Edit report at https://bugs.php.net/bug.php?id=64928&edit=1
ID: 64928
Comment by: thz at plista dot com
Reported by: thz at plista dot com
Summary: Summing floats yields different result when order of
summands is changed
Status: Not a bug
Type: Bug
Package: Math related
Operating System: Ubuntu 12.04
PHP Version: 5.5Git-2013-05-27 (Git)
Block user comment: N
Private report: N
New Comment:
Thanks for the clarification, thinking about it again, I realize that this is
due to rounding errors. I guess I didn't know everything about floats after all.
Previous Comments:
------------------------------------------------------------------------
[2013-05-27 10:26:11] larue...@php.net
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://www.floating-point-gui.de/
Thank you for your interest in PHP.
------------------------------------------------------------------------
[2013-05-27 10:22:08] thz at plista dot com
Description:
------------
First of all: I am aware of how floats work and that they are only
approximations of their ideal values.
I found an inconsistency in how they are handled when added, please see the
code below. Variable $a and $b are created by summing up three floats <1.0. The
only difference is the order of the summands. Both variables should have
exactly the same value, yet $a evaluates to 1.0 and $b to <1.0.
Test script:
---------------
<?php
$a = array_sum(array(0.6, 0.3, 0.1));
$b = array_sum(array(0.1, 0.3, 0.6));
var_dump($a); // float(1)
var_dump($b); // float(1)
var_dump($a == $b); // bool(false)
var_dump(intval($a)); // int(0)
var_dump(intval($b)); // int(1)
$c = $a - 1;
var_dump($c); // float(-1.1102230246252E-16)
Expected result:
----------------
float(1)
float(1)
bool(true)
int(1)
int(1)
float(0)
Actual result:
--------------
float(1)
float(1)
bool(false)
int(0)
int(1)
float(-1.1102230246252E-16)
------------------------------------------------------------------------
--
Edit this bug report at https://bugs.php.net/bug.php?id=64928&edit=1
