Edit report at https://bugs.php.net/bug.php?id=53045&edit=1
ID: 53045
Comment by: 282130106 at qq dot com
Reported by: remy at wh dot nl
Summary: broken result of intval()
Status: Bogus
Type: Bug
Package: *Math Functions
Operating System: FreeBSD
PHP Version: 5.2.14
Block user comment: N
Private report: N
New Comment:
echo intval(pack('a*',log(8,2)));
or
echo floor(pack('a*',log(8,2)));
It's gives 3,i don't now why.
Previous Comments:
------------------------------------------------------------------------
[2010-10-18 01:26:52] [email protected]
Closing as, most likely, this is not related to PHP.
------------------------------------------------------------------------
[2010-10-13 09:37:12] remy at wh dot nl
Ok, php 5.2.12 gives the same (broken) result...
------------------------------------------------------------------------
[2010-10-13 09:30:55] remy at wh dot nl
[remy@xxx]~> cc -o a.test a.c -lm
[remy@xxx]~> ./a.test
2.9999999999999996
I'm not entirely sure, but I did not notice this problem in 5.2.12.
I'll try to recompile 5.2.12 to see if the problem occurred too...
------------------------------------------------------------------------
[2010-10-13 09:00:14] [email protected]
Not sure there is a whole lot we can do about this. It is a typical IEEE-754
precision problem. Try this simple C program, for example:
[a.c]
#include <stdio.h>
#include <math.h>
int main(int argc, char *argv[]) {
printf("%.16f\n",log(8) / log(2));
}
LDFLAGS=-lm make a
./a
I get: 3.0000000000000000
I bet you get 2.9999999999999996
or something along those lines.
We might be able to do something clever with PHP's precision setting internally
and round to that precision before doing the truncation there, but in reality
you are dealing with floating point numbers here and whenever you deal with
floating point numbers when programming you have to treat them as
approximations, never as exact values. Unless you are using arbitrary
precision
functions such as the gmp or bcmath function (which are orders of magnitude
slower) you are going to have to account for these imprecisions in your code.
------------------------------------------------------------------------
[2010-10-13 08:38:16] remy at wh dot nl
php -i output:
phpinfo()
PHP Version => 5.2.14
System => FreeBSD xxx.xxx.com 7.3-STABLE FreeBSD 7.3-STABLE #7: Thu Jul 15
20:09:49 CEST 2010 [email protected]:/usr/obj/usr/src/sys/XXX i386
Build Date => Sep 22 2010 20:07:20
Configure Command => './configure' '--with-libxml-dir=/usr/local/lib'
'--with-config-file-path=/home/www/php' '--with-zlib' '--with-mysql'
'--with-pgsql' '--enable-calendar' '--enable-mbstring'
'--with-apxs2=/home/www/bin/apxs' '--enable-ftp' '--with-gd' '--enable-mbregex'
'--enable-gd-native-ttf' '--with-png-dir=/usr/local/lib'
'--with-jpeg-dir=/usr/local/lib' '--with-xpm-dir=/usr/X11R6/lib'
'--with-freetype-dir=/usr/local/lib' '--with-iconv' '--with-curl' '--with-bz2'
------------------------------------------------------------------------
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