Sibling Relationship Prolog

2
votes

I'm new to Prolog so I'm completely unaware of the errors i get. I'm making a code that describes the relationship between any two family members. i.e if two are married, sisters, brothers, sister and brother...etc.

wife(hussein,amina).
mother(amina,amira).
mother(amina,amany).
mother(amina,amna).
mother(amina,hassan).
mother(amina,hossam).

father(hussein,amira).
father(hussein,amany).
father(hussein,amna).
father(hussein,hassan).
father(hussein,hossam).

parent(X,Y):-
   mother(X,Y) ; father(X,Y).

married(X,Y) :- wife(X,Y).

sibling(X,Y) :-
   mother(Out1,X),
   father(Out2,X),
   mother(Out3,Y),
   father(Out4,Y),
   Out1 is Out3,Out2 is Out4.

The code works fine for the parent,married rules but i can't get why the sibling isn't working. I get for X and Y variables their mothers and make sure they're the same, their fathers and make sure they're the same to confirm they're siblings. but i get this error :

ERROR: is/2: Arithmetic: `amina/0' is not a function
1
prolog

1 Answers

1
votes

is/2 is for arithmetc; with is you can write something like

X is 3 + Y

so X is unified with 3 plus the arithmetic value of Y.

But, in you case, the terms you're using (hussein, amina, amira, etc.) aren't numbers so you can't use is for unify they.

You, in sibling/2, can "unify" Out1 with Out3 and Out2 with Out4 using a equal (= operator)

sibling(X,Y) :- 
  mother(Out1,X),
  father(Out2,X),
  mother(Out3,Y),
  father(Out4,Y),
  Out1 = Out3,
  Out2 = Out4.

or better (IMHO) usign the same variable (using Out1 instead of Out3 and Out2 instead Out4)

sibling(X,Y) :- 
  mother(Out1,X),
  father(Out2,X),
  mother(Out1,Y),
  father(Out2,Y).