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From: simon@its63b.ed.ac.uk (ECSC68 S Brown CS)
Newsgroups: net.lang.c
Subject: Re: Re: Expression sequencing query
Message-ID: <79@its63b.ed.ac.uk>
Date: Sat, 11-Oct-86 13:46:37 EDT
Article-I.D.: its63b.79
Posted: Sat Oct 11 13:46:37 1986
Date-Received: Mon, 13-Oct-86 00:42:21 EDT
References: <760@oakhill.UUCP> <111@titan.UUCP> <353@cullvax.UUCP> <2076@ecsvax.UUCP>
Reply-To: simon@its63b.ed.ac.uk (ECSC68 S Brown CS)
Distribution: net
Organization: I.T. School, Univ. of Edinburgh, U.K.
Lines: 51
Keywords: expression, order, evaluation

In article <2076@ecsvax.UUCP> bet@ecsvax.UUCP (Bennett E. Todd III) writes:
>In article <353@cullvax.UUCP> drw@cullvax.UUCP (Dale Worley) writes:
>>> In article <760@oakhill.UUCP> tomc@oakhill.UUCP (Tom Cunningham) writes:
>>> >	/* a = b + b + b */
>>> >	a = ((b=1),b) + ((b=2),b) + ((b=3),b)
>>> >
>>> >I expected the result to be 6.  With the Microsoft C compiler and the
>>> >compiler on the Sun 3, the result is 9.
>>Dec VAX Ultrix gives 9.
>>Lattice C 3.00 for MS-DOS gives 7!!!  (Yes, that's "7", not a typo!)
>Microsoft C 3.0 small memory model gives 6 and DeSmet C 2.51 small
>memory model gives 7.
>
>Looks like this falls in the "don't count on the order of evaluation of
>subexpressions with side-effects" bucket, even though it doesn't look
>illegal.
>
>Certainly, anything that produces widely different answers under
>different popular implementations of C should be avoided; it is all well
>and good to try to say "such and so is RIGHT, and anything that does
>different is wrong" but that doesn't help portability.
>

Ok, this seems good advice for expressions where each subexression
_must_ be evaluated at _some_ time.
But, how about
	c = (b=1,a==1) || (b=2,a==0) || (b=3,a==3) || (b=4,a==2);
as a quick nice way of saying
	switch (a) {
	    case 1: b=1; c=1; break;
	    case 0: b=2; c=1; break;
	    case 3: b=3; c=1; break;
	    case 2: b=4; c=1; break;
	    default: c=0;
	}
where the evaluation will "break off" at the point where one of
the comparisons first succeeds.

(I actually wanted to use something a bit like this a few days ago,
but now I'm not too sure its that portable at all, considering
all the problems with "+").

Of course, compilers on which _this_ doesn't work should be
considered even *more* wrong than those as described above, but
that's not too constructive a complaint...


--
Simon Brown
Dept. of Computer Science, University of Edinburgh.
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