pointer to sub-row of Eigen MatrixXd that behaves like a VectorXd

Question

I have an Eigen MatrixXd and need a pointer to some subsequent entries of some row. I would like to be able to use this pointer. I have something like this:

Eigen::MatrixXd* matrix = new MatrixXd(3, 3);
(*matrix) <<    1, 2, 3,
                4, 5, 6,
                7, 8, 9;

Block<MatrixXd, 1, Dynamic, false, true> full_row = (*matrix).row(1);
// this gives me the full row. I am interested only in the row containing 5 6.

Block<MatrixXd> part_row = (*matrix).block(1, 1, 1, 2);
// this gives me the partial row that I want, but now i need two indices to 
// access an element. 

part_row(0, 1) = 3; // works
part_row(1) = 3; // gives compiler error

I would like to be able to directly access the partial row, without having to copy the values. This is really important, since it has to be done often and I cannot afford to copy vectors back and forth. (I believe I cannot expect the compiler to optimize out the copying, since the sizes of the matrices are generally unknown). Any help is greatly appreciated. Cheers!

Answer 1

You need to specify that your submatrix is a vector:

Block<MatrixXd,1,Dynamic> part_row(*matrix, 1, 1, 1, 2);