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Bug #33576 19.6*100 != 19.6*10*10
Submitted: 2005-07-05 20:11 UTC Modified: 2005-07-06 15:39 UTC
From: rick at ninjafoo dot com Assigned:
Status: Not a bug Package: Math related
PHP Version: * OS: *
Private report: No CVE-ID: None
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From: rick at ninjafoo dot com
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 [2005-07-05 20:11 UTC] rick at ninjafoo dot com 
Description:
------------
100* certain values is not the same as 10*10* the same 
value. 
 
The following code should produce no output. I see output. 

Reproduce code:
---------------
<?php 

$value1 = 19.6*100;
$value2 = 19.6*10*10;

if ($value1 != $value2) {
	echo 'funny, i thaught 100x was exactly the same as 10x10x....<br>';
	echo 'value1='.$value1.', value2='.$value2.',';
}
?>

Expected result:
----------------
I expect nothing to happen. 

Actual result:
--------------
funny, i thaught 100x was exactly the same as 10x10x.... 
value1=1960, value2=1960,

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 [2005-07-05 20:13 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.

If you would like to know more about "floats" and what IEEE
754 is read this:
http://docs.sun.com/source/806-3568/ncg_goldberg.html
 
Thank you for your interest in PHP.

.
 [2005-07-05 20:56 UTC] rick at ninjafoo dot com 
Yes, I understand that. My bug report was not to point out   
the joys of floating point math.   
   
My bug report was to illustrate a problem with php's   
internal casting.    
   
19.6*100 != 1960  
19.6*100 != (int)1960   
19.6*100 != (float)1960   
19.6*100 != (string)1960  
 
echo gettype(19.6*100) returns 'double', However .....  
  
19.6*100 != (double)1960  
  
 
19.6*100 cannot be compaired to anything without manually 
casting it as something else first. 
 
(string)(19.6*100) == (string)1960
 [2005-07-05 21:34 UTC] helly@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

.
 [2005-07-06 10:42 UTC] rick at ninjafoo dot com 
Your right, this isnt a bug 
 
(double)(19.6*100) !== (double)1960 
 
This is just plain broken.
 [2005-07-06 15:39 UTC] rasmus@php.net 
I am not sure why you think casting 1960 to a float suddenly would make 19.6*100 equal to exactly 1960.0.  Floating point math is not exact.  19.6*100 is never going to be exactly 1960.0.  It will be 1959.999999999999999999999 or 1960.00000000000000000001 regardless of any casting you do to it.  You can round() it, or add a fuzz factor and floor() it.
 
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