Re: [ViennaCL-devel] Behavior of norm_* on vector<int>

[Philippe T. <phi...@gm...>]

Hi,


2014-06-23 11:06 GMT+02:00 Karl Rupp <ru...@iu...>:

> Hi,
>
>  > I've noticed in the source code that the result for norm_*(vector<int>)
> > was also an int. Am I correct?
>
> Yes, you are.
>
> > If yes,I think that it should be changed
> > because this easily violates the axioms of a norm : we can have
> > norm(alpha*v) != alpha*norm(v) because of the rounding.
>
> This will usually be the case even if we change it. There are good
> reasons why Clang emits warnings when using != or == for floating point
> comparisons ;-)
>
>
I know that ;) But I'd say that the error we make for norm2 using float is
still stable. For integers, I doubt it is :p


> > I think that norm_*(vector<int>) should be changed to float
> > norm_*(vector<int>). Any thoughts?
>
> There is no need to change it for anything for norm_1 and norm_inf. So,
> the only relevant implementation case is norm_2, for which ublas uses
> the same type convention we use now (at least that's what I found when I
> looked it up). Although a floating point return type is probably more
> often desired than an integer type, it would certainly complicate the
> implementation. Moreover, it would introduce inconsistency, which I'm
> not very fond of. The other thing, of course, is that it complicates the
> implementation considerably (which floating point type to return? float
> is not great in terms of precision, but double may not be available on
> the GPU...).
>
> I'm open to using a different approach than what we have now, but I'd
> like to hear solid arguments in favor of a change ;-)
>

Well, this implementation problem already exists! The sqrt() functions only
takes float/double as input (except std11's sqrt which casts to double). As
a result, norm_2() is actually disabled for integers (I had not noticed it
in my first e-mail) ;). This makes a lot of sense to disable it, indeed.
This leads to another  question, though. Should we add on the todo list
some casting operators such as :
viennacl::norm_2(viennacl::cast<float>(v_int)).
For opencl, these can be easily handled by the generator.

Philippe



> Best regards,
> Karli
>
>
>
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