I don't like the idea of pushing the adding, comparing, etc. into the
weights. I like the idea of having operations external to the weights
that take care of that stuff.
On Tue, Dec 13, 2011 at 12:35 PM, Claudio Squarcella
<squarcel@dia.uniroma3.it> wrote:
> Hey Simone,
>
>
> On 12/12/2011 18:35, Simone Tripodi wrote:
>>
>> Hi James!
>> yes, actual Dijkstra implementation[1] uses Double number to
>> accumulating the total path weights...
>> I think Having an accumulator would be helpful! How do you would
>> modify the current implementation  even with pseudocode?
>
>
> trying to put it all together (thanks James, Matthew):
>
> * Weighted<W> is fully generic without restrictions on the type of
> weight W
> * different properties of weights are specified with interfaces: e.g.
> Summable, HasZero, Comparable...
> * each algorithm requires the weights to implement one or more of the
> above interfaces based on needs, and only works with related methods
> abstracting from the actual type of weight. For example sum(W
> weight) for Summable.
>
> Now, IFF you like that... what shall we do with Double, Integer, etc?
>
> Claudio
>
>
>
>> TIA, all the best,
>> Simo
>>
>> [1]
>> https://svn.apache.org/repos/asf/commons/sandbox/graph/trunk/src/main/java/org/apache/commons/graph/shortestpath/Dijkstra.java
>>
>> http://people.apache.org/~simonetripodi/
>> http://simonetripodi.livejournal.com/
>> http://twitter.com/simonetripodi
>> http://www.99soft.org/
>>
>>
>>
>> On Mon, Dec 12, 2011 at 6:27 PM, James Carman
>> <james@carmanconsulting.com> wrote:
>>>
>>> Why do you need doubles for Dijkstra? Accumulating the total path
>>> weights? Why not introduce an Accumulator interface?
>>> On Dec 12, 2011 9:32 AM, "Claudio Squarcella"<squarcel@dia.uniroma3.it>
>>> wrote:
>>>
>>>> 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/howtoadd**twojavalangnumbers<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<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<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<http://www.dia.uniroma3.it/~squarcel>
>>>>>>>
>>>>>>>
>>>>>>> ****
>>>>>>> 
>>>>>>> To unsubscribe, email:
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>>>>>>>
>>>>>>> ****
>>>>>
>>>>> 
>>>>> To unsubscribe, email:
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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<http://www.dia.uniroma3.it/~squarcel>
>>>>
>>>>
>>>>
>>>> ****
>>>> To unsubscribe, email:
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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
> For additional commands, email: devhelp@commons.apache.org
>

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