我以前见过它，但我不记得如何用相同长度的 a 有效地初始化Eigen::Vector已知长度的 a std::vector。这是一个很好的例子：
std::vector<double> v1 = {1.0, 2.0, 3.0};
Eigen::Vector3d v2; // Do I put it like this in here: v2(v1) ?
v2 << v1[0], v1[1], v1[2]; // I know you can do it like this but
// I am sure i have seen a one liner.
我已经仔细阅读了有关高级矩阵初始化的页面，但没有明确说明执行此操作的方法。
根据 Eigen Doc，Vector是 Matrix 的 typedef，而 Matrix 有一个具有以下签名的构造函数：
Matrix (const Scalar *data)构造一个使用从 data 开始的系数初始化的固定大小的矩阵。
向量参考定义std::vector::data为：
std::vector::data T* data(); const T* data() const;返回指向用作元素存储的基础数组的指针。指针使得范围
[data(); data() + size())始终是有效范围，即使容器为空。
因此，您可以将向量的数据作为Vector3d构造函数参数传递：
Eigen::Vector3d v2(v1.data());
此外，从 Eigen 3.2.8 开始，上述构造函数定义为：
template<typename _Scalar, int _Rows, int _Cols, int _Options, int _MaxRows, int _MaxCols>
inline Matrix<_Scalar, _Rows, _Cols, _Options, _MaxRows, _MaxCols>
::Matrix(const Scalar *data)
{
this->_set_noalias(Eigen::Map<const Matrix>(data));
}
如您所见，它也使用Eigen::Map，正如@ggael 和@gongzhitaao 所指出的那样。
只是为了扩展@ggael的答案，以防其他人没有注意到：
float data[] = {1,2,3,4};
Map<Vector3f> v1(data); // uses v1 as a Vector3f object
Map<ArrayXf> v2(data,3); // uses v2 as a ArrayXf object
Map<Array22f> m1(data); // uses m1 as a Array22f object
Map<MatrixXf> m2(data,2,2); // uses m2 as a MatrixXf object
以下单行应该更正确：
#include <Eigen/Dense>
#include <Eigen/Core>
std::vector<double> a = {1, 2, 3, 4};
Eigen::VectorXd b = Eigen::Map<Eigen::VectorXd, Eigen::Unaligned>(a.data(), a.size());
我通过此链接找到了更好的答案：
https://forum.kde.org/viewtopic.php?f=74&t=94839
基本上首先创建一个指向 std 向量的指针，然后使用 Map 将指针和长度传递给构造函数。
此方法适用于 Eigen 中的动态 Vector 对象。当我尝试使用 std 向量中的 .data() 函数作为第一个答案建议时，它给了我一个错误：静态断言失败：YOU_CALLED_A_FIXED_SIZE_METHOD_ON_A_DYNAMIC_SIZE_MATRIX_OR_VECTOR
但是使用这种方法它可以工作！
我只是从此处的链接中复制并粘贴相关代码：
std::vector<double> v(4, 100.0);
double* ptr = &v[0];
Eigen::Map<Eigen::VectorXd> my_vect(ptr, 4);
这是我在数组/向量和 Eigen::Vector/Eigen::Matrix 之间转换的简单代码。
#include <iostream>
#include <vector>
#include <Eigen/Dense>
using namespace std;
using namespace Eigen;
void array2eigenMat();
void eigenMat2array();
void array2eigenVec();
void eigenVec2array();
void vector2eigenMat();
void eigenMat2vector();
void vector2eigenVec();
void eigenVec2vector();
int main()
{
cout << "hello world" <<endl;
array2eigenMat();
eigenMat2array();
array2eigenVec();
eigenVec2array();
vector2eigenMat();
eigenMat2vector();
vector2eigenVec();
eigenVec2vector();
return 0;
}
void array2eigenMat()
{
cout << "-------------------------- array2eigenMat --------------------------" << endl;
int array[9];
for (int i = 0; i < 9; ++i) array[i] = i;
cout << "array = [ "; for (int i = 0; i < 9; ++i) cout << array[i] << " "; cout << "]" << endl;
cout << "colMajor matrix = \n" << Map<Matrix3i>(array) << endl; // map a contiguous array as a column-major matrix
cout << "rowMajor matrix = \n" << Map<Matrix<int, 3, 3, RowMajor>>(array) << endl; // map a contiguous array as a row-major matrix
cout << "stride matrix = \n" << Map<MatrixXi, 0, OuterStride<>>(array, 3, 3, OuterStride<>(2)) << endl;
