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Request #6019 document precision of floating point values
Submitted: 2000-08-08 08:59 UTC Modified: 2006-04-03 09:38 UTC
From: thomas at netropolis dot dk Assigned:
Status: Closed Package: Feature/Change Request
PHP Version: 4.0.1pl2 OS: NT4.0
Private report: No CVE-ID: None
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From: thomas at netropolis dot dk
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 [2000-08-08 08:59 UTC] thomas at netropolis dot dk 
When comparing certain numbers using the equals operator (==) it returns true even though numbers are not equal. It doesnt matter if number is duoble or string.
It only happens when number is 16 digit. 15 & 17 digits is OK. 
Using strcmp() doesnt produce this error.

Example:

<?php

$one = 9834567891230000; //double
$two = 9834567891230000; //double
print(($one == $two)); //returns 1 (OK)

$one = 9834567891230000; //double
$two = 9834567891230001; //double
print(($one == $two)); //returns 1 (wrong)

$one = "9834567891230000"; //string
$two = "9834567891230001"; //string
print(($one == $two)); //returns 1 (wrong)

$one = 9834567891230000; //double
$two = 9834567891230002; //double
print(($one == $two)); //returns 0 (OK)

$one = 1834567891230000; //double
$two = 1834567891230001; //double
print(($one == $two)); //returns 0 (OK)

?>

I have tried under diffrent versions of php both under linux and NT4.0 with the same result.
latest: 4.0.1pl1 NT4.0 precompiled downloaded from php.net

Can send php.ini if you want.

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History

AllCommentsChangesGit/SVN commitsRelated reports
 [2000-08-08 09:07 UTC] stas@php.net 
1. Use === when you are interested in string-to-string comparison.
2. PHP has precision limits, if you need more precision please use bc extension.
 [2000-08-08 09:25 UTC] waldschrott@php.net 
verified, replacing == with === in the third block will
produce 0, what is correct

for any reason the comparison is done corretly withing
strings, I think you won?t have to switch to bcmaths
functions for these tiny doubles, using var_dump() reveals
that they?re correctly stored in $GLOABLS

resum?: PHP performs return only 0,2,4 differences not 1,3,5
dealing with large numbers
example:
echo 9834567891230000-9834567891229999; // return2 s, should 1
echo 9834567891230000-9834567891230001; // returns 0, should -1
and so on
 [2000-08-08 09:37 UTC] waldschrott@php.net 
I think this limitation isn?t mentioned anywhere clearly,
transforming into docu bug
In fact I don?t know where to look for it and I think the
php.ini precision settings has nothing to do with it...
 [2000-08-08 10:34 UTC] hholzgra@php.net 
the precisions are machine-/build-dependant
so we can't document them in the manual

maybe we can add a section to phpinfo()
and/or some constants as in limits.h
for MIN/MAX values for integer and float 
values

(passed on to feature request)
 [2002-04-28 18:16 UTC] jimw@php.net 
updating summary.
 [2006-04-03 09:38 UTC] tony2001@php.net 
See http://php.net/float
 
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