[gecode-users] MinimizeScript: Beginners Question

[Josef Eisl]

On 01/17/2012 04:59 PM, Christian Schulte wrote:
> Hi Josef,
> 
> Thanks for your observations as far as MPG is concerned. I'll think them
> through and look how to improve. The information being spread out can't
> really be helped much as each location takes a different angle and different
> depth. But I promise I'll think...
> 
> Then printing each solution is actually what you want (I guess): normally,
> finding a best solution takes so long that you will have to settle for the
> solution that has been found last after the time is up. But maybe there
> could be an additional switch which would only make sense in a best solution
> search scenario: -print-last where only the last solution found is printed,
> and if a time out is encountered then also the last and not necessarily best
> solution is printed. Would that make you happy?

Yes, such an option would be great :).

Thanks again for your time!

br
Josef

> 
> Christian
> 
> --
> Christian Schulte, www.ict.kth.se/~cschulte/
> 
> 
> -----Original Message-----
> From: users-bounces at gecode.org [mailto:users-bounces at gecode.org] On Behalf
> Of Josef Eisl
> Sent: Tuesday, January 17, 2012 3:52 PM
> To: cschulte at kth.se
> Cc: users at gecode.org
> Subject: Re: [gecode-users] MinimizeScript: Beginners Question
> 
> Hello!
> 
> On 01/17/2012 02:31 PM, Christian Schulte wrote:
>> Hi,
>>
>> Please let me take your questions in turn.
>>
>> 1. Changing the relation from strict (that is, "<") to a non-strict 
>> (that is, "<=") will typically not work. It would entail that you do 
>> not only find all _optimal_ solutions but all solution for each and 
>> every cost value! So normally that will be way too many solutions! Try 
>> it out with Gist on a small example and you will see that this will make
> things break down.
> 
> Ups, my bad. Clearly adding a non strict constraint does not make much
> sense.
> 
>> Should I add a tip in MPG that the order must be strict for things to
> work?
> 
> Maybe, but I my eyes it is more confusing that MinimizeSpace/MinimizeScript
> return non-optimal solutions, at least for a new user. Some other things
> about MPG and optimization that are not straight forward in my opinion:
> 
> 1. Information is spread over several chapters:
>    - 2.5 Best solution search
>    - 3.2 Using a cost function
>    - 6.3 Support for cost-based optimization
>    - 7.4 Search engines
> 
> 2. Best Solution Search engine
> It is mentioned in 2.5 and 7.4 but not 3.2 and 6.3 and there are no links to
> the other sections. Maybe it should be stated more clearly that BAB or
> Restart must be used for best solution search.
> 
> 3. constrain()
> In 2.5: "Note that every space defines a default constrain() member function
> (to keep the design of models simple). If a model does not re-define the
> constrain() member function, the default function will be called which
> throws an exception of type SpaceConstrainUndefined (as this is a modeling
> error)".
> In my eyes the second sentence is not true as OptimizeSpace provides a valid
> constrain().
> 
> These are rather beginner problems than problems of MPG. Gecode is one of
> the best documented free software I've seen. I just want to share my
> experience.
> 
>> 2. To find all optimal solutions, I would first find _one_ optimal 
>> solution as normal. Then I would find all solutions (starting a new 
>> search) where the cost value is equal to the optimal const value.
> 
> I see. That makes sense.
> 
>> 3. You see that printing "all" optimal solutions with respect to 1. and 2.
>> does not really make much sense.
> 
> Ok using -solutions for optimal solutions does not work. But how about
> printing only the last (optimal if -solutions 0) solution with
> MinimizeScript?
> 
>> I hope that helps
> 
> Thank you very much for your quick and detailed answers!
> 
> Josef
> 
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