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.

    References - https://stackoverflow.com/questions/5930340/aggregate-3-in-swi-prolog/5930420

  • 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]

  • Checking whether a predicate exists - https://stackoverflow.com/questions/12886179/prolog-how-to-check-if-a-predicate-exists.

    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.

In true Prolog fashion, one can find out all the operators which are curently definied using something like follows,

which will list all the opeartors with their respective precedence and type.

  • Permutations,

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