Re: [eigen] Assigning row vector expression to VectorXd

["Jean-Claude Monnin" <jc_monnin@xxxxxxxxxxxxx>]

Hi Christoph,

Thanks for your reply.

I didn't realize the size mismatch was on the right hand side of the expression with q being a column vector. I wrongly assumed the problem was on the assignment. Therefore, following expression also works:

    VectorXd v4 = (m.row(i) - m.row(j)).cwiseQuotient(q.transpose());

As you say, the assignments work in a consistent way, which makes much more sense.

Cheers,
Jean-Claude

> On 14 Aug 2019, at 14:04, Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> 
> Hi!
> 
> This is not a problem of implicitly converting a row-vector to a column vector, but that you are mixing a column and a row-vector in the same expression. I.e., expressions like
> 
>  row.cwiseQuotient(col); row + col;
> 
> much more likely are user-errors, so they are prohibited (this has nothing to do with assigning them). Also, for `row+col` what should be the dimensions of the result (should it result in a row or column)?
> 
> If one of the dimensions is Dynamic, it is only possible to check this at run-time, since Dynamic could be 1 at runtime. And runtime checks are disabled when compiled with `-DNDEBUG` (or `-DEIGEN_NO_DEBUG`).
> 
> Allowing to assign row-expressions to VectorXd was an early design-decision which can't be changed without breaking a lot of existing code. I would generally suggest to always make sure that dimensions match exactly. Adding a `.transpose()` does not add any runtime overhead (when compiled with optimizations enabled).
> 
> I can't find the exact point where (or if) this behavior (the implicit transposition) is documented.
> Related: http://eigen.tuxfamily.org/bz/show_bug.cgi?id=1218
> 
> 
> Cheers,
> Christoph
> 
> 
> On 14/08/2019 11.30, Jean-Claude Monnin wrote:
>> Hi,
>> When assigning a row vector into VectorXd (eg. a column vector), it does an implicit transpose in most cases (like in the line `VectorXd tmp = m.row(i) - m.row(j);` below).
>> I'm wondering why it doesn't behave the same way in line `VectorXd v3 = (m.row(i) - m.row(j)).cwiseQuotient(q);`. This gives the the wrong result in release mode (but calls`abort` in debug mode due to size mismatch).
>> Why does the assignment of `v3` behaves differently than the assignment of `tmp` in the example below? Is this documented behavior?
>> Cheers,
>> Jean-Claude
>>  {
>>  using namespace Eigen;
>>  MatrixXd m = MatrixXd::Random(10, 4);
>>  VectorXd q = VectorXd::Random(m.cols());
>>  Index i = 3;
>>  Index j = 8;
>>  VectorXd tmp = m.row(i) - m.row(j);
>>  VectorXd v1 = tmp.cwiseQuotient(q);
>>  VectorXd v2 = (m.row(i) - m.row(j)).transpose().cwiseQuotient(q);
>>  VectorXd v3 = (m.row(i) - m.row(j)).cwiseQuotient(q);
>>  std::cout << "v1 rows:" << v1.rows() << ", cols:" << v1.cols() << std::endl;
>>  std::cout << "v2 rows:" << v2.rows() << ", cols:" << v2.cols() << std::endl;
>>  std::cout << "v3 rows:" << v3.rows() << ", cols:" << v3.cols() << std::endl;
>>  std::cout << "v1 = " << v1.transpose() << std::endl;
>>  std::cout << "v2 = " << v2.transpose() << std::endl;
>>  std::cout << "v3 = " << v3.transpose() << std::endl;
>>  }
>> Output in release mode:
>> v1 rows:4, cols:1
>> v2 rows:4, cols:1
>> v3 rows:1, cols:1
>> v1 = -0.0375015 1.61562 -1.3097 -0.394064
>> v2 = -0.0375015 1.61562 -1.3097 -0.394064
>> v3 = -0.0375015
> 
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>  Dr.-Ing. Christoph Hertzberg
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