[SOLVED] '210010106140040100' == '210010106140040101' ==> true

[liviococco]

Debugging this simple line of a PHP script

 

if($a == $b){ } 

 

I've found that with value of

$a = '210010106140040100' (type = string)

$b = '210010106140040101' (type = string)

 

the result of the comparison is TRUE

 

the same if i compare 2 integer

 

$a = 210010106140040100 (type = int)

$b = 210010106140040101 (type = int)

 

$a == $b  ( true )

 

If I use the === operator

 

'210010106140040100' ===  '210010106140040101'  (comparison of strings: result is FALSE)

210010106140040100  ===  210010106140040101  (comparison of numbers: result is TRUE)

 

I think it could be caused by the fact that the string is 'like' integer, but exceed the standard size of integer number.

But the final result remain a mistery for me: I've tested some others 'big' numbers and I've found for example:

 

18014398509481980==18014398509481981 (true)  !!!!

18014398509481982==18014398509481981 (false) !!!!

 

In my specific script i can handle this specific comparison using the === operator, because i can assume that the variable are string.

 

But i'd like to know why this happen, and i'd appreciate suggestions about how to efficiently compare 'big' integers.

 

Thanks!

 

I use PHP 5.2 with IIS

 

 

 

[Mchl]

You're correct thinking it's because these numbers are larger than maximum integer limit. Once a number exceeds this limit, PHP stores it as double precision number which - despite its promising name - is not precise.

 

http://en.wikipedia.org/wiki/Double_precision_floating-point_format

 

[added]

 

You can use mbstring bc math extension for dealing with large integers.

[liviococco]

You're correct thinking it's because these numbers are larger than maximum integer limit. Once a number exceeds this limit, PHP stores it as double precision number which - despite its promising name - is not precise.

 

http://en.wikipedia.org/wiki/Double_precision_floating-point_format

 

[added]

 

You can use mbstring bc math extension for dealing with large integers.

 

In my example i needed to compare strings, and the 'numeric' value of strings was just a coincidence that broke my code ( so BC extension is not a choice ) .

 

I've fixed it using '===' operators, but thanks a lot for helping discover the cause.

 

Please mark as Resolved.