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).