generalized switch

[Chris Torek]

In article <15093 at ucbvax.BERKELEY.EDU> kos at ernie.Berkeley.EDU
(Joshua Kosman) writes:
>One of my only memories from a brief stint as a PL/I programmer years ago
>was the pleasure of using a more general switch structure than C allows.

[In particular, the compiler allowed `cases' that might have to be
determined at run time, or at any rate were not simple constants.]

>#define IS_TYPE_A(c)	(c==this||c==that)
>#define IS_TYPE_B(c)	((c==the_other||c==your_mother) && c != rambo)
>and so on. Now what I need is
>	switch () {
>		IS_TYPE_A(c):
>			do_type_a(c);
>			break;
>		IS_TYPE_B(c):
>			do_type_b(c);
>			break;
>		}

One of the key `features' in C is that most constructs have a very
simple relationship to the machine code that will be generated for
a given statement.  switch `wants' to become a computed branch,
much like the old FORTRAN

	GOTO (l1, l2, l3, ..., ln), var

`switch' wins readability points over the computed GOTO, but not
much else: it still requires constant expressions, so that the
compiler can convert it back to one.  In C, when one wants a run-time
test, one must write a run-time test.

In your particular example, you probably do not care how the test
is implemented.  Unfortunately (?), C requires you to care.  This
example appears to require run-time evaluation, but in another case
(pardon the pun) you might be able to get away with this:

	#define IS_TYPEA(x) ((x) > 3 && (x) < 7)
	#define CASES_TYPEA	case 4: case 5: case 6:
	#define IS_TYPEB(x) (!IS_TYPEA(x))
	#define CASES_TYPEB	default:

You could then use

		if (IS_TYPEA(n)) {
			statements;
		} else if (IS_TYPEB(n)) {
			morestatements;
		}

and

		switch (n) {

		CASES_TYPEA:
			statements;
			break;

		CASES_TYPEB:
			morestatements;
			break;
		}

but not

		switch (n) {

		CASES_TYPEB:
			morestuff;
			break;
		/* default is to ignore */
		}

So this approach too has its pitfalls.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 1516)
UUCP:	seismo!umcp-cs!chris
CSNet:	chris at umcp-cs		ARPA:	chris at mimsy.umd.edu