Path: utzoo!utgpu!water!watmath!clyde!rutgers!husc6!yale!bunker!garys
From: garys@bunker.UUCP (Gary M. Samuelson)
Newsgroups: comp.lang.c
Subject: Re: comma operator
Message-ID: <3203@bunker.UUCP>
Date: 19 Jan 88 14:57:50 GMT
References: <3819@sigi.Colorado.EDU> <5080013@hpfcdc.HP.COM> <7120@brl-smoke.ARPA> <3887@sigi.Colorado.EDU>
Reply-To: garys@bunker.UUCP (Gary M. Samuelson)
Organization: Bunker Ramo, an Olivetti Company, Shelton, Ct
Lines: 33

In article <3887@sigi.Colorado.EDU> swarbric@tramp.Colorado.EDU (SWARBRICK FRANCIS JOHN) writes:
>In article <7120@brl-smoke.ARPA> gwyn@brl.arpa (Doug Gwyn (VLD/VMB) <gwyn>) writes:
>>By the way, I don't think there IS a good way to write a generic swap
>>macro.  Didn't we go over this about a year ago?

I don't think it was that long ago.

>#define swap(a,b) ((a) = ((b) = ((a) = (a) ^ (b)) ^ (b)) ^ (a))
>
>Try that, and tell me what you think.  A super-genius friend of mine figured
>it out.  He says it will work for anything (int, char *, etc.).  But it
>sure boggles my mind...

Who told him he was a genius?  Self-proclaimed, no doubt.

How about float or double? Structures?
Suppose either a or b have side effects? (e.g., swap( *x++, *y++ ))
What is the value of a macro which performs 6 operations, instead
of the three I want?  (t = a; a = b; b = t)
Suppose a and b are aliases?
How is the exclusive-or of two pointers defined?  (It isn't, at
least not portably).

Yes, it's a clever macro.  Yes, under some conditions it does the
right thing.  I even know why it works.  But I wouldn't use it;
the straightforward approach is much better.  So it takes another
variable; so what?  Are people afraid we'll run out?

My first maxim of programming is this:  Code will be read more
than written; therefore it is more important to be easily read
than easily written.

Gary Samuelson