fabs(x) vs. (x) < 0 ? -(x) : (x)

[jtr485 at umich.UUCP]

In article <628 at sdchema.sdchem.UUCP>, tps at sdchem.UUCP (Tom Stockfisch) writes:
> >You could implement fabs() as a macro as follows:
> >    #define fabs(X)     ((_fabs = (X)), (_fabs < 0? -_fabs : _fabs))
> >if _fabs were declared as a float in the math library.
> This might not work for
> 	fabs( x ) + fabs( y )
> because _fabs gets assigned twice in one expression and the two
> comma expressions which result might get interleaved.  There was
> a major discussion in this group recently on whether the sub-expressions
> a,b,c,d in
> 	(a,b) + (c,d)
> could be evaluated in the order
> 	a c b d
> Since the current defacto standard (K&R) is ambiguous on this point, I
> think your method is not safe.
> || Tom Stockfisch, UCSD Chemistry	tps%chem at sdcsvax.UCSD

This fixes that objection:

#define fabs(X)     (((_fabs = (X)) < 0? -_fabs : _fabs))

Again it requires the 'hidden' definition of _fabs but it does not have
order of evaluation problems.

--j.a.tainter