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From: jtr485@umich.UUCP
Newsgroups: comp.lang.c
Subject: Re: fabs(x) vs. (x) < 0 ? -(x) : (x)
Message-ID: <81@umich.UUCP>
Date: Thu, 19-Feb-87 09:58:35 EST
Article-I.D.: umich.81
Posted: Thu Feb 19 09:58:35 1987
Date-Received: Sat, 21-Feb-87 20:07:32 EST
References: <4943@mimsy.UUCP> <2550005@hpisod2.HP> <628@sdchema.sdchem.UUCP> <1070@dg_rtp.UUCP>
Distribution: world
Organization: EECS, University of Michigan
Lines: 18

In article <1070@dg_rtp.UUCP>, meissner@dg_rtp.UUCP writes:
> > #define fabs(X)     (((_fabs = (X)) < 0? -_fabs : _fabs))
> It may solve that problem, but:
> 	fabs( fabs( y ) - 10.0 )
> would still get the wrong answer.
> 	Michael Meissner

No.  This will still get the right answer.  The only thing to ever worry about
would be a side effect explicitly manipulating _fabs.  Such as:
    fabs( _fabs++ )
which expands to:
    (((_fabs = (_fabs++)) < 0? -_fabs : _fabs))
And now there is no way of knowing when the ++ gets done relative to the
expressions after the '?'.

But only direct manipulation of the 'hidden' variable can do this.

--j.a.tainter