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NNTP-Posting-Date: Sun, 27 May 2012 16:49:08 -0500
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Date: Sun, 27 May 2012 17:49:07 -0400
From: pete <pfiland@mindspring.com>
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Subject: Re: Open letter to Ian Collins
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Keith Thompson wrote:
> 
> "BartC" <bc@freeuk.com> writes:
> > "jacob navia" <jacob@spamsink.net> wrote in message
> > news:jpu2h5$16r$1@speranza.aioe.org...
> >> Le 27/05/12 19:55, pete a écrit :
> >>> I think there's about a zillion combinations of (a) and (b),
> >>> where (a - b) has problems.
> >>
> >> Sure.
> >>
> >> You want to say then that it is not a good idea in C to write
> >>
> >> a - b
> >>
> >> ??
> >>
> >> If you think that overflow could be a problem in a comparison you can
> >> write something more complicated, expressions can be of any length, it
> >> is only important that they be consequent.
> >
> > Maybe he means there might be more problems using:
> >
> >  (a-b<=0)
> >
> > than just using:
> >
> >  (a<=b)
> >
> > for example. Especially when a and b are unsigned. (Assuming the way
> > comparisons will be done is along the lines of that former expression.)
> 
> My point is simply that "a - b" is not a reliable way to *compare*
> two arbitrary integer values.  Using a wider type is not a general
> solution.
> 
> If you *don't* think that overflow could be a problem in a
> comparison, then you've probably missed something.
> 
> The language doesn't have a built-in 3-way comparison operator,
> but it does have < and >.  Just use them.

My prefered 3-way, to compare 2 integers, is

#define COMPAR(A, B) (B > A ? -1 : B != A)

-- 
pete