Path: utzoo!utgpu!water!watmath!clyde!cbosgd!ucbvax!amdcad!tim
From: tim@amdcad.AMD.COM (Tim Olson)
Newsgroups: comp.lang.c
Subject: Re: comma operator
Message-ID: <19995@amdcad.AMD.COM>
Date: 17 Jan 88 21:30:52 GMT
References: <3819@sigi.Colorado.EDU> <5080013@hpfcdc.HP.COM>
Reply-To: tim@amdcad.UUCP (Tim Olson)
Organization: Advanced Micro Devices
Lines: 26

In article <5080013@hpfcdc.HP.COM> boemker@hpfcdc.HP.COM (Tim Boemker) writes:
| > Perhaps the most common use is when you're writing a macro:
| > 	#define SWAP(a, b, temp)  temp=a, a=b, b=temp
| > You can't write this the "obvious" way:
| > 	#define SWAP(a, b, temp)  {temp=a, a=b, b=temp}
| > because, someone will write
| > 	if (test(a, b)) SWAP(a, b, temp);
| > 	else abort();
| 
| OK, I'm feeling like a brick... Why can't I write it the
| "obvious" way?


Because it would expand to

	if (test(a, b)) {temp=a, a=b, b=temp}; <-- note semicolon
	else abort();
	
The semicolon at the end of the swap is a null statement, which makes
the 'else' construct illegal, since it doesn't directly follow the statement of
the if.

	-- Tim Olson
	Advanced Micro Devices
	(tim@amdcad.amd.com)