Logic programming: more Prolog examples

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c go java js lua py vbs rkt scm pro

Here are some more examples of Prolog programs.

Here is the Prolog code.

prints([]):- nl.
prints([X|Rest]):- print(X), prints(Rest).

go:-
	print("Hello"), nl,
	print('Hello'), nl.

:- solve(go).

Here is the output of the Prolog code.

Hello
Hello

The mathematical definition of factorial is defined as follows for non-negative integers.

fact(0) = 1 fact(n) = n * fact(n-1), n > 0

Here is a Prolog implementation of the Factorial function.

Here is the Prolog code.

prints([]):- nl.
prints([X|Rest]):- print(X), prints(Rest).

fact(0, 1).
fact(N1, X1):-
	N2 is N1 - 1,
	fact(N2, X2),
	X1 is N1 * X2.

go:-
	N3 is 7,
	fact(N3, X3),
	prints(['fact(',N3,') is ',X3,'.\n']).

:- solve(go).

Here is the output of the Prolog code.

fact(7) is 5040.\n

Here is the Prolog code.

prints([]):- nl.
prints([X|Rest]):- print(X), prints(Rest).

data(156).

toBinary2(0,[]).
toBinary2(X1,[R1|S1]):-
	R1 is X1 mod 2,
	Q1 is X1 div 2,
	toBinary2(Q1,S1).

reverse([],Xs,Xs).
reverse([X|Xs],Ys,Zs):-
	reverse(Xs,Ys,[X|Zs]).

combine(Xs,S):-
	atom_chars(S,Xs).

toBinary1(0,[0]).
toBinary1(X1,S2):-
	toBinary2(X1,S1),
	reverse(S1,S2,[]).

go:-
	data(X1),
	toBinary1(X1,S1),
	combine(S1,S2),
	prints(['Value ',X1,'d in binary is ',S2,'b.\n']).

:- solve(go).

Here is the output of the Prolog code.

Value 156d in binary is 10011100b.\n

Note that to do what might be simple in other imperative languages might be somewhat cumbersome (though straight-forward) in Prolog.

If a problem fits Prolog's computation and resolution model then Prolog is a good choice.

Otherwise, there is some extra effort involved.

Prolog lacks modularity (e.g., modules, namespaces, etc.).

A Prolog program is typically a collection of flat (non-nested) definitions.