Prolog Cookbook
  • Classic factorial function,
    factorial(0, 1).
    factorial(N, N_Fact) :-
    N > 0,
    M is N-1,
    factorial(M, M_Fact),
    N_Fact is M_Fact*N.
  • Fibonacci functions
    ;; Non tail recursive version - is going to stack overflow quickly
    fib(0, 0).
    fib(1, 1).
    fib(N, N_Fib) :-
    N > 1,
    M is N-1,
    T is N-2,
    fib(M, M_Fib),
    fib(T, T_Fib),
    N_Fib is M_Fib + T_Fib.
    ​
    ;; Tail recursive version
    fibonacci(N, N_Fib) :- tfib(N, 0, 1, N_Fib).
    tfib(0, A, _, A).
    tfib(N, A, B, N_Fib) :-
    N > 0,
    Next_N is N-1,
    Next_A is B,
    Next_B is A + B,
    tfib(Next_N, Next_A, Next_B, N_Fib).
  • Aggregate counting in SWI-Prolog,
    likes(jimmy, anna).
    likes(paul, anna).
    likes(jimmy, paul).
    ​
    popular(X) :- aggregate(count, Y^likes(Y,X), N), N > 1.
    Here, popular function checks whether given person is liked by more than one person. In the definition Y is existentionally qualified.
  • Higher order functions,
    double(X, Y) :- Y is X + X.
    pow2(X, Y) :- Y is X * X.
    ​
    map([], _, []).
    map([X|Xs], P, [Y|Ys]) :-
    call(P, X, Y),
    map(Xs, P, Ys).
    ​
    ;; Examples
    map([1, 2, 3], double, Xs). ;; Xs = [1, 4, 6]
    map([1, -2, 3], pow2, Xs). ;; Xs = [1, 4, 9]
  • current_predicate(map/3). ;; checks whether map which takes 3 parameters exists
  • Define contains based on append function,
    append([], Ys, Ys).
    append([X|Xs], Ys, [X|Zs]) :- append(Xs, Ys, Zs).
    ​
    prefix(P, L) :- append(P, _, L).
    suffix(S, L) :- append(_, S, L).
    ​
    contains(SubL, L) :- suffix(S, L), prefix(SubL, S), !.
  • Palindrome check in Prolog is quite nice,
    palindrome(Xs) :- reverse(Xs, Xs).
    In a restricted set, we can even use this declarative definition to generate palindromes, as in the following example!
    member(X, [1, 2, 3]),
    member(Y, [10, 11, 12]),
    member(Z, [1, 2]),
    palindrome([X, Y, Z]).
  • Fizz buzz in Prolog,
    ```prolog print_fizz_buzz(N) :- ( 0 is mod(N, 15) -> write("fizzbuzz"),nl ; 0 is mod(N, 3) -> write("fizz"), nl ; 0 is mod(N, 5) -> write("buzz"), nl ; write(N), nl ).
fizz_buzz(N) :- aux_fizz_buzz(0, N). aux_fizz_buzz(M, N) :- M < N, print_fizz_buzz(M), M1 is (M + 1), aux_fizz_buzz(M1, N). aux_fizz_buzz(M, N) :- M >= N, !, nl.
- `current_op` can be used to find out precedence and type of an operator.
Example, to find out these information about `mod`,
​
```prolog
current_op(Precedence, Type, mod).
In true Prolog fashion, one can find out all the operators which are curently definied using something like follows,
current_op(Precedence, Type, Op).
which will list all the opeartors with their respective precedence and type.
  • Permutations,
    take([H|T], H, T).
    take([H|T], R, [H|S]) :- take(T, R, S).
    ​
    perm([], []).
    perm(List, [H|T]) :- take(List, H, R), perm(R, T).
Last modified 2yr ago
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