Re: [eclipse-clp-users] constraint on indexed elements

Benoit Hoessen wrote:
> Hello,
> 
> I am currently learning to use eclipse and I'm facing a problem. I
> have a list of couples that needs to be ordered according some
> constraints. I tried the following model:
> 
> ArrayCouples is the array of my couples [Couple0, Couple1, ... ]
> NbCouples is the number of couples
> OrderedList is a list of integers where each integer represent the
> index of a Couple in ArrayCouples. Those integers are the variables of
> my problem and their domain is [0..NbCouples - 1]
> 
> And the constraints should be (with a c-like syntax)
> for (i = 0 ; i < NbCouples-2; i++){
>     CoupleA = ArrayCouples[i];
>     CoupleB = ArrayCouples[i+1];
>     //constraint on the element of CoupleA & CoupleB
> }
> 
> But it seems that I can't model this with eclipse with my current
> knowledge or it's not the way I should model my problem.
> 
> Can somebody tell me what my error is?

Hi Benoit,

It is difficult to give any specific answer, as it is not clear what 
your exact difficult is. You may indeed want to learn more about 
programming in ECLiPSe. Another general comment I can say is that it is 
probably not a very good idea to try and specify the program you want to 
write in C and then try to translate this to ECLiPSe. C does not map 
very well to ECLiPSe (or Prolog in general), and in any case you should 
be thinking about modelling your problem at a higher level...

One possible issue is the data structures you are using: you mention 
lists -- these do not map well to C arrays. ECLiPSe (and Prolog) does 
provide something similar to C arrays -- structures, which allows random 
access to its elements. In addition, ECLiPSe provides syntax that is
more convenient to use (than what is standard in Prolog), and which also 
looks more like traditional array syntax that you find in C. This array 
notation is described in the manual:

http://www.eclipse-clp.org/doc/userman/umsroot024.html

One issue you should be aware of is the index starts from 1, rather than 0.

On the other hand, for the specific example you show in C, it is not 
clear that you do need random access to the elements, so a list may be 
the right data structure to use. Your example can be dnne in ECLiPSe 
using a do-loop (which is simply a different way of writing recursion, 
the standard way in Prolog to write "loops"):

(fromto(NbCouples, [CoupleA,CoupleB|Rest],[CoupleB|Rest], [_]) do
     /* constraint on CoupleA and B */
)

If you also need the index, you can add another iteration specifier like
count(I,Min,Max).

Cheers,

Kish



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