Path: utzoo!mnetor!uunet!husc6!hao!ames!oliveb!sun!betelgeuse!jamesa
From: jamesa%betelgeuse@Sun.COM (James D. Allen)
Newsgroups: comp.lang.c
Subject: Re: comma operator
Message-ID: <39121@sun.uucp>
Date: 17 Jan 88 18:20:53 GMT
References: <3819@sigi.Colorado.EDU>
Sender: news@sun.uucp
Lines: 42
Summary: A real example

The comma operator is one of the things that makes C what it is. For example
	while (A, B)
is equivalent to
	for (A; B; A)
and generates less code on most compilers.  The latter may be considered
more readable, but that seems like evidence of a mental block.

Apropos of "comma in macro definition" here are some excerpts from
"real code".  The program is used to solve the "Logic Puzzles" in
Dell Crosswords magazine although it could be adapted for related problems.
Logic propositions are represented, converted directly into compiled C,
and all solutions are found (through backtracking).

Logic propositions can be nested arbitrarily.  'AND' is implied by
concatenation, 'OR' requires an "EITHER...OR...OR...OR...END_EITHER"
as in the example:

/*
 * Clue 3) Tom and Janice were on their honeymoon but one of them
 * spent the whole time eating.
 */
	MRS_AND_MR(JANICE, TOM)
	EITHER
		IS_TRUE(TOM, EAT)
	OR
		IS_TRUE(JANICE, EAT)
	END_EITHER

Here are some of the relevant "#defines":

#define	EITHER		if (Psol[1] = Psol[0], ! setjmp((++Psol)->jb)) {
#define	OR		} else EITHER
#define	TRY_AGAIN	longjmp((Psol--)->jb, 1)
#define	END_EITHER	} else TRY_AGAIN;

The interesting macro is "EITHER".  It copies and pushes a large
structure representing the problem state:  "Psol[1] = Psol[0], ++Psol",
and sets a checkpoint for later backtracking:  "setjmp()".  I didn't
see a way to do this without the "," operator. (avoid variable number of
"}" in END_EITHER).

If there's interest I'll post the program.