Initialise Eigen::vector with std::vector
C++EigenC++ Problem Overview
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.
C++ Solutions
Solution 1 - C++
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 - C++
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 - C++
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 - C++
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);
Solution 5 - C++
Here is my simple code that converts between array/vector and Eigen::Vector/Eigen::Matrix.
#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;
}
More details are Here.