Prolog Examples

Prolog programs are shown indented and in bold, user input is shown in bold red. The rest is Prolog output

How to quit Prolog :)

Family tree basic example

% A number of facts stored in file 'facts.pro'
parent(kim,holly).
parent(margaret,kim).
parent(herbert,margaret).
parent(john,kim).
parent(felix,john).
parent(albert,felix).

The grandparent and greatgrandparent rule

% This is added at the end of file 'facts.pro'.

% grandparent rule
grandparent(GP,GC) :-
       parent(GP,P), parent(P,GC).

% greatgrandparent rule
greatgrandparent(GGP,GGC) :-
       parent(GGP,GP), parent(GP,P), parent(P,GC).

The recursive ancestor rule

% ancestor rule base case
ancestor(X,Y) :-
       parent(X,Y).

% ancestor rule recursion
ancestor(X,Y) :-
       parent(Z,Y), ancestor(X,Z).

The three versions of the sibling rule

% This rule does not work because sibling(kim,kim) is true!
sibling(X,Y) :-
       parent(Z,X), parent(Z,Y).


% This rule does not work because the first goal will always fail!
sibling(X,Y) :-
       not(X=Y), parent(Z,X), parent(Z,Y).


% This rule works
sibling(X,Y) :-
       parent(Z,X), parent(Z,Y), not(X=Y).

The append rule

% base case
append([], B, B).

% recursive case
append([ Head | TailA ], B, [ Head | TailC ]) :-
       append(TailA, B, TailC).

Numerical Computation

The list length rule

% base case
mylength([], 0).

% recursive case
mylength([ _ | Tail ], Len) :-
       mylength(Tail, TailLen),
       Len is TailLen + 1.

Towers of Hanoi

% The goal is to move disks from one peg to another, using only one % extra peg, while never putting a larger disk on a smaller disk. Initially % all disks are piled on one single peg. % % Basic move, just to print out what happens
move(A,B) :-
       nl, write('Move top disk from '),
       write(A), write(' to '), write(B).

% The rule transfer(N,A,B,X) is designed to move N disks from peg A to peg B using peg X

% transfer rule base case
transfer(1,A,B,X) :-
       move(A,B).

% transfer rule recursive case
transfer(N,A,B,X) :-
       M is N-1,
       transfer(M,A,X,B),
       move(A,B),
       transfer(M,X,B,A).

The Farmer, Goat, Cabbage, etc.