Prolog and List

[effatha]

Hi,

I'm trying to implement an algorithm in Prolog, in which I have a list L = [b1, b2, b3] and facts of the genre Liga2 (b1, 5), Liga2 (b2, 1), Liga2 (b3, 3). My aim is to sort the list L according to the argument of Liga2, getting as a result L = [b2, b3, b5].

Anyone can help me?

Thanks

 

[joel76]

Use predsort/3 of SWI-Prolog.

 

[effatha]

Hi. Thanks by your answer. I'm using LPA and not SWI-Prolog. LPA doesn't have a predicate like predsort. However i implemented the follow code:


ordena2(L,LO):-findall(t(N,X),(member(X,L),liga2(X,N)),LL),
sort(LL,LLS),extrai_bs2(LLS,LO).

extrai_bs2([],[]).
extrai_bs2([t(_,X)|L],[X|L1]):-extrai_bs2(L,L1).

 

[joel76]

If you want to implement your own sort you can do like that (the list is very small), for a quicksort, the idea is the same. :

selection([], L, L).

selection([H | T], L, LF) :-
	extract(H, T, H1, [], T1),
	selection(T1, [H1 | L], LF).

% extract the greatest number
extract(H, [], H, L, L).

extract(H1, [H2 | T], HF, LC, LF) :-

	% I get the value of H1 and H2
	liga2(H1, V1),
	liga2(H2, V2),

	(   V1 =< V2 ->
	    extract(H2, T, HF, [H1 | LC], LF);
	    extract(H1, T, HF, [H2 | LC], LF)).