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Subject: Re: [boost] Review Request: Creasing (Sequence Properties)
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2010-01-27 15:42:52

Simonson, Lucanus J wrote:
> Stewart, Robert wrote:
> > Simonson, Lucanus J wrote:
> >>>
> >> I'm a little confused why there is such a narrow focus on
> >> ordering relationships here. What seems to be the most
> >
> > Why is it confusing? The OP focused the thread and no one
> > generalized it beyond the is_ordered idea.
> >
> >> general solution to this is a generic range adaptor that
> >> given a range over elements of type T returns an adapted
> >> range over pairs of elements of type T that are adjacent to
> >> eachother in the input range. I'm constantly writing such
> >> adaptors to convert polygon vertices to polygon edges as
> >> pairs of vertices. Combine that with a foreach or just a
> >> while loop and you can apply the predicate. We can
> >
> > You say that like there's no code involved.
> I'm paraphrasing, but given the adaptor and foreach the
> solution is a one liner:
> foreach_with_early_termination(a in
> pair_range_adapt(input_range)), predicate());

To write that requires knowledge of foreach_with_early_termination, pair_range_adapt, whatever "a in pair_range_adapt(input_range)" means in code, and the correct way to wire them together. is_ordered looks quite compelling by contrast.

That code looks almost write only. I'd expect a comment explaining it each time it appeared, so why not create a named algorithm that makes the code self documenting?

> >> As it happens we
> >> have boost libraries that provide a basis for each of these
> >> generally useful constructs. It seems a little pointless to
> >> propose a library to combine them to solve a very specific
> >> problem. Nobody is at a loss as to how to code up something
> >> to figure out whether elements in an iterator range are
> >> sorted, and nobody would or even should look in boost for
> >> such a simple thing.
> >
> > If you say so. I don't happen to agree. Unless one does such a
> > thing with any regularity, one must find a recipe or reinvent it
> > again each time. Your statement seems rather like suggesting that
> > std::for_each shouldn't be in the Standard Library because
> "nobody is
> > at a loss as to how to code up something" to do such a loop.
> You can take my statement and apply the falicy of taking to
> an absurd extreme, but that doesn't prove anything. In your
> example for_each is a more general feature than the proposed
> library. We could similarly say that for loops are not
> needed in the language because while loops can always get the
> job done and nobody is at a loss as to how. This misses the
> point of what I was saying.

I don't think I was being absurd or extreme, or that I missed the point. I was trying to make the point that something specific is often much easier to use -- and find, for that matter -- than a more generalized solution.

> > Any time a library can provide a useful, robust, *named* solution to
> > a common problem, it seems worthwhile. As could be seen from this
> > thread, using std::adjacent_find was not immediately obvious, so
> > there is room to write less than optimal solutions, making a Boost
> > library solution helpful. Furthermore, if there are any
> > compiler-specific quirks that should arise, the library can account
> > for those and user code need not do so.
> >
> > Have I missed something in your concern? Do you still disagree with
> > including such tools in Boost, possibly in addition to your
> > generalization?
> Creating a named solution for a problem incurrs a cost in
> complexity in addition to a benefit. The benefit of
> convenience of having a named solution for a problem people
> face regularly needs to be greater than the cost of learning
> the name for all the people who need the solution. In this

That's not quite accurate. Those that know the general tools and find their assembly obvious and straightforward, can always use the general tools. Those that only want to determine whether a sequence is ordered are likely to look for is_ordered or use some search terms that easily locate it. That algorithm will be easy to understand for the simple purpose at hand.

> case I think the proposed library fails the test. I find it
> easier to figure out how to code up whether a iterator range
> is sorted based on the stl alone, or on general boost
> capabilities than to remember a name, order of arguments and
> header file to include to get a library solution. In this

You propose that all must follow that approach though not all will find things as easy to figure out as you? Most folks I know of would not even begin to think in the terms you've outlined.

> particular case all four uses for creasing are covered by
> std::is_sorted directly with predicate less<T>, greater<T>,
> less_equal<T> and greater_equal<T>. If you pass
> greater_equal<T> to is_sorted it returns true only if all
> elements in the range are decreasing and there are no
> duplicates. Now, I agree that is_sorted(b, e,
> greater_equal<T>()) is kind of hard to understand as meaning
> decreasing with
> no duplicates, rather than write a new library function it
> would be better to put /*decreasing with no duplicates*/
> preceding that line in your code. So the named solution in
> this case is to the problem that people don't know to use the
> existing named solution? I hardly think adding more named
> solutions is going to solve that problem.

It could well be that is_ordered is sufficient, as Joachim suggested, given the set discussed before you joined the thread, but only if there were specific examples in the documentation to show how and why to use those four predicates. However, is_ordered remains far easier to grasp and use than what you described above.

Rob Stewart robert.stewart_at_[hidden]
Software Engineer, Core Software using std::disclaimer;
Susquehanna International Group, LLP

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