Re: [eigen] Matrix2i mean

[Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>]

Adding an additional template method would not be a problem (minimal 
example: https://godbolt.org/z/ZB9WDI), though I don't see any real 
advantage vs writing

    m.cast<int>().sum();

Sure, `m.sum<int>();` would be shorter, but also more confusing.

Your "another template parameter for the matrices" formulation sounded 
to me like you wanted to add another template parameter to 
`Eigen::Matrix` (which is not an option).

Any patches clarifying the documentation are welcome!

Christoph


On 05/11/2019 16.44, Petr Kubánek wrote:
> Hi,
>
> I know what's the problem. I am just looking either to document it or
> for a solution.
>
> Why would:
>
> template <typename dt> dt sum()
>
> break API/ABI? You will be able to call either sum() or sum<int32_t>(),
> shouldn't you? I will try that and submit a patch.
>
> Having sum<dt>() working, one can hopefully create mean<dt_sum,
> dt_mean>(), so one can code:
>
> mean<int32_t,double>()
>
> Petr
>
> On Tue, 2019-11-05 at 16:36 +0100, Christoph Hertzberg wrote:
> > On 05/11/2019 16.11, Peter wrote:
> > > [...]
> > >
> > > actually, this would also be interesting for the scalar products
> > > in
> > > general, namely a different
> > > type for accumulating the sums within a scalar product, e.g. as
> > > yet
> > > another template parameter for the matrices.
> >
> > Adding another template parameter to basic types is not an option.
> > This
> > would break ABI and API compatibility (even if the parameter has a
> > default).
> >
> > You could create your own custom type `my_int16` for which `my_int16
> > *
> > my_int16` results in a `my_int32` (this needs to be told to Eigen,
> > similar to how real*complex products are handled).
> >
> > > > [...]
> > >
> > > I think it's more subtle than that.
> > > Even
> > >
> > >     int16_t  Two =  2;
> > >     int16_t  Max =  INT16_MAX;
> > >     int16_t mean = ( Max/2 + Max + Two + Two ) / int16_t(4);
> > >
> > > doesn't produce an overflow.
> >
> > Yes, because `Max/2` gets implicitly converted to `int`. Actually,
> > even
> > adding two `int16_t` get implicitly converted to `int` (search for
> > integer promotion rules -- I don't know them entirely either). And
> > dividing an `int` by an `int16_t` results in an `int` which is only
> > afterwards converted to `int16_t`.
> >
> > In contrast, Eigen::DenseBase::sum() does something (more or less)
> > equivalent to:
> >
> >       T sum = T(0);
> >       for(Index i=0; i< size(); ++i)
> >           sum+=coeff(i);
> >       return sum;
> >
> > i.e., after each addition the result gets reduced to the scalar type
> > of
> > the matrix, thus it will immediately overflow.
> >
> > And mean() essentially just takes the return value of `sum()` and
> > divides it by the size.
> >
> >
> > Christoph

--
 Dr.-Ing. Christoph Hertzberg

 Besuchsadresse der Nebengeschäftsstelle:
 DFKI GmbH
 Robotics Innovation Center
 Robert-Hooke-Straße 5
 28359 Bremen, Germany

 Postadresse der Hauptgeschäftsstelle Standort Bremen:
 DFKI GmbH
 Robotics Innovation Center
 Robert-Hooke-Straße 1
 28359 Bremen, Germany

 Tel.:     +49 421 178 45-4021
 Zentrale: +49 421 178 45-0
 E-Mail:   christoph.hertzberg@xxxxxxx

 Weitere Informationen: http://www.dfki.de/robotik
  -------------------------------------------------------------
  Deutsches Forschungszentrum für Künstliche Intelligenz GmbH
  Trippstadter Strasse 122, D-67663 Kaiserslautern, Germany

  Geschäftsführung:
  Prof. Dr. Jana Koehler (Vorsitzende)
  Dr. Walter Olthoff

  Vorsitzender des Aufsichtsrats:
  Prof. Dr. h.c. Hans A. Aukes
  Amtsgericht Kaiserslautern, HRB 2313
  -------------------------------------------------------------