Question

I haven’t been able to solve this prolog exercise. I was hoping someone here could give me some hints or post a solution. Thanks in advance.

Database:

lig(super, porto).  
lig(super, benfica).  
lig(super, sporting).  
lig(honra, feirense).  
lig(honra, guimaraes).  

jog(sporting, ricardo, gr).  
jog(guimaraes, cleber, de).  
jog(feirense, edgar, me).  
jog(porto, quaresma, av).  
jog(porto, helton, gr).  
jog(benfica, simao, av).  
jog(sporting, moutinho, me).

The sample output:

?- calcula(Lista).
Lista = [super-[porto-[quaresma,helton], benfica-[simao], sporting-
[moutinho,ricardo]], honra-[ feirense-[edgar], guimarães-[cleber]]].

My procedure:

calcula(Lista) :-
    findall(Lig-[Eq-[X]],
            (lig(Lig, Eq), findall(Jog, jog(Eq, Jog, _), X)),
            Lista).

My output (which is wrong!).

Lista = [super-[porto-[[quaresma, helton]]], super-[benfica-[[simao]]], super-[sporting-[[ricardo, moutinho]]], honra-[feirense-[[edgar]]]

Answer 1

I see in the zfm’s solution, the predicate lig(Lig, _) becomes true 5 times so there is some duplication in the final list. You can use the predicate setof/3 and existential quantified variable Eq0^ to remove duplication:

calcula(T) :- setof(Lig-X, Eq0^(lig(Lig, Eq0), 
              findall(Eq-U, (lig(Lig,Eq), findall(Jog, jog(Eq, Jog, _), U)), X)), T).