Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.csd.uwm.edu!gem.mps.ohio-state.edu!ginosko!uunet!virtech!cpcahil
From: cpcahil@virtech.UUCP (Conor P. Cahill)
Newsgroups: comp.lang.c
Subject: Re: Obfuscated SWAP: not portable!
Message-ID: <1134@virtech.UUCP>
Date: 6 Sep 89 14:01:13 GMT
References: <784@skye.ed.ac.uk> <1267@levels.sait.edu.au> <149@cpsolv.UUCP> <1400@levels.sait.edu.au>
Organization: Virtual Technologies Inc
Lines: 35

In article <1400@levels.sait.edu.au>, CCDN@levels.sait.edu.au (DAVID NEWALL) writes:
> In article <149@cpsolv.UUCP>, rhg@cpsolv.UUCP (Richard H. Gumpertz) writes:
> > The exchange of x and y that has been under discussion:
> >     x ^= y ^= x ^= y
> > is non-portable because it depends on right-to-left evaluation which is NOT
> > specified in the C standard.
> 
> I think it is specified.  (Perhaps it depends on whose standard you follow?)
> 
> K&R I, page 19:
>      "The line
>         nl = nw = nc = 0;
>     sets all three variables to zero.  This is not a special case, but a
>     consequence of the fact that an assignment has a value and assignments
>     associate right to left."

But, you cannot know what the value of x is at the far left of the
equation prior to evaluating the "^=".  It may be the original value
of x, or it may be the latest value of x (modified by the right hand
side of the equation.  As far as I know, there is no specification
as to when the x will be evaluated.









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