I frequently find myself attempting to resize a matrix block before copying it to a fixed-size matrix or using it in a function call. Something like

Matrix<double, 9, Dynamic> A;
Matrix3d B = A.col(0).conservativeResize(3, 3);

This is not allowed so my current work-around is to make B a dynamic matrix:

MatrixXd B = A.col(0);
B.conservativeResize(3, 3);

If I really want a fixed-size matrix, I have make another copy,

Matrix3d C = B;

Is there a better way to accomplish this?

What you want is to "reshape" an expression, not resizing it. You can workaround using Map on the lhs or rhs, e.g.:

Code: Select all

Matrix3d B;
Map<VectorXd>(B.data(), 9) = A.col(0);