Re: [eigen] Returning Tensor Expressions

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I am using C++14. Thank you, I had completely forgotten that auto worked for plain functions as well as lambdas.


I will respond to Eugene’s e-mail with a minimal working example for my second point.




From: David Tellenbach <david.tellenbach@xxxxxxxxxxxxx>
Reply to: "eigen@xxxxxxxxxxxxxxxxxxx" <eigen@xxxxxxxxxxxxxxxxxxx>
Date: Tuesday, 17 December 2019 at 17:15
To: "eigen@xxxxxxxxxxxxxxxxxxx" <eigen@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [eigen] Returning Tensor Expressions


Hi Toby,


could you specify which C++ standard you are using?


The possible implementations for a free function that replaces your lambda depends on this.

E.g. since C++14 you should be able to just declare the function as


  auto func(...)




On 17. Dec 2019, at 13:15, Wood, Tobias <tobias.wood@xxxxxxxxx> wrote:




I am trying to write a finite difference function for Eigen::Tensors. Currently I am using a lambda:


auto diff = [](Eigen::Tensor<std::complex<float>, 3> const &a, Eigen::Index const d) {

  Dims3 const sz{a.dimension(0) - 2, a.dimension(1) - 2, a.dimension(2) - 2};

  Dims3 const st1{1, 1, 1};

  Dims3 fwd{1, 1, 1};

  Dims3 bck{1, 1, 1};

  fwd[d] = 2;

  bck[d] = 0;


  return (a.slice(fwd, sz) - a.slice(bck, sz)) / a.slice(st1, sz).constant(2.f);



This works okay. However, I would like to do two things:


1 – Change this from a lambda into a free function. What should the return type of the function be, so that it returns the _expression_/operation and does not evaluate the tensor into a temporary?

2 – I would prefer to pass in a TensorRef, so I can pass in a .chip() from a 4D tensor without a temporary. When I try to do this with the current lambda, and I am assigning to a slice, e.g.


b.chip<3>(0).slice(st1, sz) = diff(a, 0);


I get the following error:


TensorRef.h:413:51: error: cannot initialize return object of type 'Eigen::TensorEvaluator<const Eigen::TensorRef<Eigen::Tensor<std::__1::complex<float>, 3, 0, long> >, Eigen::ThreadPoolDevice>::Scalar *' (aka 'std::__1::complex<float> *') with an rvalue of type 'const Eigen::TensorRef<Eigen::Tensor<std::__1::complex<float>, 3, 0, long> >::Scalar *' (aka 'const std::__1::complex<float> *')


This appears to be complaining that I can’t assign a `const std::complex<float> *` to a `std::complex<float> *`?


Thanks in advance,



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