RE: [boost] Interval Library and comparison operators

From: David Bergman (davidb_at_[hidden])
Date: 2002-09-06 10:21:58

Guillaume,

I have used tribools, as well as fuzzy logic, a lot, and understand its
importance in certain applications. It is just that for this general
interval abstraction we are discussing here it does not seem to
warranted.

And, I know that by transitivity a1 < b2 when [a1, a2] < [b1, b2]. I
just wanted to avoid cycles. After writing that "extra conjunct", I
realized that I had been wandering into graph-land. You are right, that
"extra" is not needed, since we antisymetry. Thanks. 

/David

-----Original Message-----
From: boost-bounces_at_[hidden]
[mailto:boost-bounces_at_[hidden]] On Behalf Of Guillaume Melquiond
Sent: Friday, September 06, 2002 5:54 AM
To: boost_at_[hidden]
Subject: RE: [boost] Interval Library and comparison operators

On Thu, 5 Sep 2002, David Bergman wrote:

> So, back to the ordering. Which one is the most natural choice? Hmm, 
> there are obviously three orderings that popup:
[...]
> 3. The "complete position" ordering (have no proper name for it...), 
> where
>      ' [a1, a2] < [b1, b2] ' iff ' a2 < b1 && a1 < b1 '
>    the extra conjunct ' a1 < b1 ' is needed to ensure that all end 
> points are in the same chain, since we else can get A < B && B < A, if

> the points lie in a circle.

Two years ago, when an interval library was already discussed on this
mailing-list, some people suggested the library should handle intervals
with inverted bounds. However, during this review, it seems everybody
has agreed that the two bounds of an interval are ordered.

So I don't understand your point. We have 'a1 <= a2'; and the order,
even if it is a partial order, has the transitivity property. So we are
sure that 'a2 < b1' iff 'a2 < b1 && a1 < b1'.

> Partial orderings always have, and should have, these "undefined 
> regions". What is the problem with that? Forcing those regions to be 
> defined by either (1) introducing inconsistencies, such as defining 
> '<=' to be '!>', or (2) using obscure tribools will not help.

Concerning your first point, you are mistaken if you think defining '<='
to be '!>' was meant to force these regions to be defined. In fact,
these still exists some undefined region, since you can easily find two
intervals x and y such as we don't have 'x < y' nor 'x = y' nor 'x > y'
(and that's got nothing to do with the definition of '<=').

Using "obscure tribools" is really a great help when solving inequation
systems (it's my current business) or doing static analysis. For
example, you are trying to solve 'f(x) > 0' where x and 'f(x)' are
intervals. There are three possibilities. First, the comparison result
may be tribool::true; in that case, you know that each element of x is a
solution to the inequation. The result may also be tribool::false; in
that case, you are sure no element of x is a solution. Finally, the
result may be tribool::maybe and you need to do additional computations.

> We all have to realize that at least two of the orderings above are 
> partial, there is not much we can do about it.
> 
> No matter what ordering chosen, '<=' should be regarded as syntactic 
> sugar for '< || ==' in the case there exists an equivalence, such as 
> here...

Yes, '<=' should probably be seen as '< || =='. But such a relation is
completely useless in my opinion. So I'm more and more driven to forsake
'<=' and '>='.

> I must be extremely stupid/ignorant not to see the problem by these 
> fairly straight-forward definitions, so I hope someone will enlighten 
> me, so I also can join the "tribool and complex ordering" choire ;-)
> 
> David

Regards,

Guillaume

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