Re: Bug #984 Updated: round() works bad

Bug Database <php-dev@lists.php.net> writes: > ID: 984 > Updated by: zeev > Reported By: segi@active.ch > Status: Closed > Bug Type: Misbehaving function > Assigned To: > Comments: > > This has to do with the fact that floating point numbers > have limited accuracy. > > 0.5 is never represented as 0.5000000... with zeros all the way > to infinity, but usually as something like > 0.49999999999999999998 > > 1.5 may be represented as 1.50000000000002, etc. > > Simply, you cannot rely on 0.5 being accurate, and thus > the behavir of round() can be considered undefined. > > Full Bug description available at: > http://ca.php.net/bugs.php3?id=984 Power-of-two fractions such as 0.5 *are* represented exactly in floating point as long as there is enough precision, and there is certainly enough to represent say 1.5 which only requires 2 bits. At the risk of spreading further disinformation, I think round() is more likely using a convention where 0.5 will be always be rounded to an even whole number. Since both rounding 0.5 up and rounding it down are equally valid, this rule meanes 50% of the time it is rounded up and 50% of the time it is rounded down so any nasty numeric effects might balance each other out somewhat instead of accumulating. Tom. -- PHP Development Mailing List http://www.php.net/ To unsubscribe send an empty message to php-dev-unsubscribe@lists.php.net For help: php-dev-help@lists.php.net