complex<double>,mtl::rectangle<>,mtl::compressed<>,mtl::row_major >::type MyMatrix;

and I can only cover the matix elements using

for (int i=0; i<n,i++){
    for (int j=0; j<n,j++){
        x = A(i,j);
    }
}

it is very time expensive because my matrix is sparse and i want only get the nonzero elements.
Do you have any idea how I can solve this problem using interator. thank you for any hints!

Regards,
Ricardo

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