Re: [eigen] Behavior of sum() and prod() for empty vectors

[Gael Guennebaud <gael.guennebaud@xxxxxxxxx>]

you are right, problem fixed for sum(), prod() and indirectly for
trace() and dot().

I also made sure an assertion is triggered for other reductions (that
was not the case).

cheers,

gael



On Fri, Jul 16, 2010 at 1:34 PM, Martin Senst <martin.senst@xxxxxx> wrote:
> Dear developers,
>
> I just stumbled upon a nasty issue with reductions of empty (zero-sized)
> vectors. I'm currently using Eigen3 Beta 1.
> Consider the following program:
>
>
> #include <Eigen/Core>
> #include <iostream>
>
> int main()
> {
>  Eigen::VectorXd v = Eigen::VectorXd::Random(4);
>
>  std::cout << v.head(0).sum() << std::endl; // outputs v(0)
>  std::cout << v.tail(0).sum() << std::endl; // outputs 0
>  std::cout << v.segment(2, 0).sum() << std::endl; // outputs v(2)
>
>  Eigen::VectorXd w;
>  std::cout << w.sum() << std::endl; // segfaults
> }
>
>
> I think that calling sum() on an empty vector should be a perfectly legal
> operation, and should return 0. Similar with prod() and 1. That's according to
> the principle of least surprise, and it's also consistent with Matlab and
> Octave.
> The same code with a VectorXcd, by the way, at least triggers the assertion in
> Redux.h:186, but with double and int as scalar type, v.head(0).sum() just
> silently return v(0). It was pure coincidence that I noticed this behavior.
>
> The same argument of course also applies to trace() and det() of zero-sized
> matrices.
>
> I would have liked to submit a patch, but honestly, I got lost in all the
> template magic... I would be really grateful if one of the gurus could fix
> this.
>
> If you wonder why someone would want to take the sum of an empty vector in the
> first place; well, of course I don't have code like v.head(0).sum() with a
> literal 0, but I do have something like
>
> double f(const VectorXd& v, int n)
> {
>  assert(n >= 0 && n <= v.size());
>  return foo(v.head(n).sum()) + bar(v.tail(v.size() - n).sum());
> }
>
> which should be legal for n == 0 and n == v.size(), and in these cases I
> naturally expect sum() to return 0 for an empty segment.
> If reductions of empty vectors remain unsupported, I would have to clutter my
> code with tons of ifs and special-case-treatments.
>
> Cheers
> Martin
>
>
>