Hi Marco, Yes, you are right. The ultimate purpose of this constraint is symmetry breaking. I 'll try what you suggest. Thanks! Regards, Panagiotis On 27-Mar-23 12:51 PM, Marco Gavanelli wrote: > Hi Panagiotis, > > this constraint reminds me of a symmetry breaking labeling proposed by > Pedro Meseguer. > > Anyway, what about: > > L[0] = 1 > > forall i>0 > L[i] <= maxlist(L[0..(i-1)]) + 1 > ? > > I hope the intuition is clear, with L[0..(1-i)] I mean the sublist of > the first i elements of the list L. > > Best, > Marco > > > On 27/03/2023 11:34, Panagiotis Stamatopoulos wrote: >> Hello Everybody, >> >> I am seeking ideas on how to implement in ECLiPSe a specific >> constraint in a simple, if possible, and efficient way. >> >> Let L be a list of length N with domain variables ranging >> in 1..M. Acceptable lists are the ones that ... >> 1. ... contain values from 1 up to K (K =< M), but not any >> values from K+1 up to M (K is not given). >> 2. ... satisfy the condition that the first occurrences of >> the values from 1 to K appear in this order in the list. >> >> For example, let N = 8 and M = 5. The lists [1,1,2,1,2,3,2,1] >> and [1,2,1,3,2,4,3,1] are valid. The first one has K = 3 (only >> items 1, 2, 3 appear in the list) and the second one has K = 4 >> (just 5 is missing from the list). In the first list, the first >> occurrences of 1, 2, 3 are in positions 1, 3, 6 and in the second >> list, the first occurrences of 1, 2, 3, 4 are in positions 1, 2, >> 4, 6 in the lists. All fine! >> >> I believe that the requirement 1 above could be implemented >> easily with the occurrences constraint (one for each number in >> 1..M) and a set of implication (=>) constraints, stating that >> if the number of occurrences of x in 1..M is 0, then the numbers >> of occurrences of x+1, x+2, ... in the list should also be 0. >> I cannot predict the propagation level of this approach, but >> it seems that, at least, declaratively can be stated. >> >> I don't have any good ideas for the requirement 2. I tried >> something that exploits again the occurrences constraint (for >> every number in 1..M and every prefix list of the given list) >> and then the lex_le constraint. It worked, but if N is around >> 50 or more, the efficiency is unacceptable. >> >> Any ideas on the above would be more than welcome. >> >> Best Regards, >> Panagiotis >> >> >> _______________________________________________ >> ECLiPSe-CLP-Users mailing list >> ECLiPSe-CLP-Users_at_lists.sourceforge.net >> https://lists.sourceforge.net/lists/listinfo/eclipse-clp-users >Received on Mon Mar 27 2023 - 09:58:48 CEST
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