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Bug #29516 Mathematical calculations give unexpected result
Submitted: 2004-08-04 05:49 UTC Modified: 2004-08-04 07:56 UTC
From: beckwest22 at yahoo dot com Assigned:
Status: Not a bug Package: Math related
PHP Version: 5.0.0 OS: Windows XP
Private report: No CVE-ID: None
 [2004-08-04 05:49 UTC] beckwest22 at yahoo dot com
Description:
------------
I have been reading that doing some mathematical calculations with floats and integers in php gives some unexpected results.  I am not sure if this is a bug or not. When I do a combination of division and subtraction with the number 19, I get unexpected results.  I would appreciate any light you could shine on this subject.

Modules compiled with PHP: 
I did not compile any modules with PHP.

The only thing different in my php.ini file is that I uncommented the extension=php_mysql.dll line in the file.



Reproduce code:
---------------
<?php
$num = 19;
$res = $num/10;
echo $res;
echo "<br>";
$res1 = ($num%10)/10;
echo $res1;
echo "<br>";
$res2 = $res - $res1;
echo $res2;
$final_result = $res2 - 1;
echo "<br>";
echo $final_result;
?>


Expected result:
----------------
This is what I expect from the calculations:

$res = 19/10; // equals 1.9
$res1 = (19%10)/10; // equals 0.9
$res2 = 1.9 - 0.9; // equals 1
$final_result = 1 - 1; // equals 0

expected $final_result to be 0

Actual result:
--------------
$final_result equals -1.11022302463E-016

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History

AllCommentsChangesGit/SVN commitsRelated reports
 [2004-08-04 07:56 UTC] derick@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.
 
Thank you for your interest in PHP.

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