In MATLAB, the line below converts a Matrix to a Vector.It flattens the matrix column by column into a vector.
myvar(:)
How do I do that with Eigen? The solution should work for any dimension of Matrix.
MatrixXd A(3,2);
VectorXd B(6);
A << 1,2,3,4,5,6;
B << A.col(0), A.col(1);
//This isn't general enough to work on any size Matrix
Eigen matrices are stored in column major order by default, so you can use simply use Eigen Maps to store the data column by column in an array:
MatrixXd A(3,2);
A << 1,2,3,4,5,6;
VectorXd B(Map<VectorXd>(A.data(), A.cols()*A.rows()));
If you want the data ordered row by row, you need to transpose the matrix first:
MatrixXd A(3,2);
A << 1,2,3,4,5,6;
A.transposeInPlace();
VectorXd B(Map<VectorXd>(A.data(), A.cols()*A.rows()));
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]
Another way to do it is:
...
VectorXd B = A;
B.resize(B.cols()*B.rows(), 1);
or, if you want the vector in row order:
...
VectorXd B = A.transpose();
B.resize(B.cols()*B.rows(), 1);
Regards.
