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Date: Fri, 2 May 2008 13:38:47 -0400
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Subject: RE: [PHP-DEV] Float comparison
From: jbondc@openmv.com ("Jonathan Bond-Caron")

Thanks Pierre, 
I realize it's not a "bug" and I've read some "bogus" reports :) 

But since (float)(string)$test == (double)1960 is 'correct'

My question is: can php do the conversion internally?

It would seem *very* convenient for users, 
If not changing the float behavior, I'm advocating for a (numeric) type that
does this magic conversion.

(numeric) == (float)(string) in en_US locale / convert to string with a "."
seperator

Let's say it's hard to promote PHP as enterprise ready without dealing so
well with numbers.

I don't know the float-to-string internals but it seems to do 'right' thing.
We get '1960', then converting back to a float, all accuracy bits are 0, so
(numeric)$test == (double)1960

I would guess adding a new type is a big amount work?

-----Original Message-----
From: Pierre Joye [mailto:pierre.php@gmail.com] 
Sent: May 2, 2008 1:05 PM
To: Jonathan Bond-Caron
Cc: internals@lists.php.net
Subject: Re: [PHP-DEV] Float comparison

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

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