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Organization: Parliament Hill Computers Ltd
Subject: Re: [PHP-DEV] Float comparison
From: addw@phcomp.co.uk (Alain Williams)

On Fri, May 02, 2008 at 01:03:45PM -0700, Todd Ruth wrote:
> On Fri, 2008-05-02 at 11:41 -0700, Rasmus Lerdorf wrote:
> > Stefan Walk wrote:
> > > And you'll quickly see that the "cast to string before comparision" is a 
> > > bad idea, because:
> > > $ php -dprecision=1 -r 'var_dump((string)1.4 == (string)1.1);'
> > > bool(true)
> > > 
> > > Having display settings affect comparisions seems like a really bad idea 
> > > to me ;)
> > 
> > Yup, it would be a fatal flaw in the language if that ever came to be.
> > 
> > -Rasmus
> 
> Most people don't care about floating numbers beyond a certain 
> number of digits.  If precision were the only ini setting that
> gave us an idea of how much the person cared, I think it would
> be fair to use the setting or introduce a new setting.  As it
> happens, there is another setting.  How would you feel about
> this returning true:
> 
> php -dserialize_precision=1 -r 'var_dump(1.4 == 1.1);'
> 
> Setting serialize_precision to 1 is a very strong statement of
> not caring about those digits.  Personally, I don't care about
> more than about 6 digits and I don't do much math, so I could
> set it to 8 digits and be happy.  Not everyone has the luxury
> of controlling ini settings, but for those that do, this
> would be much nicer than having

Hmmm, what you are talking about is implying:

	$a == $b

be implemented as:

	abs($a - $b) < (max(abs($a), abs($b)) / EPSILON)

Where EPSILON is some sort of accuracy factor. It might be
possible to implement this easier/faster at a machine code level.

The trouble is that doing something like this would be global and
might mess up some code written elsewhere. Mind you: any programmer
going '$a == $b' on true floating point has got to expect rubbish.

This is making things easier for the naive/novice programmer. In this case
I don't think that it realy helps since the naive programmer isn't
going to have a clue about choosing a good value for EPSILON.

Best leave it the way that it is.

-- 
Alain Williams
Linux Consultant - Mail systems, Web sites, Networking, Programmer, IT Lecturer.
+44 (0) 787 668 0256  http://www.phcomp.co.uk/
Parliament Hill Computers Ltd. Registration Information: http://www.phcomp.co.uk/contact.php
Chairman of UKUUG: http://www.ukuug.org/
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