Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!gem.mps.ohio-state.edu!ginosko!uunet!mcsun!ukc!edcastle!aiai!ken
From: ken@aiai.ed.ac.uk (Ken Johnson)
Newsgroups: comp.lang.c
Subject: XOR operator (was Re: C history question)
Keywords: C design, XOR
Message-ID: <866@skye.ed.ac.uk>
Date: 11 Sep 89 16:25:15 GMT
References: <575@calmasd.Prime.COM>
Reply-To: ken@aiai.UUCP (Ken Johnson)
Followup-To: comp.lang.c
Organization: AIAI, University of Edinburgh, Scotland
Lines: 27

In article <575@calmasd.Prime.COM> wlp@calmasd.Prime.COM (Walter Peterson) writes:

>C has bitwise operators for AND (&), OR (|) and XOR (^) and boolean 
>operator for AND (&&) and OR (||), but not for XOR (^^). [...] Why
>make people go to the trouble of writing something like
>(a || b) && (!(a && b)) when a ^^ b is so much "cleaner".

I can't tell you why ^^ isn't in the language, but note that A^^B would
probably have a property that the above longer expression does not have,
namely: A && B and A || B each evaluate the second argument only if
necessary, and at most once. 

In some cases, evaluating A ^^ B by your suggested method would require
evaluating A or B more than once.  So a more suitable expression might
be

int X1, Y1;
if ((X1 = A) && (!(X2 = B))) || (X2 && !X1) {...}

Or you could just use goto's -- I think they would be justified here if
you explain what they do. 

-- 
Ken Johnson, AI Applications Institute, 80 South Bridge, Edinburgh EH1 1HN
E-mail ken@aiai.ed.ac.uk, phone 031-225 4464 extension 212
`I have read your article, Mr.  Johnson, and I am no wiser than when I
started.' -- `Possibly not, sir, but far better informed.'