[Qt-interest] strange qFuzzyCompare implementation

[Thiago Macieira]

Peter Radagast wrote:
>Hello!
>I think all of us got used to high quality of the Qt design and
> implementation. And I was very disappointed, when I saw the
> qFuzzyCompare implementation:

That's because you didn't understand the purpose behind those functions.

>static inline bool qFuzzyCompare(double p1, double p2)
>{
>    return (qAbs(p1 - p2) <= 0.000000000001 * qMin(qAbs(p1), qAbs(p2)));
>}
>
>static inline bool qFuzzyCompare(float p1, float p2)
>{
>    return (qAbs(p1 - p2) <= 0.00001f * qMin(qAbs(p1), qAbs(p2)));
>}
>
>Do you consider these hard-coded numbers etc effective, precise and
> portable? 

Yes. They work fine for the intent that these functions are meant to have.

> I supposed to see smth like:
>static inline bool qFuzzyCompare(double p1, double p2)
>{
>    return (qAbs(p1 - p2) <= std::numeric_limits::epsilon<double>());
>}

That would have the opposite effect of what is wanted. And it would not 
work.

For one thing, no two numbers can have a difference that is smaller than 
epsilon / order of magnitude. They can either be equal (exactly), differ by 
one epsilon/order of magnitude or more. Remember that epsilon is the 
smallest number that, added to 1, makes it different from 1. But if you had 
1e-9 instead of 1, you the difference would be epsilon * 1e-9. Even if you 
took the order of magnitude into consideration in your function, it would 
compare only if the two numbers are exactly equal or differ by the smallest 
unit that they could differ by.

Like I said, that's not the intent of the function. The intent of the 
function is to verify if the two numbers are sufficiently close that they 
should be considered equal for the code in question. The problem is that 
the many floating point operations accumulate rounding errors: the more 
operations you make, the greater that error is. It's definitely greater 
than one epsilon.

So your code would definitely not work.

We could have chosen to make the code compare to a multiple of epsilons 
(like 1000 epsilon), but instead we chose to make our lives easier and 
just write a very small number.

-- 
Thiago Macieira - thiago.macieira (AT) nokia.com
  Senior Product Manager - Nokia, Qt Software
      Sandakerveien 116, NO-0402 Oslo, Norway
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 189 bytes
Desc: This is a digitally signed message part.
Url : http://lists.qt-project.org/pipermail/qt-interest-old/attachments/20090412/f7b5be0c/attachment.bin