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 <[email protected]>
> 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
>
> --
> Dr.-Ing. Christoph Hertzberg
>
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