Always false float comparisons

[Walter Bright via Digitalmars-d]

On 5/15/2016 2:36 PM, Ola Fosheim Grøstad wrote:
> On Sunday, 15 May 2016 at 21:01:14 UTC, Walter Bright wrote:
>>> Err... these kind of problems only applies to D.
>>
>> Nope. They occur with every floating point implementation in every programming
>> language. FP math does not adhere to associative identities.
>
> No. ONLY D give different results for the same pure function call because you
> bind the result to a "const float" rather than a "float".

It's a fact that with floating point arithmetic, (x+y)+z is not equal to 
x+(y+x), on every programming language, whether or not the result is bound to a 
const float or a float.


> Yes. Algorithms can break because of it.

So far, nobody has posted a legitimate one (i.e. not contrived).


>> Ironically, the identity is more likely to hold with D's extended precision
>> for intermediate values than with other languages.
> No, it is not more likely to hold with D's hazard game. I don't know of a single
> language that doesn't heed a request to truncate/round the mantissa if it
> provides the means to do it.

Standard C provides no such hooks for constant folding at compile time. Neither 
does Standard C++. In fact I know of no language that does. Perhaps you can 
provide a link?


> I care about algorithms working they way I designed them to work and what I have
> tested them for. If I request rounding to a 24 bit mantissa then I _expect_ the
> rounding to take place.

Example, please, of how you 'request' rounding/truncation.


> And yes, it can break algorithms if you don't.

Example, please.

----

Something you should know from the C++ Standard:

"The values of the floating operands and the results of floating expressions may 
be represented in greater precision and range than that required by the type; 
the types are not changed thereby"  -- 5.0.11

D is clearly making FP arithmetic *more* predictable, not less. Since you care 
about FP results, I seriously suggest studying 
http://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html and what the C++ 
Standard actually says.