Re: Bitwise NOT operator?

'Twas brillig, and Andy McKenzie at 20/08/10 16:10 did gyre and gimble: > 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 output looks correct to me, so I think it's your expectations that are incorrect. You are assuming that you are working with a 2-bit number the in actual fact you are working with a 32-bit number. If you did $bin = 2; $notbin = ~$bin & 3; Then this should get the results you want. The bitwise & 3 bit essentially limits things to a 2-bit number. (3 in binary is represented by 1's for the two LSBs and 0's for all other bits). FWIW, the fact that ~2 == -3 is due to twos compliment binary notation: http://en.wikipedia.org/wiki/Twos_Compliment Col -- Colin Guthrie gmane(at)colin.guthr.ie http://colin.guthr.ie/ Day Job: Tribalogic Limited [http://www.tribalogic.net/] Open Source: Mandriva Linux Contributor [http://www.mandriva.com/] PulseAudio Hacker [http://www.pulseaudio.org/] Trac Hacker [http://trac.edgewall.org/]