Message-ID: <494D0C66.1000200@biostat.ku.dk>
Date: 2008-12-20T15:16:54Z
From: Peter Dalgaard
Subject: understanding lexical scope
In-Reply-To: <494CE2BF.9060202@idi.ntnu.no>
Wacek Kusnierczyk wrote:
> Peter Dalgaard wrote:
>> One distinction is if you have things like
>>
>> #define f(x) 2*x
>> #define g(y) f(y+2)
>>
>> (in the C language preprocessor syntax), then you end up with g(y) as
>> y+2*2 (i.e., y+4), whereas the corresponding function calls give
>> 2*(y+2). Also, and the flip side of the original question: Macros have
>> difficulties with encapsulation; with a bit of bad luck, arguments
>> given to f() can modify its internal variables.
>
> using c macros, you end up with g(y) substituted by 2*y+2, rather than
> y+2*2, as you say (and rather than 2*(y+2), which you'd effectively get
> using a function).
Oops. Yes. I suppose I had x*2 there at some point....
> that's why you'd typically include all occurences of all macro
> 'parameters' in the macro 'body' in parentheses:
>
> #define f(x) 2*(x)
>
> some consider using c macros as not-so-good practice and favour inline
> functions. but macros are not always bad; in scheme, for example, you
> have a hygienic macro system which let's you use the benefits of macros
> while avoiding some of the risks.
>
> vQ
>
--
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