Turbo Prolog: 420 PROLOG.ERR missing

Question

I'm quite new in Prolog. I'm trying to find the nth term and sum of a Fibonacci Series.

/* Fibonacci */

predicates
    fibonacci(integer, integer, integer)

clauses
fibonacci(1,1,1):-!.
fibonacci(2,1,2):-!.

fibonacci(N, Term, Sum):-
    N1 = N - 1,
    N2 = N - 2,
    fibonacci(N1, Term1, Sum1),
    fibonacci(N2, Term2, Sum2),
    Term = Term1 + Term2,
    Sum = Term + Sum.

However while compiling in Turbo Prolog I'm getting 420 PROLOG.ERR missing on

fibonacci(N2, Term2, Sum2),

Why is this happening? Any help is appreciated. Thanks in advance.

Answer 1

Is that really the entire error message? It does not say what is missing?

EDIT: According to comments below, Turbo Prolog's = does indeed correspond to is/2, so the remarks below, which are correct for Prolog, don't apply. According to comments on the original question, the terrible error message might be a singleton warning for Sum2.

In any case: Assuming that Turbo Prolog's clauses part corresponds to standard Prolog, none of N1, N2, Term and Sum will be integers in your program. = means unification, not arithmetic evaluation. If you call fibonacci(3, Term, Sum), then inside the call N1 will be bound to the uninterpreted term 3 - 1, not to the integer 2. The same goes for your other uses of =.

For the arithmetic part, you will want to use is/2: N1 is N - 1, N2 is N - 2 etc. This will evaluate the right-hand side as an arithmetic expression and actually bind these variables to integers.

Without thinking about it too hard, it's not clear to me if this will result in a useful computation for Term.

Answer 2

i guessing turbo cant find some file with error descriptions. looks like tp incorrectly installed? correct this and you get more informative message.

look at http://rosettacode.org/mw/index.php?title=Fibonacci_sequence&action=edit&section=399 and modify it for not only finding Nth but Sum also.

you get something like: ----

% fibsum(i, n, fib(i-2), fib(i-1), fib(i), sum(i-1), sum(i))
fibsum(N, N, Fi2, Fi1, F, Si1, S) :-
    F is Fi2 + Fi1,
    S is Si1 + F.
fibsum(I, N, Fi2, Fi1, F, Si1, S) :-
    In is I + 1,
    Fn is Fi2 + Fi1,
    Sn is Si1 + Fn, !,
    fibsum(In, N, Fi1, Fn, F, Sn, S).


% fibs(i, fib(i), sum(i))
fibs(1, 1, 1).
fibs(2, 1, 2).
fibs(C, N, S) :-
    C > 2,
    fibsum(3, C, 1, 1, N, 2, S). % Generate from 3rd on

--- (barely tested on http://swish.swi-prolog.org/)

Answer 3

Turbo Prolog cannot find the error messages file PROLOG.ERR. That is normally installed in Turbo Prolog installation directory.

If the file is there, chek that the application path is correctly set under Setup->Directories->Turbo Directory