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Bug #35656 Strange behaviour when multiplying floats with one multiplicator being 0
Submitted: 2005-12-13 11:00 UTC Modified: 2005-12-13 11:11 UTC
From: php at lipfi dot ch Assigned:
Status: Not a bug Package: Scripting Engine problem
PHP Version: 5.1.1 OS: Gentoo Linux 2005.1
Private report: No CVE-ID: None
View Developer Edit
 [2005-12-13 11:00 UTC] 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)

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 [2005-12-13 11:02 UTC] 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 11:10 UTC] php at lipfi dot ch 
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
 [2005-12-13 11:11 UTC] php at lipfi dot ch 
make that example calculation

0*-0.02 = -0
-0.02*0 = 0

Sorry. I forgot the - sign in the previous post.

Philip
 
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