On Tue, Apr 8, 2008 at 6:19 AM, Tony Collings <tony@xxxxxxxxxxxxxxxx> wrote: > The humble tilde (~). I came across it the other day in some PHP code > [code]~E_ERROR[/code] That comes from C, not postpositional math. It's bitwise negation. That is, the number 47 in binary is 110001, with leading 0's out to whatever the word size is, usually 32 or 64 bits. The ~ flips the bits, so on a 32-bit system ~47 is binary 11111111111111111111111111001110, which is -48 if you treat it as a signed value and 429496248 as unsigned. The tilde is most often seen in the company of flag values, where each bit represents an option that is on or off. Typically, you have a bunch of constants defined as the individual bit values, like say E_DEBUG. Then ~E_DEBUG means "turn on everything except E_DEBUG". Often used with bitwise AND (&) as a mask to turn a particular bit off, as in <?php $flags &= ~E_DEBUG ?> which turns off E_DEBUG while leaving the other bits in $flag unchanged. -- Mark J. Reed <markjreed@xxxxxxxx> -- PHP General Mailing List (http://www.php.net/) To unsubscribe, visit: http://www.php.net/unsub.php
