给定布尔向量，如何从C ++特征数组中删除某些系数？

Question

I have a Eigen::VectorXd v and I would like to create another vector w containing only the coefficients from v meeting a certain criteria, like positiveness, something like

Eigen::VectorXd v(3);
v << -1, 0, 1;
w = v(v > 0);
std:: cout << v; // prints  -1, 0, 1
std:: cout << w; // prints 1

How can I do this in Eigen?

Answer 1

Copy v in w , sort w with std::sort and then find the first non-positive value with std::find_if . That will be the end of your list of conforming values.

You can use any other criteria with std::sort .

Answer 2

using namespace std;
using namespace Eigen;

template<typename T>
void push_back(VectorXd &v, T val, T ref)
{
    if (val > ref /*Or Your condition*/) {
        v.conservativeResize(v.size() + 1);
        v[v.size() - 1] = val;
    }
}

int main()
{
    VectorXd v(3); 
    v << 1, 2, 3; 

    const double ref = 0.0f;
    push_back(v, 4.0, ref);
    for (size_t i = 0; i < v.size(); i++)
        cout << v(i) << "  ";
    return 0;
}

You get the idea, you can even templatize the first argument.