有没有更好的方法来使用 Eigen/C++ 实现 matlab 的逻辑索引？

Question

我一直在寻找一种更 "eigen" 的方法来实现 Matlab 的逻辑索引功能。这是我能想到的最好的。（为了简单起见，这里关注一个 int 数组）

//an attempt at matlab-style Logical Indexing
//equivalent to the matlab:
//  original = [1,2,3,4]
//  subset = original(original < 3)

using namespace Eigen;
using std::cout;
using std::endl;

IOFormat OctaveFmt(StreamPrecision, 0, ", ", " ", "", "", "[", "]");

ArrayXi original(4);
original << 1,2,3,4;
cout<<"Original with bad values:"<<endl
    <<original.format(OctaveFmt)<<endl;

Array<bool, Dynamic,1> selections = original < 3;
cout<<"One if it's a good value:"<<endl
    <<selections.format(OctaveFmt)<<endl;

std::vector<int> picked;
for(int i = 0; i < selections.size(); i++ )
{
    if(selections(i))
    {
        picked.push_back(original(i));
    }
}

//put the vector values back into an eigen array
ArrayXi theGoodStuff = Map<ArrayXi, Unaligned>
                       (picked.data(), picked.size());

cout<<"Just the good stuff:"<<endl
    <<theGoodStuff.format(OctaveFmt)<<endl;

这是我得到的输出：

Original with bad values:
[1  2  3  4]
One if it's a good value:
[1  1  0  0]
Just the good stuff:
[1  2]

有谁知道如何以更 'eigen' 的方式或比遍历数组更快的方式做到这一点？

Answer 1

这是一个旧的但是....Eigen 3.4 使它更干净。可悲的是，我不能谈论表演。我主要对可读性感兴趣。我做了这个：

class logical
{
private:
    const Index new_size;
    Array<Index, Dynamic, 1> old_inds;

public:
    logical(const Array<bool, Dynamic, 1> &keep) : new_size(keep.count()), old_inds(new_size)
    {
        for (Index i = 0, j = 0; i < keep.size(); i++)
            if (keep(i))
                old_inds(j++) = i;
    }
    Index size() const { return new_size; }
    Index operator[](Index new_ind) const { return old_inds(new_ind); }
};

使用方式如下：

logical inds(ind_to_keep);
Y_slice = Y(inds); // vector
H_slice = H(inds, all); // remove some rows
R_slice = R(inds, inds); // remove the same rows as columns

本页底部有 Eigen 帮助： https://eigen.tuxfamily.org/dox-devel/group__TutorialSlicingIndexing.html

有没有更好的方法来使用 Eigen/C++ 实现 matlab 的逻辑索引？

Is there a better way to implement matlab's Logical Indexing using Eigen/C++?

c++

arrays

matlab

slice

eigen3