CODEWITHSUNDEEP
prologprimesswi-prologprolog-toplevelgoldbach-conjecture
BonWaGG
BonWaGG

Reputation: 43

Type error: `character_code' expected with prolog

I am new to prolog. I was writing a code with Goldbach’s Conjecture problem which I have to list all possible groups of one even number. I found a code like this:

is_prime(2).
is_prime(3).
is_prime(P) :- integer(P), P > 3, P mod 2 =\= 0, \+ has_factor(P,3).  

has_factor(N,L) :- N mod L =:= 0.
has_factor(N,L) :- L * L < N, L2 is L + 2, has_factor(N,L2).

goldbach(4,[2,2]) :- !.
goldbach(N,L) :-
  N mod 2 =:= 0,
  N > 4,
  goldbach(N,L,3).
goldbach(N,[P,Q],P) :-
  Q is N - P,
  is_prime(Q), P < Q.
goldbach(N,L,P) :-
  P < N,
  next_prime(P,P1),
  goldbach(N,L,P1).



next_prime(P,P1) :- P1 is P + 2, is_prime(P1), !.
next_prime(P,P1) :- P2 is P + 2, next_prime(P2,P1).

However when I execute the program, the program prints the first group successfully, but then comes up with error.

1 ?- goldbach(28, L).
L = [5, 23]
ERROR: Type error: `character_code' expected, found `-1' (an integer)
ERROR: In:
ERROR:   [11] char_code(_4206,-1)
ERROR:   [10] '$in_reply'(-1,'?h') at c:/program files/swipl/boot/init.pl:779

I do not know how this error occurs, and I look for information in Internet but get nothing. Does someone have any idea? Thank you for your answering.

Upvotes: 1

Views: 2813

Answers (1)

Isabelle Newbie
Isabelle Newbie

Reputation: 9378

After Prolog prints the first answer, it waits for your input to tell it whether you are interested in more answers. To a first approximation, valid keys to press are ; (semicolon) or Space or n if you want more answers, or . (period) or Enter or y if you are not interested in any more answers.

It looks like you are using some other, unexpected, key or key combination. If you are trying to type ; but your keyboard doesn't have a single key for it, try Space or n instead.

I can successfully get several answers out of the code you posted by pressing ; after each answer:

?- goldbach(28, L).
L = [5, 23] ;
L = [11, 17] ;
false.

Edit: As you are on Windows, the answer from After the first answer, Prolog shows the error "char_code/2: Cannot represent due to 'character_code'" to use swipl-win.exe instead of swipl.exe might also be useful.

Upvotes: 1

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