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Date: Fri, 02 May 2008 10:36:33 -0700
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Subject: Re: [PHP-DEV] Float comparison
From: truth@proposaltech.com (Todd Ruth)

I'm afraid you'll find Pierre's response representative.  The
php devs seem to consider the fact that a theoretically perfect
"==" doesn't exist to mean that the improvements that would 
cover 99.9% of user issues with float == shouldn't be made.

It could be worse, however.  At least the cast to string works
as one would expect.  Just imagine if those who oppose
improving "==" had their logic extended to the cast to string.
Php might to the following:

<?php
$x=.3+.4+.5;
print "$x\n";
?>

might output "1.19999999999999996447286321199499070644378662109375"

If someone filed a bug report, they could refer you to that
paper and tell you there's no bug because you're trying to get
a string from float and you never know what you might get.
So we can at least be glad that casts to string work as the
average person would expect.  We can hope that someday the
fact that the cast to string works as expected will inspire
them believe that making "==" work as expected wouldn't be 
a bad thing.

Seriously though, the biggest issue I see with improving "=="
is the implication for other features.  For example, what
should the following code do?

$a[.3+.4+.5] = "hello";
print isset($a[1.2])

Should floats be allowed as array indices?  I would say no because
I'd rather floats be converted to strings before being used as
array indices.  That would make the above code work as the
average person would expect.  Unfortunately, that would be a huge
BC break.  However, perhaps the perfect shouldn't be the enemy of
the good and we shouldn't let the inability to fix floats as
array indices be a reason not to improve "==".

- Todd

On Fri, 2008-05-02 at 19:04 +0200, Pierre Joye wrote:
> On Fri, May 2, 2008 at 6:52 PM, Jonathan Bond-Caron <jbondc@openmv.com> wrote:
> > Hi,
> >
> >
> >
> >  I'm new to the PHP internals list and I'm posting an issue I'm sure has been
> >  mentioned before -- float comparison
> >
> >
> >
> >  I'd like to know if there are reasons not to change the default behavior
> >  when comparing floats.
> >
> >
> >
> >  i.e. This would seem logical to me:
> >
> >
> >
> >                 $test = 19.6*100;
> >
> >                 if((float)(string)$test == (double)1960)
> >
> >                                 echo "GOOD BEHAVIOR";
> >
> >
> >
> >                 // Implicitely convert the float to string then float (en_US
> >  locale)
> >
> >                 if($test == (double)1960)
> >
> >                                 echo "THIS SHOULD PASS by casting
> >  (float)(string)$test internally...";
> >
> >                 else
> >
> >                                 echo "NOT GOOD BEHAVIOR";
> >
> >
> >
> >                 // Exact comparison would still fail
> >
> >                 if($test !== (double)1960)
> >
> >                                 echo "GOOD BEHAVIOR";
> >
> >
> >
> >  Any reason why $test == (double)1960 should fail? I realized we are
> >  comparing two floats, but couldn't php internally convert to string then
> >  float - for the non strict (===) case
> 
> From our bug report:
> "
> Floating point values have a limited precision. Hence a value might
> not have the same string representation after any processing. That also
> includes writing a floating point value in your script and directly
> printing it without any mathematical operations.
> 
> If you would like to know more about "floats" and what IEEE
> 754 is read this:
> http://docs.sun.com/source/806-3568/ncg_goldberg.html
> 
> Thank you for your interest in PHP.
> "
> 
> We use this text as automatic reply for bugs about floating points (aka bogus).
> 
> Cheers,
> -- 
> Pierre
> 
> http://blog.thepimp.net | http://www.libgd.org
>