Always false float comparisons

[Joakim via Digitalmars-d]

On Thursday, 19 May 2016 at 11:00:31 UTC, Ola Fosheim Grøstad 
wrote:
> On Thursday, 19 May 2016 at 08:37:55 UTC, Joakim wrote:
>> On Thursday, 19 May 2016 at 08:28:22 UTC, Ola Fosheim Grøstad 
>> wrote:
>>> On Thursday, 19 May 2016 at 06:04:15 UTC, Joakim wrote:
>>>> In this case, not increasing precision gets the more 
>>>> accurate result, but other examples could be constructed 
>>>> that _heavily_ favor increasing precision.  In fact, almost 
>>>> any real-world, non-toy calculation would favor it.
>>>
>>> Please stop saying this. It is very wrong.
>>
>> I will keep saying it because it is _not_ wrong.
>
> Can you then please read this paper in it's entirety before 
> continuing saying it. Because changing precision breaks 
> properties of the semantics of IEEE floating point.
>
> What Every Computer Scientist Should Know About Floating-Point 
> Arithmetic
>
> https://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html#3377
>
> «Conventional wisdom maintains that extended-based systems must 
> produce results that are at least as accurate, if not more 
> accurate than those delivered on single/double systems, since 
> the former always provide at least as much precision and often 
> more than the latter. Trivial examples such as the C program 
> above as well as more subtle programs based on the examples 
> discussed below show that this wisdom is naive at best: some 
> apparently portable programs, which are indeed portable across 
> single/double systems, deliver incorrect results on 
> extended-based systems precisely because the compiler and 
> hardware conspire to occasionally provide more precision than 
> the program expects.»

The example he refers to is laughable because it also checks for 
equality.

>> The notion that "error correction" can fix the inevitable 
>> degradation of accuracy with each floating-point calculation 
>> is just laughable.
>
> Well, it is not laughable to computer scientists that accuracy 
> depends on knowledge about precision and rounding... And I am a 
> computer scientists, in case you have forgotten...

Computer scientists are no good if they don't know any science.