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Re: [xml-dev] XML design of ((a and b) or c)
• From: Rick Marshall <rjm@zenucom.com>
• To: Tei <oscar.vives@gmail.com>
• Date: Sat, 16 Dec 2006 09:32:47 +1100

```That's correct for C (and many C-like languages), but the use of & is
very language dependent - I was only using to avoid overloading "and" in
the discussion.

Not every language allows you to fiddle "bits" so & and && (and | and
||) aren't always needed.

Rick

Tei wrote:
> On 12/15/06, Rick Marshall <rjm@zenucom.com> wrote:
>> Hi Ken
>>
>> The point here is quite subtle and VERY important.
>>
>> Let's just take "a AND b" or "a & b" to reduce the overloading of the
>> word and.
>>
>> Now if a and b are atomic and & is commutative the the order of
>> evaluation is unimportant and optimisers can play around to suit
>> themselves.
>>
>> However, if a or b are not atomic and the calculation of either has side
>> effects then "a & b" is no longer commutative and optimisers cannot stop
>> the calculation on detection of a or b being false.
>>
>> Unless of course we agree to use slightly different mathematics: "a & b"
>> is not commutative; it is evaluated left to right; and evaluation stops
>> if a is false. This can then be extended to "a & b & c ...".
>>
>> Now the "a & b" of computer languages is fundamentally different to the
>> "a & b" of mathematics although they degenerate to the same thing if a
>> and b are atomic.
>>
>
> Just to be sure,  &  is different from && on C.
>
> & is the and operation betwen bits, the result is a integer
> && is the and operation betwen booleans, the result is a boolean
>
> 0x0F & 0xF0 result to 0
> 0x0F && 0xF0 result to true
>
> You will not use & for shortcuts, but &&.
>
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```

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