Edit report at https://bugs.php.net/bug.php?id=62240&edit=1
ID: 62240
Updated by: rasmus@php.net
Reported by: zuallauz at gmail dot com
Summary: 5.4.3 regression, converting from float to int gives
incorrect output
Status: Not a bug
Type: Bug
Package: *General Issues
Operating System: Ubuntu 12.04 32-bit
PHP Version: 5.4.3
Block user comment: N
Private report: N
New Comment:
That's interesting because that is exactly what is happening when you call PHP's
log() function. log(8,2) in PHP ends up being log(8)/log(2) in C. Did you compile
your PHP yourself? If not, could you try grabbing the 5.4.3 tarball and doing a
simple: ./configure && make
Building just the cli version is enough. Then run your log(8,2) test with
sapi/cli/php test.php
Previous Comments:
------------------------------------------------------------------------
[2012-06-07 08:48:37] zuallauz at gmail dot com
Hmm, I get the same output with 64 zeros:
3.0000000000000000000000000000000000000000000000000000000000000000
------------------------------------------------------------------------
[2012-06-07 08:08:25] rasmus@php.net
Well, float to int conversion issues isn't a PHP-specific thing. Try this little
C program on your machine, for example:
#include <stdio.h>
#include <math.h>
int main(char *argv[], int argc) {
printf("%.64f\n",log(8)/log(2));
}
If you name the file, "a.c" you can just type: make a
then run it with: ./a
My output on 64-bit Ubuntu:
10:07am x220:~> ./a
3.0000000000000000000000000000000000000000000000000000000000000000
It would be interesting to see your output.
------------------------------------------------------------------------
[2012-06-07 07:50:15] zuallauz at gmail dot com
Actually I may just use the code below now that I've been reading up on the perils of
converting from float to int in PHP:
$num = log(8, 2);
$num = (int) number_format($num, 0, '.', '');
------------------------------------------------------------------------
[2012-06-07 07:25:04] zuallauz at gmail dot com
I have put in a quick hack for now by converting from float to string first then converting to int.
This seems to get the correct output:
$num = (int) (string) log(8, 2);
var_dump($num);
Output: int(3)
I don't think the problem lies in the log() function at all as that correctly retrieves the
float(3) result. It's the conversion from the float to integer that is doing the incorrect
calculation. Likely the log(8, 2) does not result in a perfect 3, it might actually be
2.9999999999999999 so there could be a conversion error happening.
My test machine is running Ubuntu server 32-bit, so maybe the 64-bit edition has enough precision to
do the integer conversion properly. I was talking to a colleague today and he mentioned there were
some float to integer optimisations done in 5.3.11 that changed things?
------------------------------------------------------------------------
[2012-06-06 14:05:13] rasmus@php.net
Yes, I doubt upgrading to 5.4.4 is likely to fix your case. I have no idea why
your libmath log() call isn't returning a more accurate value here. I tested this
on a 64-bit Ubuntu 12.04 and couldn't reproduce it.
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