[Wolves] Bitwise AND

[Matthew Revell]

Cheers for that, Aq. Very helpful!


Aquarius said:
> Matthew Revell spoo'd forth:
>> if ($n & 1 = 1) return "odd";
>>
>> then the reason it can let you know if a number is odd or even is
>> because
>> it can only ever match all the numbers with 1 if the number is odd.
>
> Sort of. Odd numbers in binary form always have a 1 at the end, because
> they will always have a (1*1) in their binary makeup. (You add together
> 1s, 2s, 4s, 8s, 16s, etc to get the number, and all the things you add
> are even except the 1, so any odd number must have the 1 in it).
> Therefore, any number AND 1 will be 1 if it has a binary 1 in it (i.e.,
> is odd) and 0 if it doesn't (i.e., is even).
>
>> So, that's cool, but what else would you want to use it for?
>
> It's useful if you want to compress a set of flags into one number.
> Imagine that we wanted to show who was at a LUG meeting. YOu might do
> this by defining a load of flags:
> FLAG_MATT_REVELL  = 1
> FLAG_AQ           = 2
> FLAG_JONO         = 4
> FLAG_SPARKES      = 8
> FLAG_PETER_OLIVER = 16
> FLAG_LEE_JORDAN   = 32
> FLAG_HELEN        = 64
>
> and then add up the flags to get a number for each LUG meeting. Say
> that you, I, and sparkes were at a meeting: the number for that meeting
> would be 8 + 2 + 1 = 11. Now, you can then find out whether a given
> person was at the meeting by ANDing the meeting number with the flag
> for that person. So 11 AND FLAG_JONO = 0, so Jono wasn't there.
> 11 AND FLAG_AQ = FLAG_AQ, so I was there. Useful little trick.
>
> Aq.
>
> --
> UTF-8 has a certain purity in that it equally annoys every nation, and
> is nobody's default encoding.
> 	   -- Andy Robinson, 10 Apr 2000
>
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