Eigen :: Ref用于连接矩阵 [英] Eigen::Ref for concatenating matrices

问题描述

如果我要连接两个矩阵 A 和 B ，我会这样做

If I want to concatenate two matrices A and B, I would do

using Eigen::MatrixXd;
const MatrixXd A(n, p);
const MatrixXd B(n, q);
MatrixXd X(n, p+q);
X << A, B;

现在，如果 n ， p ， q 很大，以这种方式定义 X 意味着创建副本 A 和 B 。

Now if n, p, q are large, defining X in this way would mean creating copies of A and B. Is it possible to define X as an Eigen::Ref<MatrixXd> instead?

谢谢。

推荐答案

否， Ref 不是为此设计的。我们/您将需要为此定义一个新表达式，该表达式可以称为 Cat 。如果只需要水平连接两个矩阵，则在Eigen 3.3中，可以用不到十几行代码作为空表达式来实现，请参见一些示例有。

No, Ref is not designed for that. We/You would need to define a new expression for that, that could be called Cat. If you only need to concatenate two matrices horizontally, in Eigen 3.3, this can be implemented in less than a dozen of lines of code as a nullary expression, see some exemple there.

编辑：包含的示例表明可以混合矩阵和表达式：

here is a self-contained example showing that one can mix matrices and expressions:

#include <iostream>
#include <Eigen/Core>

using namespace Eigen;

template<typename Arg1, typename Arg2>
struct horizcat_helper {
  typedef Matrix<typename Arg1::Scalar,
    Arg1::RowsAtCompileTime,
    Arg1::ColsAtCompileTime==Dynamic || Arg2::ColsAtCompileTime==Dynamic
    ? Dynamic : Arg1::ColsAtCompileTime+Arg2::ColsAtCompileTime,
    ColMajor,
    Arg1::MaxRowsAtCompileTime,
    Arg1::MaxColsAtCompileTime==Dynamic || Arg2::MaxColsAtCompileTime==Dynamic
    ? Dynamic : Arg1::MaxColsAtCompileTime+Arg2::MaxColsAtCompileTime> MatrixType;
};

template<typename Arg1, typename Arg2>
class horizcat_functor
{
  const typename Arg1::Nested m_mat1;
  const typename Arg2::Nested m_mat2;

public:
  horizcat_functor(const Arg1& arg1, const Arg2& arg2)
    : m_mat1(arg1), m_mat2(arg2)
  {}

  const typename Arg1::Scalar operator() (Index row, Index col) const {
    if (col < m_mat1.cols())
      return m_mat1(row,col);
    return m_mat2(row, col - m_mat1.cols());
  }
};

template <typename Arg1, typename Arg2>
CwiseNullaryOp<horizcat_functor<Arg1,Arg2>, typename horizcat_helper<Arg1,Arg2>::MatrixType>
horizcat(const Eigen::MatrixBase<Arg1>& arg1, const Eigen::MatrixBase<Arg2>& arg2)
{
  typedef typename horizcat_helper<Arg1,Arg2>::MatrixType MatrixType;
  return MatrixType::NullaryExpr(arg1.rows(), arg1.cols()+arg2.cols(),
                                horizcat_functor<Arg1,Arg2>(arg1.derived(),arg2.derived()));
}

int main()
{
  MatrixXd mat(3, 3);
  mat << 0, 1, 2, 3, 4, 5, 6, 7, 8;

  auto example1 = horizcat(mat,2*mat);
  std::cout << example1 << std::endl;

  auto example2 = horizcat(VectorXd::Ones(3),mat);
  std::cout << example2 << std::endl;
  return 0;
}

这篇关于Eigen :: Ref用于连接矩阵的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！