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From Sébastien Brisard <>
Subject Re: svn commit: r1348721 - in /commons/proper/math/trunk/src: main/java/org/apache/commons/math3/linear/ test/java/org/apache/commons/math3/linear/
Date Sat, 16 Jun 2012 07:38:19 GMT
Hi Gilles,

2012/6/16 Gilles Sadowski <>:
> Hi.
>> >>
>> >> > [...]
>> >> >
>> >> >> I fully agree. We could also opt for a less correct, but more
>> >> >> efficient solution: we do not store the sign of zero, and return
>> >> >> each time v / zero occurs. The result should be NaN anyway, because
>> >> >> its sign is undecidable. This specificity would be clearly stated
>> >> >> the javadoc. What do you think?
>> >> >
>> >> > We had this sort of discussion with the "Complex" class, where there
>> >> > such issues as "standard" vs "correct" vs "documented" etc. E.g. some
>> >> > computations return "NaN" where it should be "infinity" (or vice-versa,
>> >> > don't remember exactly). See MATH-667:
>> >> >
>> >> >
>> >> > Maybe that we must be guided with some use-cases for this class.
>> >> > Maybe in actual uses, the problems discussed here do not appear, and
>> >> > additional check would destroy the usefulness of this class in such
>> >> > (just a wild guess). Maybe that we should drop support for sparse vectors!
>> >> >
>> >> I fully agree with you. Speaking quite honestly as a user of the
>> >> linear package, even if I know that IEEE floats handle infinite values
>> >> well, I tend to dish results that contain those values, and try to
>> >> cure the cause of these unwanted values. So, as a user, whether the
>> >> method returns NaN of infinity does not matter to me (as long as it
>> >> does not return zero --or any normal value-- quietly): in any case, I
>> >> would do a computation again, this time with "correct" values.
>> >>
>> >> I also think that in this case, strict adherence to IEEE standards
>> >> (which were written for scalar values, not vectors) should not
>> >> compromise the performance of the class.
>> >
>> > Am I mistaken, or is this at odds with a fix that will make the "division"
>> > test pass? [I.e. If there is no way to distinguish between +0 and -0 as the
>> > default entry then "1 / default" will always be "+inf".]
>> >
>> Do you mean I am bending the rules in favour of this specific case?
> I'm just referring to that failure:
> ---
> java.lang.AssertionError: entry #14, left = 1.0, right = -0.0 expected:<-Infinity>
but was:<Infinity>
> ---
> I.e. you can never make this test pass if you do not keep the sign.

>> In
>> fact, it could be argued that the returned value *should* be NaN,
>> since its sign is not decidable. Isn't that the reason why NaNs were
>> created in the first place? If we clearly state that for sparse
>> vectors with zero default value, the sign is lost, then the logical
>> conclusion is that ebeDivide should return NaN instead of infinity.
>> This is only my point of view, and I'm quite happy with any decision
>> (since none is going to be perfect...).
> I'm not convinced; I'd rather throw an exception.
OK, that's an option.

>> >>
>> >> I would be more cautious with your suggestion about getting rid of
>> >> sparse vectors. I think it's nice to have them, even if the support is
>> >> limited (we should of course concentrate on array-based vectors
>> >> first). As long as we clearly document these limits, I'd like to keep
>> >> them. Besides, with the new set of abstract tests being constructed
>> >> for any vector class, it should be possible in the future to propose
>> >> and test a better implementation. The problem we have is clearly lack
>> >> of feedback on these features (remember the debates we had on the
>> >> "sparseIterators"-- not perfect, but experience only can tell how to
>> >> improve).
>> >
>> > Well, if we know that it doesn't work well and we don't know how to fix it
>> > while at the same time keep its usefulness (efficiency-wise), we have to
>> > raise the issue of the necessity to keep supporting it even though it seems
>> > that nobody uses it.
>> >
>> > For the time being (staying compatible), we should at least fix the failing
>> > unit test problem; hence decide for example (as you said before) that the
>> > purpose is to save memory, but the trade-off is loss of performance.
>> >
>> Yes, but the problem is that the inefficient fix has already been
>> implemented... And it does not fix all bugs.
>> Anyway, we could probably aim at the most correct solution (still to
>> be defined!), at the expense of performance, since a more efficient
>> implementation can easily be implemented through the newly implemented
>> visitor pattern.
> If the visitor can compute a correct answer more efficiently, then IMO we
> should readily deprecate the method and remove it ASAP.
Sorry, that wasn't what I meant. I meant that a visitor can compute a
less correct answer more efficiently. So if users can cope with
special values incorrectly handled, this opportunity is offered to
them, while the default implementation is more correct.

>> > Maybe that we could also impose that it is forbidden to divide by a sparse
>> > vector whose default value is zero. This will avoid a check on all entries
>> > (at the expense of forbidding legitimate divisions, when all the entries
>> > have a non-default value; but then one would wonder why using a sparse
>> > vector...).
>> >
>> That's also an idea to explore. I have to say I still do not see
>> clearly what we should do. I quickly looked at BLAS, they apparently
>> do not provide an element-by-element division. I should look at
>> octave, scilab, numpy...
> Gilles
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Gilles, I have the feeling that your prefered option is to store the
sign of the default value. In theory, that would solve the problem. I
see two objections
1. sparse vectors would be less sparse (most users do not care about
the sign of the zero entries!)
2. there is a threshold to decided whether or not an entry is actually
zero. So the current implementation already assumes the sign is lost.

I'm really confused about the best solution to adopt.


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