Re: Help with some clever bit operations

The example starting values $existing = 181; # = 10110101 $new = 92; # = 01011100 $mask = 15; # = 00001111 Get the bits that will be changed $changing = $new & $mask; # = 12 = 00001100 Get the bits that won't be changed $staying = $existing & ~$mask; # = 176 = 10110000 Combine them together $result = $changing ^ $staying; # = 188 = 10111100 David Niels wrote: > Hi, > > I have a problem I can solve with some loops and if-thens, but I'm sure it > can be done with bit operations -- that would be prettier. I've tried to > work it out on paper, but I keep missing the final solution. Maybe I'm > missing something obvious... > > The problem: A function tries to update an existing value, but is only > allowed to write certain bits. > > There are 3 variables: > A: the existing value, eg. 10110101 > B: what the function wants to write, eg. 01011100 > C: which bits the function is allowed to write, eg. 00001111 > > With these examples, 10111100 should be written. > > How do I combine A, B and C to get that result? > > > Thanks, > Niels >