How to access multiple elements in c++ Eigen array?

Question

I want to retrieve certain elements in an Eigen array and return them as a vector. I use the following code:

Eigen::ArrayXXi test;
test.resize(5,5);
test.setRandom();
Eigen::Matrix<int, 2, 3> inds;
inds<<0, 2, 3, 2, 3, 1;
auto res = test(inds.row(0), inds.row(1));
std::cout<<"test: \n"<<test <<std::endl;
std::cout<<"inds: \n"<<inds <<std::endl;
std::cout<<"res: \n"<<res <<std::endl;

The output is:

  test:
  730547559  -649503489  -48539462    893772102  -1038736613
 -226810938  -353856438   276748203   291438716  -552146456
  607950953   576018668  -290373134   466641602  -779039257
  640895091  -477225175   28778235   -769652652   653214605
  884005969   115899597   971155939   229713912  -737276042

  inds:
  0 2 3
  2 3 1

  res:
 -48539462   893772102  -649503489
 -290373134  466641602   576018668
  28778235  -769652652  -477225175

The result is a matrix. I note that the diagonal of the matrix is the result I want. I could use res.diagonal() to retrieve the vector. However, I am still wondering if I can do the same thing in a more efficient way.

Answer 1

You can reshape the test Array to a column and then use the single-index access operator:

auto res = test.reshaped()(inds.row(0) + test.rows() * inds.row(1));

Generally, be careful when using auto with Eigen expressions (this case is fine, as long as test and inds are still valid when res is used).