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Bug #67501 strange operation of comparison of same value
Submitted: 2014-06-23 11:15 UTC Modified: 2014-06-24 06:51 UTC
From: blazexerit at yahoo dot co dot jp Assigned:
Status: Not a bug Package: *General Issues
PHP Version: 5.5.13 OS: Linux
Private report: No CVE-ID: None
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 [2014-06-23 11:15 UTC] blazexerit at yahoo dot co dot jp 
Description:
------------
after Test script

php > var_dump($now,$next);
int(1060)
double(1060)
php > if($now == $next) echo 1,PHP_EOL; else echo 0,PHP_EOL;
0


but,

php > if((int)1060 == (double)1060) echo 1,PHP_EOL; else echo 0,PHP_EOL;
1



Test script:
---------------
$r = 1060;
$now = 0; $next = 0;
$per = $r/50;

$i=0;
while($i!=$r){
	$now++;
	while($now >= $next){
		$next += $per;
	}
	$i++;
}

var_dump($now,$next);
if($now == $next) echo 1,PHP_EOL; else echo 0,PHP_EOL;


Expected result:
----------------
$now is 1060.
$next is 1060.
So, "if($now==$next)" will return true.

Actual result:
--------------
"if($now==$next)" returns false.

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 [2014-06-23 17:26 UTC] requinix@php.net
-Status: Open +Status: Not a bug
 [2014-06-23 17:26 UTC] requinix@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.
 [2014-06-24 04:29 UTC] blazexerit at yahoo dot co dot jp 
Thank you for answer.
I understand cause of this issue.

But, in JavaScript(V8), when I dump `next', 1060.000000000001 is printed.
So, I request that var_dump of php prints 1060.000000000001 instead of 1060, as well.
 [2014-06-24 04:54 UTC] requinix@php.net 
That's controlled by your "precision" php.ini setting.

php > var_dump(1060 + 1/50);
float(1060.02)
php > ini_set("precision", 20);
php > var_dump(1060 + 1/50);
float(1060.0199999999999818)
 [2014-06-24 06:51 UTC] blazexerit at yahoo dot co dot jp 
Thank you for answer.
The problem has been solved.
 
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