Eigen Array indexing - c++

Question

I would like to create new Eigen::Array using an index.
I know it is possible for Eigen::Matrix, a code is given here.
A similar question has also been posted on stackoverflow
The question is how to update the following code to work with Eigen::Array
#include <iostream>
#include <stdio.h>
#include <Eigen/Core>
using namespace Eigen;
template<class ArgType, class RowIndexType, class ColIndexType>
class indexing_functor {
const ArgType &m_arg;
const RowIndexType &m_rowIndices;
const ColIndexType &m_colIndices;
public:
typedef Matrix<typename ArgType::Scalar,
RowIndexType::SizeAtCompileTime,
ColIndexType::SizeAtCompileTime,
ArgType::Flags&RowMajorBit?RowMajor:ColMajor,
RowIndexType::MaxSizeAtCompileTime,
ColIndexType::MaxSizeAtCompileTime> MatrixType;
indexing_functor(const ArgType& arg, const RowIndexType& row_indices, const ColIndexType& col_indices)
: m_arg(arg), m_rowIndices(row_indices), m_colIndices(col_indices)
{}
const typename ArgType::Scalar& operator() (Index row, Index col) const {
return m_arg(m_rowIndices[row], m_colIndices[col]);
}
};
template <class ArgType, class RowIndexType, class ColIndexType>
CwiseNullaryOp<indexing_functor<ArgType,RowIndexType,ColIndexType>, typename indexing_functor<ArgType,RowIndexType,ColIndexType>::MatrixType>
indexing(const Eigen::MatrixBase<ArgType>& arg, const RowIndexType& row_indices, const ColIndexType& col_indices)
{
typedef indexing_functor<ArgType,RowIndexType,ColIndexType> Func;
typedef typename Func::MatrixType MatrixType;
return MatrixType::NullaryExpr(row_indices.size(), col_indices.size(), Func(arg.derived(), row_indices, col_indices));
}

Answer 1

With #ggael's help, I made this little test program that uses Eigen master branch.
First I fetch eigen repo, that points on master branch, and includes new indexing features, that are not present in tagged version 3.3.5.
git clone https://github.com/eigenteam/eigen-git-mirror
Here is the program that I compile with g++ -O3 -isystem eigen-git-mirror -o eigen_indexing eigen_indexing.cpp
#include <iostream>
#include <Eigen/Dense>
int main()
{
Eigen::ArrayX3d array = Eigen::ArrayX3d::Random(8, 3);
Eigen::ArrayXi indices(5);
indices << 4, 7, 0, 2, 1;
Eigen::Array3i cols(0,1,2);
std::cout << "array = " << std::endl << array << std::endl << std::endl;
std::cout << "indices = " << std::endl << indices << std::endl << std::endl;
std::cout << "cols = " << std::endl << cols << std::endl << std::endl;
std::cout << "array(indices,cols) = " << std::endl << array(indices,cols) << std::endl << std::endl;
std::cout << "Equivalent but Eigen::placeholders::all syntax may evolve " << std::endl << std::endl;
std::cout << "array(indices, Eigen::placeholders::all) = " << std::endl << array(indices,Eigen::placeholders::all) << std::endl << std::endl;
std::cout << "Assignment also works array(indices,cols) = 0.0;" << std::endl<< std::endl;
array(indices,cols) = 0.0;
std::cout << "array = " << std::endl << array << std::endl << std::endl;
std::cout << "Assignment also works array(indices,cols) = 10 * Eigen::ArrayX3d::Random(5, 3);" << std::endl<< std::endl;
array(indices,cols) = 10 * Eigen::ArrayX3d::Random(5, 3);
std::cout << "array = " << std::endl << array << std::endl << std::endl;
return 0;
}
If the program can not compile, here is the version I have used
cd eigen-git-mirror
# git checkout master
git checkout a31719c75f244673c962ee65f279606bee6fc7ef
cd ..
Here is the output of the program
array =
-0.999984 0.358593 0.342299
-0.736924 0.869386 -0.984604
0.511211 -0.232996 -0.233169
-0.0826997 0.0388327 -0.866316
0.0655345 0.661931 -0.165028
-0.562082 -0.930856 0.373545
-0.905911 -0.893077 0.177953
0.357729 0.0594004 0.860873
indices =
4
7
0
2
1
cols =
0
1
2
array(indices,cols) =
0.0655345 0.661931 -0.165028
0.357729 0.0594004 0.860873
-0.999984 0.358593 0.342299
0.511211 -0.232996 -0.233169
-0.736924 0.869386 -0.984604
Equivalent but Eigen::placeholders::all syntax may evolve
array(indices, Eigen::placeholders::all) =
0.0655345 0.661931 -0.165028
0.357729 0.0594004 0.860873
-0.999984 0.358593 0.342299
0.511211 -0.232996 -0.233169
-0.736924 0.869386 -0.984604
Assignment also works array(indices,cols) = 0.0;
array =
0 0 0
0 0 0
0 0 0
-0.0826997 0.0388327 -0.866316
0 0 0
-0.562082 -0.930856 0.373545
-0.905911 -0.893077 0.177953
0 0 0
Assignment also works array(indices,cols) = 10 * Eigen::ArrayX3d::Random(5, 3);
array =
-8.1607 5.24396 2.65277
-1.68001 -9.05071 9.82075
3.07838 -4.75094 5.12821
-0.0826997 0.0388327 -0.866316
6.92334 4.02381 4.72164
-0.562082 -0.930856 0.373545
-0.905911 -0.893077 0.177953
0.538576 8.20642 -3.43532