approxEqual() has fooled me for a long time...

[so]

It is in phobos/std/math.d, where approxEqual is.

On Wed, 20 Oct 2010 18:22:23 +0300, Craig Black <cblack at ara.com> wrote:

>
> "Don" <nospam at nospam.com> wrote in message  
> news:i9mhvd$2l5f$1 at digitalmars.com...
>> Lars T. Kyllingstad wrote:
>>> (This message was originally meant for the Phobos mailing list, but  
>>> for some reason I am currently unable to send messages to it*.   
>>> Anyway, it's probably worth making others aware of this as well.)
>>>
>>> In my code, and in unittests in particular, I use  
>>> std.math.approxEqual()
>>> a lot to check the results of various computations.  If I expect my
>>> result to be correct to within ten significant digits, say, I'd write
>>>
>>>   assert (approxEqual(result, expected, 1e-10));
>>>
>>> Since results often span several orders of magnitude, I usually don't
>>> care about the absolute error, so I just leave it unspecified.  So far,
>>> so good, right?
>>>
>>> NO!
>>>
>>> I just discovered today that the default value for approxEqual's  
>>> default
>>> absolute tolerance is 1e-5, and not zero as one would expect.  This
>>> means that the following, quite unexpectedly, succeeds:
>>>
>>>   assert (approxEqual(1e-10, 1e-20, 0.1));
>>>
>>> This seems completely illogical to me, and I think it should be fixed
>>> ASAP.  Any objections?
>>
>> I'm personally pretty upset about the existence of that function at all.
>> My very first contribution to D was a function for floating point  
>> approximate equality, which I called approxEqual.
>> It gives equality in terms of number of bits. It gives correct results  
>> in all the tricky special cases. Unlike a naive relative equality test  
>> involving divisions, it doesn't fail for values near zero. (I _think_  
>> that's the reason why people think you need an absolute equality test  
>> as well).
>> And it's fast. No divisions, no poorly predictable branches.
>>
>> Unfortunately, somebody on the ng insisted that it should be called  
>> feqrel(). Stupidly,  I listened. And now nobody uses my masterpiece  
>> because it has a totally sucky name.
>
> Where can I find this function that you wrote?  I would be interested in  
> using it.
>
> -Craig


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