#35656 [Bgs]: Strange behaviour when multiplying floats with one multiplicator being 0
ID: 35656
User updated by: php at lipfi dot ch
Reported By: php at lipfi dot ch
Status: Bogus
Bug Type: Scripting Engine problem
Operating System: Gentoo Linux 2005.1
PHP Version: 5.1.1
New Comment:
I fear your decision to mark this as bogus was a very quick one.
http://www.php.net/manual/en/language.operators.arithmetic.php
does not tell me anywhere that the commutative law is not valid in PHP.
Also note that I'm very well aware of floats having limited precision,
but -0 is no defined value and certainly not something you'd expect to
get as a result of a multiplication. And certainly not depending on the
operands order.
0*0.02 = -0
0.02*0 = 0
where var_dump() certainly knows that -0 == 0, but the engines core
seems not to.
I certainly expect the arithmetics in PHP to work like it's supposed
to. This means adhering to the commutative law which basically means
that I can interchange the operands in a multiplication.
Besides: -0 is IMHO not a valid value. It's either 0 or not 0, but not
-0.
I'd really appreciate a further comment of why the behaviour whitnessed
here is really working as intended and where that is documented in the
manual.
Thank you
Philip
Previous Comments:
------------------------------------------------------------------------
[2005-12-13 11:02:53] sniper@php.net
Thank you for taking the time to write to us, but this is not
a bug. Please double-check the documentation available at
http://www.php.net/manual/ and the instructions on how to
report
a bug at http://bugs.php.net/how-to-report.php
------------------------------------------------------------------------
[2005-12-13 11:00:33] php at lipfi dot ch
Description:
------------
When multiplying two float values, one being 0, one being negative,
it's possible to either get 0 or -0 (!) as a result depending on the
operands order.
This is IMHO not how math is supposed to work.
Additionally, I'm seing discrepancies between the variables value and
what var_dump() things it contains. The problem is very simple to
reproduce
Reproduce code:
---------------
<?php
$v = 0*-0.02;
var_dump($v);
echo "$v\n";
$v = floatval($v);
echo "$v\n\n";
$v = -0.02*0;
var_dump($v);
echo "$v\n";
?>
Expected result:
----------------
float(0)
0
0
float(0)
0
Actual result:
--------------
float(0)
-0
-0
float(0)
0
(the -0 obviously being the problem here)
------------------------------------------------------------------------
--
Edit this bug report at http://bugs.php.net/?id=35656&edit=1
Thread (4 messages)