Reputation: 459
My database is similar to this:
% happy(Person,Happiness)
happy(1,10).
happy(2,5).
happy(3,8).
happy(4,1).
I want to sort people w.r.t. their happiness.
I coded the following and it does what I want. However it looked cumbersome to me. Any improvements?
? - sortPeople(Ts).
Ts = [1, 3, 2, 4].
My solution:
getFirst([],R,R).
getFirst([[H1,_]|T],F,R) :-
append([H1],F,R1),
getFirst(T,R1,R).
compareHappiness(X, [_,S1], [_,S2]) :- compare(X, S1, S2).
sortPeople(Ts) :-
findall([X,Y], happy(X,Y), List),
predsort(compareHappiness, List, SortedList),
getFirst(SortedList,[],Ts).
Upvotes: 0
Views: 314
Reputation: 1712
-Here is what I got :
sort(Rez) :- findall([Happiness,PId],happy(PId,Happiness),List),
msort(List,LSorted),
findall(PersonID,member([_,PersonID],LSorted),Sorted),
reverse(Sorted,Rez).
Upvotes: 1
Reputation: 40778
Consider using more descriptive and declarative predicate names, for example:
person_happiness(1, 10).
person_happiness(2, 5).
person_happiness(3, 8).
person_happiness(4, 1).
To sort people by happiness, consider using the built-in keysort/2
, which is more efficient than predsort/3
. You only need to build key-value pairs, for which by convention the functor -/2 is used, and instead of your auxiliary predicate, consider using the SWI-Prolog built-ins pairs_values/2
and reverse/2
:
descending_happiness(Ps) :-
findall(H-P, person_happiness(P, H), HPs),
keysort(HPs, HPs1),
pairs_values(HPs1, Ps1),
reverse(Ps1, Ps).
Example query:
?- descending_happiness(Ps).
Ps = [1, 3, 2, 4].
Upvotes: 2