How can I create a matrix slice from a dynamic structur?

I need something like that

matrix_vector_slice<matrix<double> > mvsRow, mvsCol

for (iterate over matrix element)

    if (matrix element in condition) {

mvsRow.push( row );

mvsCol.push( col );    

}

     

matrix.assign(mvsRow, mvsCol) = newvalue

Thanks

Phil