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

[Benoit Jacob <jacob.benoit.1@xxxxxxxxx>]

Thanks Gael for fixing this; did you add a unit test?

Benoit

2010/7/16 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
>>
>>
>>
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