Initialise Eigen :: vector with std :: vector [英] Initialise Eigen::vector with std::vector

问题描述

我已经看过它之前，但我不记得如何有效地初始化 Eigen :: Vector 已知长度的 std :: vector 的长度相同。这是一个很好的例子：

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.

我已经阅读了 此 页面，但没有关于执行此操作的方法的清楚说明。

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

推荐答案

根据Eigen Doc，向量是Matrix的typedef，而且Matrix具有构造函数< a>具有以下签名：

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

Matrix (const Scalar *data)

构造一个用数据开始的系数初始化的固定大小的矩阵。

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

并且向量引用将 std :: vector :: data 定义为：

std::vector::data

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

返回指向作为元素存储的底层数组的指针。
指针是这样的，范围 [data（）;

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.


$ b（$ data（）+ size（））总是一个
有效范围。 $ b

所以，你可以传递向量的数据作为 Vector3d 构造函数参数：

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

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

此外，从Eigen 3.2.8开始，上面提到的构造函数定义为：

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));
}

如你所见，它还使用 Eigen： ：Map ，如@ggael和@gongzhitaao所述。

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

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