CODEWITHSUNDEEP
prolog
chris kelly
chris kelly

Reputation: 11

Can anyone explain why im getting this out put for my prolog Query

% comment the next two lines out when testing - put back in to submit
:- initialization(q1).
:- initialization(q2).
:- initialization(q3).
:- initialization(end). 
% FACTS. 
mammal(kitty).
mammal(ratatat).
mammal(fido).
claws(kitty).
tail(ratatat).
bestfriend(fido).
feathers(tweety). 
% 9 RULES. 
% If X is a mammal then X has fur. 
fur(X) :- mammal(X).
% If X has fur and X has a tail then X is a rat. 
rat(X) :- fur(X),tail(X).
% If X has claws and X has fur then X is a cat.
cat(X) :- claws(X),fur(X).
% If X is a cat then X meows.
meows(X) :- cat(X).
% If X has feathers then X is a bird.
bird(X) :- feathers(X).
% If X is a bestfriend and X has fur then X is a dog.
dog(X) :- bestfriend(X),fur(X).
% If X is a dog and Y meows then X likes Y.
likes(X,Y) :- dog(X),meows(Y).
% If X is a cat and Y is a bird then X likes Y.
likes(X,Y) :- cat(X),bird(Y).
% If X is a cat and Y is a rat then X likes Y.
likes(X,Y) :- cat(X),rat(Y). 
writeln(T) :- write(T), nl. 
max(X,Y,Z) :-
  X > Y,
  X > Z,
  write(X).
max(X,Y,Z) :-
  X > Y,
  write(Z).
max(_,Y,Z) :-
  Y > Z,
  write(Y).
max(_,_,Z) :-
  write(Z).
moves(N,R) :-
N = 0, R is 0.
moves(N,R) :-
N = 1, R is 1.
moves(N,R) :-
N > 1, T1 is N - 1, T2 is N-1, moves(T1,R1), moves(T2,R2), R is R1 + R2 + 1. 
% QUERY. Uncomment after you have written all predicates.
q1 :- writeln('Who likes whom?'), 
      findall(X, (likes(X,Y), format('~w ~w~n',[X,Y])),_).
q2 :- write('Move cnt for 5: '),
      findall(X, (moves(5,X), format('~w' ,[X])),_).
q3 :-nl, write( 'Max(7,3,9)? '),
      findall(X, (max(7,3,9), format('~w ~n' ,[X])),_).
end :- writeln('done.'), halt.

output:
Who likes whom?
fido kitty
kitty tweety
kitty ratatat
Move cnt for 5: 31
Max(7,3,9)? 9_13 <---- why is it printing and underscore 13?
9_13 <--- also why is it printing this line again?
done.

not sure why its printing the way it is any info would be greatly appreciated! im thinking it has to do with my q3 query but i have been messing with it and cant figure it out thanks guys!

Upvotes: 0

Views: 99

Answers (1)

CapelliC
CapelliC

Reputation: 60034

q3 has in common with q1,q2 an unusual usage of findall/3 (I think you should use forall/2, really). Differently than other 2 queries, q3 outputs an unbounded variable (X doesn't appear in findall goal), and it does 2 times because max/3 succeeds 2 times. Here is the q3 pertinent trace, obtained (in swipl) adding a directive :- initialization((leash(-all),trace)).. In gprolog, it's :- initialization((leash(none),trace)).

   Call: (20) q3
   Call: (21) nl

   Exit: (21) nl
   Call: (21) write('Max(7,3,9)? ')
Max(7,3,9)? 
   Exit: (21) write('Max(7,3,9)? ')
^  Call: (21) findall(_G1900,  (max(7, 3, 9), format('~w ~n', [_G1900])), _G1912)
   Call: (27) max(7, 3, 9)
   Call: (28) 7>3
   Exit: (28) 7>3
   Call: (28) 7>9
   Fail: (28) 7>9
   Redo: (27) max(7, 3, 9)
   Call: (28) 7>3
   Exit: (28) 7>3
   Call: (28) write(9)
9
   Exit: (28) write(9)
   Exit: (27) max(7, 3, 9)
^  Call: (27) format('~w ~n', [_G1900])
_G1900 
^  Exit: (27) format('~w ~n', [_G1900])
   Redo: (27) max(7, 3, 9)
   Call: (28) 3>9
   Fail: (28) 3>9
   Redo: (27) max(7, 3, 9)
   Call: (28) write(9)
9
   Exit: (28) write(9)
   Exit: (27) max(7, 3, 9)
^  Call: (27) format('~w ~n', [_G1900])
_G1900 
^  Exit: (27) format('~w ~n', [_G1900])
^  Exit: (21) findall(_G1900, user: (max(7, 3, 9), format('~w ~n', [_G1900])), [_G1923, _G1920])
   Exit: (20) q3

Upvotes: 1

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