Initialise Eigen::vector with std::vector

I have seen it done before but I cannot remember how to efficiently initialize an Eigen::Vector of known length with a std::vector of the same length. Here is a good example:

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.

I have perused this page about advanced matrix initialization but there is not a clear explanation of the method to perform this action.

Solution 1:

According to Eigen Doc, Vector is a typedef for Matrix, and the Matrix has a constructor with the following signature:

Matrix (const Scalar *data)

Constructs a fixed-sized matrix initialized with coefficients starting at data.

And vector reference defines the std::vector::data as:

std::vector::data

T* data();
const T* data() const;

Returns pointer to the underlying array serving as element storage. The pointer is such that range [data(); data() + size()) is always a valid range, even if the container is empty.

So, you could just pass the vector's data as a Vector3d constructor parameter:

Eigen::Vector3d v2(v1.data());

Also, as of Eigen 3.2.8, the constructor mentioned above defined as:

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));
}

As you can see, it also uses Eigen::Map, as noted by @ggael and @gongzhitaao.

Solution 2:

Just to extend @ggael answer in case others didn't notice it:

From Quick Reference Guide: Mapping External Array:

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

Solution 3:

The following one-liner should be more correct:

#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());

Solution 4:

I found a better answer by this link:

https://forum.kde.org/viewtopic.php?f=74&t=94839

Basically first create a pointer to the std vector, and then pass the pointer and length to the constructor using Map.

This method works with dynamic Vector object in Eigen. While I tried using .data() function from std vector as the first answer suggest, it gives me an error: static assertion failed: YOU_CALLED_A_FIXED_SIZE_METHOD_ON_A_DYNAMIC_SIZE_MATRIX_OR_VECTOR

But using this method it works!

I just copy and paste the relevant code from the link here:

std::vector<double> v(4, 100.0);
double* ptr = &v[0];
Eigen::Map<Eigen::VectorXd> my_vect(ptr, 4);