I have these predicates:
male(roelof).
male(mans).
male(ronald).
male(jan).
female(chantal).
female(marie).
female(gerda).
female(dagmar).
female(denise).
female(kimberly).
parent(mans,gerda).
parent(mans,roelof).
parent(marie,gerda).
parent(marie,roelof).
parent(dagmar,denise).
parent(dagmar,kimberly).
parent(ronald,denise).
parent(ronald,kimberly).
parent(chantal,tamara).
parent(roelof,tamara).
parent(jan,chantal).
parent(jan,dagmar).
father_child(Father, Child) :-
parent(Father, Child),
male(Father).
mother_child(Mother, Child) :-
parent(Mother, Child),
female(Mother).
child_father_mother(Child, Father, Mother) :-
father_child(Father, Child),
mother_child(Mother, Child).
same_father(Child, Sibling) :-
father_child(Father, Child),
father_child(Father, Sibling).
same_mother(Child, Sibling) :-
mother_child(Mother, Child),
mother_child(Mother, Sibling).
siblings(X,Y) :-
( same_father(X, Y),
X \= Y
; same_mother(X, Y),
\+ same_father(X, Y)
).
display_siblings(Person) :-
findall(Person-Y, siblings(Person,Y), Sibs),
display_the_siblings(Sibs).
display_the_siblings([]) :-
write('Er zijn geen zussen/broers bekend').
display_the_siblings([X-Y]) :-
write('The enigste broer of zuster is '),
write(Y).
display_the_siblings([XY0,XY1|XYs2]) :-
XYs0=[XY0,XY1|XYs2],
write('Alle zusterparen zijn : \n '),
forall(( member(X - Y,XYs0)
),( format('~w en ~w. \n',[X,Y])
)).
It works fine but if you do display_siblings(X) you will see duplicates like this
Person 1 - Person 2
Person 2 - Person 1
How can I change this so the duplicates will not show up. I know I can change findall to setof but then I see no output at all.
Roelof
Edit 1: @< works but it also means that display(kimberly) fails because Kimberly is further on the alphabet then denise. Can I not use the same trick as this topic : How can I prevent duplicates in prolog.
