Eigen Convert Matrix to Vector

Question

In MATLAB, the line below converts a Matrix to a Vector.It flattens the matrix column by column into a vector.

myvar(:)

Ho

Answer 1

If you wanna change the matrix values without defining a new variable you can use

Eigen::MatrixXd A(2,3);
A.data()[3] = 1.2

in this case data()[3] will correspond to index A[1,1] of matrix, or read them by

double value = A.data()[5];

so if I write down the whole 2by3 matrix it would be like

A.data()[0]     A.data()[2]    A.data()[4]
A.data()[1]     A.data()[3]    A.data()[5]