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From: karl@haddock.ISC.COM (Karl Heuer)
Newsgroups: comp.lang.c
Subject: Re: Possible C Anomaly
Message-ID: <1313@haddock.ISC.COM>
Date: Mon, 5-Oct-87 12:39:34 EDT
Article-I.D.: haddock.1313
Posted: Mon Oct  5 12:39:34 1987
Date-Received: Fri, 9-Oct-87 00:42:10 EDT
References: <9601@brl-adm.ARPA>
Reply-To: karl@haddock.ima.isc.com (Karl Heuer)
Organization: Interactive Systems, Boston
Lines: 31

In article <9601@brl-adm.ARPA> mrd@clutx.clarkson.EDU (Michael R. DeCorte) writes:
>I might simply missing something but I still believe [ k = --k ] is
>correct.  It does not matter which is evaluted first.

It is incorrect.  The value of the expression is (oldk - 1), with TWO side
effects (decrement k, and store (oldk - 1) in k).  It would be legal for the
compiler to perform the store and then the decrement, leaving (oldk - 2) in k.

>The way I would actully code it is rather non C'ish
>if (k > 1) k--; else k = 1;

By "non C'ish" you seem to mean that it isn't obscure enough?  I think it's
both C'ish and the best way to write it if you want to preserve those exact
semantics.  But I'd just write "if (k > 1) --k;", if it's known that k > 0 (as
was implied, I think, by the original problem statement).

>And to give yet another solution you could also try [...] or
>    (k-- <= 1) ? k = 1 : ;
>Although not very readable this has the advantage that it will very
>good with many cpu's and compilers but very bad with others.

As has been pointed out already, this one is illegal; you have to either use a
dummy expression (": 0;") or use the if-else syntax (which is better anyway,
if you aren't in an expression context).

As for being "very good with many CPUs/compilers", I think you'd do better
with "if (--k <= 0) k=1;" which has the advantages of using a predecrement (so
no need to save the old value) and a compare against zero (which is often a
side-effect of decrementing anyway).

Karl W. Z. Heuer (ima!haddock!karl or karl@haddock.isc.com), The Walking Lint