Hello world!

How can i get a left matrix-vector multiplication using eigen?

Do you mean M*V or V*M?

You need to obey the way that matrix multiplication works; Eigen vectors are simply matrices with one row or one column.

Matrix<float, 1, 3> V1;
Matrix<float, 3, 1> V2;
Matrix<float, 3, 3> M;

Allows you to say

float f = V1 * M * V2;

to complete the answer, you might also need .transpose() to transform a column vector to a row one:

Matrix<float, 3, 1> V2;
Matrix<float, 3, 3> M;

V2.transpose() * M

Thanx