Bug #16366 Updated: bit shifting error

ID: 16366 Updated by: msopacua@idg.nl Reported By: adam@adeptsoftware.com Status: Closed Bug Type: Math related Operating System: WinXP PHP Version: 4.1.2 New Comment: Related to the bug only: <?php $int=2; for($i=25;$i<33;$i++) { $check=pow($int, $i); $check2=(int)$check; $check2 += 1; $check3=(int)$check; $check3 -= 1; echo("i is $i\n"); var_dump($check); var_dump($check2); var_dump($check3); echo("\n"); } ?> Gives some weird results: $ ./test_float_conversion.php i is 25 int(33554432) int(33554433) int(33554431) i is 26 int(67108864) int(67108865) int(67108863) i is 27 int(134217728) int(134217729) int(134217727) i is 28 int(268435456) int(268435457) int(268435455) i is 29 int(536870912) int(536870913) int(536870911) i is 30 int(1073741824) int(1073741825) int(1073741823) i is 31 float(2147483648) int(-2147483647) float(-2147483649) i is 32 float(4294967296) int(1) int(-1) I guess the manual should be appended, in the 'typecasting' section, to not cast to integer, when it's larger than 2^30 and that those numbers are automatically converted to floats. (http://www.php.net/manual/en/language.types.type-juggling.php#language.types.typecasting) Previous Comments: ------------------------------------------------------------------------ [2002-03-31 13:10:38] derick@php.net Can you please stop this discussion in the bug system. Fighting about what is best can be done in private mail just fine. Derick ------------------------------------------------------------------------ [2002-03-31 13:09:00] daniel@php.net I'm sorry, but this is not flipcode.com - this is web-development. Not performance, but maintaineability and readability are crucial. Always write your code in a way someone else could easily understand what you are trying to do (even if you'll never release your sourcecode). ------------------------------------------------------------------------ [2002-03-31 12:58:54] daniel@php.net But "32 columns" are the way databases work. You can give names to these columns and everybody will understand what they are good for. - If you decide to just have a (int)flags in SQL, how could someone possibly understand this without reading your documentation (if you wrote it) or reading through your source code? - Databases are meant to have columns - commands such as WHERE, ORDER BY etc.. only work on columns, but not on your homebrewn data format. - You can add or remove columns, but modifying your flags would end in a big mess. - Databases can optimize/cache requests on certain columns (at least I think they can). - Using columns is less error-prone. Why the hell should you STILL be using your custom-made flags-thingie instead? But you are right and the bug still exists. PHP has problems with floating point operations - that's why I suggested to use the "BC" package. I wrote a "BC wrapper" which wraps all BC functions to regular PHP functions. It is meant for people who want to develop their code with BC, but still want to keep compatibility to PHP installations without BC. Tell me if you want it. ------------------------------------------------------------------------ [2002-03-31 12:50:00] msopacua@idg.nl Daniel: 32 columns or 1 integer, I'd take the integer anytime. The problem is, that large integers are converted to floats: <?php $number=11; $bigint=$number * 10000000000000000000000000000000; $bigint_nocalc=1100000000000000000000000000000000; echo("Using values: $number - $bigint - $bigint_nocalc\n"); var_dump($number); var_dump($bigint); var_dump($bigint_nocalc); echo ("\nNumber shift 34\n"); echo ($number>>34); echo ("\nNumber shift 2\n"); echo ($number>>2); echo("\nBigint shift 29\n"); echo($bigint>>29); echo("\nBigint shift 30\n"); echo($bigint>>30); echo("\nBigint shift 31\n"); echo($bigint>>31); echo("\nBigint shift 32\n"); echo($bigint>>32); echo("\nBigint shift 33\n"); echo($bigint>>33); echo("\nBigint shift 1\n"); echo($bigint>>1); ?> Output: $ ./test_bit.php Using values: 11 - 1.1E+32 - 1.1E+33 int(11) float(1.1E+32) float(1.1E+33) Number shift 34 2 Number shift 2 2 Bigint shift 29 0 Bigint shift 30 0 Bigint shift 31 0 Bigint shift 32 0 Bigint shift 33 0 Bigint shift 1 0 ------------------------------------------------------------------------ [2002-03-31 12:49:34] daniel@php.net > I think I will switch to blobs or something. the problem is PHP-related. Switching to BLOBs won't solve it. Why don't you use the ENUM('0','1') data field instead? It is more humand-friendly (better readable) and somewhat more what the inventor of databases wanted to do. You can forget all database-related optimizations because the database does not understand what you want to do. ------------------------------------------------------------------------ The remainder of the comments for this report are too long. To view the rest of the comments, please view the bug report online at http://bugs.php.net/16366 -- Edit this bug report at http://bugs.php.net/?id=16366&edit=1