//mapping an array while specifying an outer stride. Here, since we're mapping as a column-major matrix,
// 'outer stride' means the pointer increment between two consecutive columns
Map<MatrixXi> eigMat1(array, 3, 3);
MatrixXi eigMat2 = Map<MatrixXi>(array, 3, 3);
array[0] = 9;
cout << "eigMat1 matrix = \n"; cout << eigMat1 << endl;
cout << "eigMat2 matrix = \n"; cout << eigMat2 << endl;
cout << "---------------------------------------------------------------------" << endl;
}
void eigenMat2array()
{
cout << "-------------------------- eigenMat2array --------------------------" << endl;
Matrix3d eigMat;
eigMat <<
1, 2, 3,
4, 5, 6,
7, 8, 9;
cout << "init eigMat = \n"; cout << eigMat << endl;
double* eigMatptr = eigMat.data();
cout << "array = [ "; for (int i = 0; i < 9; ++i) cout << eigMatptr[i] << " "; cout << "]" << endl;
eigMat(0, 0) = 9;
cout << "array = [ "; for (int i = 0; i < 9; ++i) cout << eigMatptr[i] << " "; cout << "]" << endl;
double *eigMatptrnew = new double[eigMat.size()];
Map<MatrixXd>(eigMatptrnew, eigMat.rows(), eigMat.cols()) = eigMat;
eigMat(2, 2) = 0;
cout << "init matrix = \n"; cout << eigMat << endl;
cout << "array = [ "; for (int i = 0; i < 9; ++i) cout << eigMatptr[i] << " "; cout << "]" << endl;
cout << "---------------------------------------------------------------------" << endl;
}
void array2eigenVec()
{
cout << "-------------------------- array2eigenVec --------------------------" << endl;
int array[9];
for (int i = 0; i < 9; ++i) array[i] = i;
cout << "data array = [ "; for (int i = 0; i < 9; ++i) cout << array[i] << " "; cout << "]" << endl;
Map<VectorXi> eigVec(array, 5);
cout << "eigen vector transpose = " << eigVec.transpose() << endl;
cout << "stride vector transpose = " << Map<VectorXi, 0, InnerStride<2> >(array, 4).transpose() << endl;
// map an array as a vector, specifying an inner stride, that is, the pointer increment between two consecutive coefficients
array[0] = 9;
cout << "eigen vector transpose = " << eigVec.transpose() << endl;
cout << "stride vector transpose = " << Map<VectorXi, 0, InnerStride<2> >(array, 4).transpose() << endl;
cout << "---------------------------------------------------------------------" << endl;
}
void eigenVec2array()
{
cout << "-------------------------- eigenVec2array --------------------------" << endl;
VectorXf eigvec(5);
eigvec << 0, 1, 2, 3, 4;
cout << "eigen vector transpose = " << eigvec.transpose() << endl;
float *array = new float;
array = eigvec.data();
cout << "data array = [ "; for (int i = 0; i < eigvec.size(); ++i) cout << array[i] << " "; cout << "]" << endl;
eigvec(0) = 9;
cout << "data array = [ "; for (int i = 0; i < eigvec.size(); ++i) cout << array[i] << " "; cout << "]" << endl;
array[0] = 5;
cout << "eigen vector transpose = " << eigvec.transpose() << endl;
cout << "---------------------------------------------------------------------" << endl;
}
void vector2eigenMat()
{
cout << "-------------------------- vector2eigenMat --------------------------" << endl;
