Re: [eigen] Behavior of sum() and prod() for empty vectors
• To: eigen@xxxxxxxxxxxxxxxxxxx
• Subject: Re: [eigen] Behavior of sum() and prod() for empty vectors
• From: Gael Guennebaud <gael.guennebaud@xxxxxxxxx>
• Date: Mon, 19 Jul 2010 10:44:44 +0200

```yes I know, I did not look at the determinant case. Now it's fixed too.

gael

On Sun, Jul 18, 2010 at 9:15 PM, Martin Senst <martin.senst@xxxxxx> wrote:
> Dear Gael,
>
> thanks again for your fast response! I've did some tests with the devel
> branch, and pretty much everything works now.
>
> The only comment I still have is that determinant() on an empty matrix
> triggers the assertion in DenseStorageBase.h:487, but I think that for
> consistency reasons, this should be a supported operation. Compiled with
> NDEBUG, it returns 1 as expected.
>
> Cheers
> Martin
>
>
>
> Am Freitag 16 Juli 2010, 14:06:23 schrieb Gael Guennebaud:
>> 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
>
>
>
>

```

 Mail converted by MHonArc 2.6.19+ http://listengine.tuxfamily.org/