Edit report at http://bugs.php.net/bug.php?id=51518&edit=1

 ID:               51518
 Updated by:       ras...@php.net
 Reported by:      krenshala at koboldi dot net
 Summary:          should add to zero, but gets 1.1102230246252E-16
 Status:           Bogus
 Type:             Bug
 Package:          Math related
 Operating System: Multiple
 PHP Version:      5.2.13

 New Comment:

The important thing to take away from that doc is that computers can not

fractions accurately in an efficient manner.  They are much better and
faster at 

dealing with integers.  In your particular example where you are using
0.3, that 

is one of many fractions that a computer cannot represent.  It is

stored as 0.299999999999999988897769753748434595763683319091796875 which
is very 

close to 0.3, but if you start doing math on it and any sort of exact 

comparisons, it simply won't work.  You can see this with this simple


echo 0.3;

Previous Comments:
[2010-04-09 20:40:58] krenshala at koboldi dot net

tested it on my system at home and was able to reproduce it.

$ php --version

PHP 5.2.13-pl0-gentoo (cli) (built: Apr  9 2010 03:35:37)

Copyright (c) 1997-2010 The PHP Group

Zend Engine v2.2.0, Copyright (c) 1998-2010 Zend Technologies

$ cat test.php

echo "0.9 - 0.9 returns ".(0.9 - 0.9)."\n";

echo "0.9 + (-3 * .3) returns ".(0.9 + (-3 * .3))."\n";

echo "-0.9 + (3 * .3) returns ".(-0.9 + (3 * .3))."\n";

echo "0.8 + (-3 * .3) returns ".(0.8 + (-3 * .3))."\n";

$ php test.php

0.9 - 0.9 returns 0

0.9 + (-3 * .3) returns 1.11022302463E-16

-0.9 + (3 * .3) returns -1.11022302463E-16

0.8 + (-3 * .3) returns -0.1

Also, I can understand the multiplication increasing the precision used,
from 1 to 3 significant digits in the function above. Jumping from 1E-3
to 1E-16 seems a bit much to me, however.  I'm reading the Floating
Point doc you linked Rasmus in case it does address the issue.  From the
little of it I've read so far it does seem to address at least some of
what is happening here.

My concern is with the differences between the output of the different
lines in my new script (this comment).  Assuming it is due to the
floating point operation rounding/approximations mentioned in the doc,
I'm assuming the difference shows up due to the multiplication.

... off to finish reading the Floating Point doc.

[2010-04-09 05:12:28] ras...@php.net

The fix here is to decide on your precision and do:

$output = round(0.9 + ($input * 0.3), 2);  // 2-decimal precision

[2010-04-09 05:05:12] ras...@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:
Thank you for your interest in PHP.


[2010-04-09 05:03:27] krenshala at koboldi dot net

Math error for code that should be returning zero, but instead returns
1.1102230246252E-16 on multiple systems using multiple versions of PHP.

The problem is that when the input value is -3 the output value should
be zero: 0.9 + (-3 * 0.3) = 0.9 - 0.9 = 0.  If the calculated value is
non-zero the error does not occur.

I first saw the error on a WinXP (Home SP3, 32bit) system running PHP
5.2.9-1, but it also shows up on a MAC (10.6.2) with PHP 5.3.0
installed.  I'm in the process of upgrading PHP (to 5.2.13) on my Gentoo
box to test it there but haven't had a chance to do so yet.

Test script:

function my_function($input){

  $output = 1;

  if($input <= 0)

    $output = 0.9 + ($input * 0.3);

  // the above should give 0.9 - 0.9 = 0

  echo "Verifying values: 0.9 - ".($input * 0.3)." is supposed to be

  echo "Input: $input\tOutput: $output\n";

  return $output;


Expected result:
should have received zero (0).

Actual result:
actually received: 1.1102230246252E-16


Edit this bug report at http://bugs.php.net/bug.php?id=51518&edit=1

Reply via email to