vector<int> stdvec{ 1, 2, 3, 4, 5, 6, 7, 8, 9 };
Map<Matrix<int, 3, 3, RowMajor>> eigMat1(stdvec.data());
MatrixXi eigMat2 = Map<Matrix<int, 3, 3, RowMajor>>(stdvec.data());
cout << "eigMat1 matrix = \n"; cout << eigMat1 << endl;
cout << "eigMat2 matrix = \n"; cout << eigMat2 << endl;
stdvec[0] = 9;
cout << "eigMat1 matrix = \n"; cout << eigMat1 << endl;
cout << "eigMat2 matrix = \n"; cout << eigMat2 << endl;
cout << "---------------------------------------------------------------------" << endl;
}
void eigenMat2vector()
{
cout << "-------------------------- eigenMat2vector --------------------------" << endl;
Matrix3d eigMatCol;
eigMatCol <<
1, 2, 3,
4, 5, 6,
7, 8, 9;
cout << "eigen matrix col = \n"; cout << eigMatCol << endl;
vector<double> stdvec1(eigMatCol.data(), eigMatCol.data() + eigMatCol.size());
cout << "std vector1 = ["; for (int i = 0; i < stdvec1.size(); ++i) cout << stdvec1[i] << " "; cout << "]" << endl;
Matrix<double, 3, 3, RowMajor> eigMatRow = eigMatCol;
cout << "eigen matrix row = \n"; cout << eigMatCol << endl;
vector<double> stdvec2(eigMatRow.data(), eigMatRow.data() + eigMatRow.size());
cout << "std vector2 = ["; for (int i = 0; i < stdvec2.size(); ++i) cout << stdvec2[i] << " "; cout << "]" << endl;
cout << "---------------------------------------------------------------------" << endl;
}
void vector2eigenVec()
{
cout << "-------------------------- vector2eigenVec --------------------------" << endl;
vector<int> stdvec{ 1, 2, 3, 4, 5 };
cout << "std vector = ["; for (int i = 0; i < stdvec.size(); ++i) cout << stdvec[i] << " "; cout << "]" << endl;
Map<VectorXi> eigVec1(stdvec.data(), stdvec.size());
VectorXi eigVec2 = Map<VectorXi>(stdvec.data(), stdvec.size());
cout << "eigen vector1 transpose = " << eigVec1.transpose() << endl;
cout << "eigen vector2 transpose = " << eigVec2.transpose() << endl;
cout << "stride vector transpose = " << Map<VectorXi, 0, InnerStride<2> >(stdvec.data(), 2).transpose() << endl;
stdvec[0] = 9;
cout << "eigen vector1 transpose = " << eigVec1.transpose() << endl;
cout << "eigen vector2 transpose = " << eigVec2.transpose() << endl;
cout << "stride vector transpose = " << Map<VectorXi, 0, InnerStride<2> >(stdvec.data(), 2).transpose() << endl;
cout << "---------------------------------------------------------------------" << endl;
}
void eigenVec2vector()
{
cout << "-------------------------- eigenVec2vector --------------------------" << endl;
VectorXf eigvec(5);
eigvec << 0, 1, 2, 3, 4;
cout << "eigen vector transpose = " << eigvec.transpose() << endl;
vector<float> stdvec(eigvec.data(), eigvec.data() + eigvec.size());
cout << "std vector = ["; for (int i = 0; i < stdvec.size(); ++i) cout << stdvec[i] << " "; cout << "]" << endl;
eigvec(0) = 5;
cout << "std vector = ["; for (int i = 0; i < stdvec.size(); ++i) cout << stdvec[i] << " "; cout << "]" << endl;
cout << "---------------------------------------------------------------------" << endl;
}
更多细节在这里。