PHP math strange behavior

Question

I have phantom bug in my code. It appears in one case from 10 attempts. When I try to debug it I found this strange behavior:

$a = floor(1385968017.8665 * 10000); // or (int) (1385968017.8665 * 10000)
// here $a equals to 13859680178664
$b = (1385968017.8665 * 10000);
// here $b equals to 13859680178665

I have such PHP version/configuration:

PHP 5.3.10-1ubuntu3.8 with Suhosin-Patch (cli) (built: Sep  4 2013 20:00:51) 
Copyright (c) 1997-2012 The PHP Group
Zend Engine v2.3.0, Copyright (c) 1998-2012 Zend Technologies
with Xdebug v2.2.3, Copyright (c) 2002-2013, by Derick Rethans

UPDATE: I know about floating operations precesion. Same code in C++:

#include <iostream>

int main()
{
    std::cout << std::fixed << (1385968017.8665 * 10000) << std::endl;
    std::cout << (unsigned long long) (1385968017.8665 * 10000) << std::endl;

    return 0;
}

gives me next output:

13859680178664.998047
13859680178664

And it was I expected from it. But why my PHP code shows that (1385968017.8665 * 10000) is precise equals to 13859680178665?

UPDATE 2: As user @core1024 mentioned in comments, PHP possible is rounding result of calculation (1385968017.8665 * 10000). If try php -r 'printf("%f\n",1385968017.8665 * 10000);' result will be somewhere about 13859680178664.998047. Who can explain when and why PHP do this?

I get 13859680178700 in both cases... Apache/2.2.8 (Win32) PHP/5.2.6 — 0xAli, Dec 02 '13 at 07:31
Take a look here http://stackoverflow.com/questions/588004/is-javascripts-floating-point-math-broken — core1024, Dec 02 '13 at 07:34
Exactly, but why (1385968017.8665 * 10000) is equals to 13859680178665 in my case? — Ivan Velichko, Dec 02 '13 at 07:35
PHP Is rounding it either case. Try `php -r 'printf("%f\n",1385968017.8665 * 10000);'` — core1024, Dec 02 '13 at 07:42
Check my answer, and especially follow thru the link (Which leads to a blog by Jon Skeet). Hopefully you'll be enlightened as to why the values output like that. — Malcolm Salvador, Dec 02 '13 at 07:43
@core1024 Can you explain me when PHP rounds it? And why? I a little familiar with zend engine and I very interesting within it! — Ivan Velichko, Dec 02 '13 at 07:54
Unfortunately I have no idea why. If I knew it I would post it as an answer here :) — core1024, Dec 02 '13 at 08:09

Malcolm Salvador · Answer 1 · 2013-12-02T07:39:35.327

2

It runs as expected.

You used the FLOOR() function in the first statement, which ROUNDS DOWN a value to the nearest integer.

Check here for more info on that.

As to why it rounds down like that, it's because your value 1385968017.8665 isn't exactly that. Jon Skeet explains that here ..along with Tony the Pony.

edited Dec 02 '13 at 07:39

answered Dec 02 '13 at 07:32

Malcolm Salvador

1,476
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score 0 · Answer 2 · answered Dec 02 '13 at 07:43

0

That's because of 1385968017.8665 is an infinite double in binary, and (literal 1385968017.8665) ~= (1385968017.8664998 in computer).

Is round(1385968017.8665 * 10000) what you need?

answered Dec 02 '13 at 07:43

wumch

1
1

Excactly in this case round gives me correct result. But I always want to retrive nearest integer value less or equals to "ideal division result" without floating operations magic. Using round don't save me from phantom bug in my code. – Ivan Velichko Dec 02 '13 at 07:50

score 0 · Accepted Answer · answered Dec 03 '13 at 09:33

I found explanation for this problem. My php.ini precision setting is equal to 14. If I increase precision setting by one, this stragne behavior not represented:

$a = floor(1385968017.8665 * 10000); // or (int) (1385968017.8665 * 10000)
// here $a equals to 1385968017866(!)5

PHP math strange behavior

3 Answers3