Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!nrl-cmf!ames!ucsd!chem.ucsd.edu!tps
From: tps@chem.ucsd.edu (Tom Stockfisch)
Newsgroups: comp.lang.c
Subject: Re: avoiding explicit array sizes (was: Point me in the right direction)
Message-ID: <415@chem.ucsd.EDU>
Date: 24 Feb 89 00:43:45 GMT
References: <23631@watmath.waterloo.edu> <1841@mit-caf.MIT.EDU> <807@atanasoff.cs.iastate.edu> <14647@ccicpg.UUCP>
Reply-To: tps@chem.ucsd.edu (Tom Stockfisch)
Organization: Chemistry Dept, UC San Diego
Lines: 36

In article <14647@ccicpg.UUCP> swine@ccicpg.UUCP (Peter Swain) writes:
>In article <807@atanasoff.cs.iastate.edu> hascall@atanasoff.cs.iastate.edu (John Hascall) writes:
>	[ .. week-long discussion deleted .. ]
>>>double fund_consts[] = {
>>>	3.14159265,
>>>};

>Try using 
>	#define dim(a)	(sizeof(a)/sizeof(*(a)))

This doesn't work unless the defining declaration of a[] is in the same file.
If it is in the same file, it probably should have been declared static.
Usually "a" must be declared in a header file which cannot have access to
the initialization.  The scheme I use is

	/* header.h */
	# define NELT 20
	extern int	a[NELT];

	/* extern.c */
	# include "header.h"
	int	a[] = { 42, 666, ... };
	/* compiler will complain if sizeof(a)/sizeof(a[0]) is not equal
	 * to NELT, thus ensuring that extern.c and header.h are in sync
	 */

Any file in the project can then get at the dimension of "a" by #include'ing
"header.h".  "extern.c" must NOT be

	int	a[NELT] = {...};

because if you delete a member of the initialization set, NELT becomes out
of sync without the compiler being able to complain.
-- 

|| Tom Stockfisch, UCSD Chemistry	tps@chem.ucsd.edu