Re: Bitwise NOT operator?

On 20 August 2010 17:10, Andy McKenzie <amckenzie4@gmail.com> wrote: > Hey everyone, > >  I'm really not sure what's going on here:  basically, the bitwise > NOT operator seems to simply not work.  Here's an example of what I > see. > > ============Script============ > > $ cat bintest2.php > > <?php > > $bin = 2; > $notbin = ~$bin; > > echo "Bin: " . decbin($bin) . "  !bin:  " . decbin($notbin) . > "\n"; > echo "Bin: $bin  !bin:  $notbin\n"; > > > ?> > ============================= > > > ============Output============ > > $ php bintest2.php > Bin: 10  !bin:  11111111111111111111111111111101 > Bin: 2  !bin:  -3 > > ============================= > > > Obviously that's not the expected response.  I expect to get something > more like this: > > Bin: 10  !bin:  01 > Bin: 2  !bin:  1 > > > Can anyone shed some light on this for me?  The server is running an > old version of OpenSUSE, and php --version returns: > You probably need to read up on computer architechture and CS theory - the ~ operator works fine, your understanding does not. What you're looking at is the following: $bin = 2 = 00000000000000000000000000000010 (64 bit number) ~$bin = ~2 = -3 = 11111111111111111111111111111101 (the inverse of the above) Computers generally do not operate on a couple of bits from a byte - they tend to operate on a byte, a word, a doubleword, etc (before any nitpickers interrupt: sure, they can, but most operations don't). Hence, you're not doing a not operation on just "10", you're doing it on "00000000000000000000000000000010". And as you are operating on a signed int, you'll get a negative number out of the ~ operation (seeing as you flipped the most significant bit). Regards Peter -- <hype> WWW: http://plphp.dk / http://plind.dk LinkedIn: http://www.linkedin.com/in/plind BeWelcome/Couchsurfing: Fake51 Twitter: http://twitter.com/kafe15 </hype>