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Consider the followig code, compiled in clingo.

File test.lp. Compile command: clingo 0 test.lp.

col(("rot";"blau")).

freq("rot","hell",2). freq("rot","dunkel",2). freq("rot","hell",5).
freq("blau","hell",20). freq("blau","dunkel",30). freq("blau","hell",50).

freq_sum(C,F) :- F = #sum{ X : freq(C,_,X) }, col(C).
%% does not work: >>error: syntax error, unexpected [, expecting {<<
% freq_sum(C,F) :- F = #sum[ X : freq(C,_,X) ], col(C).

#show freq_sum/2.

The code should compute the sum of frequencies for each colour. For "red", this should be 9 and for “blue" this should be 100. For "red" the value 2 is counted twice. When I replace {...}by […], which should make the list an ordered list and deliver the ordered sum, the compiler complains error: syntax error, unexpected [, expecting {. I have clingo version 4.5.4. Is there something wrong with my code or with my compiler?

Can someone please tell me, how I can perform an ordered sum in asp?

Thomas
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1 Answers1

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In case anyone comes across this issue, I found the solution. The problem is the version of clingo/gringo from version 4 does not support multisets. One can only enter in lists via {...}. Nevertheless there is a way to make prevent collapsing of multiple elements.

freq_sum(C,F) :- F = #sum{ X,H : freq(C,H,X) }, col(C).

naming the objects, over which the indexing occurs, allows each item in the list to be uniquely tagged, preventing duplicate values from being viewed as a single element. I found this solution in §2 of http://ceur-ws.org/Vol-1645/paper_9.pdf.

Thomas
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  • The formal definition of the language is in the ASP-Core-2 Standard https://www.mat.unical.it/aspcomp2013/files/ASP-CORE-2.01c.pdf ... the aggregates (like #sum) are defined syntactically in 1.3 and semantically in 2.4. Indeed in the solution above, the sum is built over the first element of the tuple "X,H", so over "X" and "H" serves only as a distinguishing element between tuples. – peschü Jan 10 '17 at 11:16
  • Thanks, yes that’s essentially what I wrote in the second half of my post. – Thomas Jan 10 '17 at 18:27