如何从本征矩阵映射/构建C ++向量？

Question

MatrixXf A = MatrixXf::Random(3, 3);

MatrixXf B = A.row(1);

std::vector<float> vec;

I want to built the vector "vec" with elements from the row Eigen matrix "B". Something like this "vec=B.data()"

Answer 1

In addition to the obvious answer (manual push_back s or pre-allocation + index-by-index assignment), you can initialize it directly using the base pointer returned by ::data() :

Eigen::MatrixXf A = Eigen::MatrixXf::Random(3, 3);
Eigen::MatrixXf B = A.row(1);

std::vector<float> vec(B.data(), B.data() + B.size());

Just be careful that Eigen may use memory alignment to take advantage of SSE-family instructions, so it may not work correctly with higher dimensions.

Answer 2

Just use a loop:

MatrixXf A = MatrixXf::Random(3, 3);
MatrixXf B = A.row(1);
std::vector<float> vec(B.size());
for(size_t row=0; row < vec.size(); row++) {
    vec[row] = B[row];
}