将特征向量/矩阵中的元素转换为原始双精度类型

Question

Basically how do i do this

Eigen::RowVectorXd param(2);
param << 1.0, 2.0;
double last_element = param.tail(1); // error: no suitable conversion...

I know i can use .coeff() however in my code some vectors are being resized, thus the size is unknown a priori

Answer 1

Would something like this work:

    Eigen::RowVectorXd param(2);
    param << 1.0, 2.0;
    double last_element = param(param.size() - 1);

or even better, using the Eigen provided last value:

    double last_element = param(Eigen::last);