Hi,
On 12/12/2011 05:39, James Carman wrote:
> Sorry, I was on my phone before when I sent that. Let me elaborate a
> bit more. I would just allow the weights to be of any type. However,
> you can create two different types of scenarios where you either use a
> Comparable derivative or you use whatever you want, but you have to
> supply a custom Comparator.
ok it definitely makes sense, thanks :)
The thing is: in case the weight is actually a number I would really
like to keep it simple on the user side, i.e. it should be usable with
something like {{Weighted<Double>}}, or {{Weighted<Integer>}}, etc. On
the other hand, some of the implemented algorithms would need to expose
one method per number type: e.g. Dijkstra (which also sums weights, so
it needs numbers) would need a method for Doubles, one for Integers,
etc. Alternatively one could use the abstract class {{Number}} once for
all  but that would not make things easier, because there is no way to
do maths directly with it (see e.g.
http://stackoverflow.com/questions/2721390/howtoaddtwojavalangnumbers).
Summing up:
* {{public interface Weighted<W>}} with method {{public W getWeight()}}
* weighted "things" ({{Edge}}, {{Vertex}}, {{Graph}}, etc) need to
implement it, e.g. {{public interface WeightedEdge<E,W> extends
Edge<E>, Weighted<W>}}
* each algorithm specifies the type of weight needed. E.g. Prim would
only require edges to have {{Comparable}} weights or a
{{Comparator}}, while Dijkstra needs edges with weights as real
numbers (maybe just {{Double}} for now), etc.
How does that sound?
Ciao,
Claudio
>
> On Sun, Dec 11, 2011 at 8:01 PM, James Carman
> <james@carmanconsulting.com> wrote:
>> I wouldn't restrict the weight to Comparable. What if the user wanted to
>> provide their own Comparator?
>>
>> On Dec 11, 2011 7:07 PM, "Claudio Squarcella"<squarcel@dia.uniroma3.it>
>> wrote:
>>> Hi all,
>>>
>>> I explored a bit more the (rather philosophical) dilemma that came from a
>>> thread from last week, quoted below
>>>> One step further. A weight is not necessarily a double: in some cases not
>>>> even a number, but rather a "comparable" of some sort. So I would suggest
to
>>>> make use of generics in some way, possibly the smartest. Suggestions are
>>>> welcome :)
>>>
>>> The question is: *what do we mean by weight when dealing with graphs?*
>>>
>>> "Real number" is a standard answer in graph theory: see, e.g.,
>>> http://www.math.jussieu.fr/~jabondy/books/gtwa/pdf/chapter1.pdf (pag. 15).
>>> What we have now in the code is a {{getWeight()}} method that returns a
>>> double. That serves well for all the algorithms currently implemented, and
>>> probably for many more to come. However it is also true that:
>>>
>>> * some domains of interest and/or algorithms might be more restrictive
>>> on the type and sign of "real number" for the weights: integers,
>>> nonnegative rationals, etc.
>>> * strictly speaking, the basic operations associated with weights are
>>> usually just a few. Comparison and sum are enough at least for the
>>> algorithms implemented so far in the project (please correct me if I
>>> am wrong). Maybe scaling? Additive inverse?
>>> * each algorithm is aware of the subset of required operations. E.g.
>>> Prim's algorithm for minimum spanning trees only requires edge
>>> weights to be comparable, so they could even be Strings or whatever...
>>> * some very abstract user might want to use a new class (not
>>> necessarily a number) as a weight, provided that it meets the
>>> requirements of the domain.
>>>
>>> So here is a highlevel view of what I propose:
>>>
>>> * the basic weight is nothing more than a {{Comparable}}, which is
>>> hopefully generic enough;
>>> * where needed, algorithms define more specific constraints on the
>>> input graph in their signature (e.g. Dijkstra can use {{Double}}).
>>>
>>>
>>> Looking forward for comments,
>>> Claudio
>>>
>>> 
>>> Claudio Squarcella
>>> PhD student at Roma Tre University
>>> Email address: squarcel@dia.uniroma3.it
>>> Phone: +390657333215
>>> Fax: +390657333612
>>> http://www.dia.uniroma3.it/~squarcel
>>>
>>>
>>> 
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> 
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>

Claudio Squarcella
PhD student at Roma Tre University
Email address: squarcel@dia.uniroma3.it
Phone: +390657333215
Fax: +390657333612
http://www.dia.uniroma3.it/~squarcel

To unsubscribe, email: devunsubscribe@commons.apache.org
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