Some free Prolog tutorials
- Prolog tutorial by J.R. Fisher
- Prolog tutorial by A. Aaby
- A short prolog tutorial
- Learn Prolog Now!
- Many more (just google it)
A support forum
One online reference for the Prolog version you're using GNU-Prolog Manual
Make a simple knowledge base. Represent some of your favorite books and authors.
wrote(charles_dickens, a_tale_of_two_cities).
wrote(j_r_r_tolkien, the_hobbit).
wrote(j_r_r_tolkien, the_lord_of_the_rings).
wrote(dan_brown, the_da_vinci_code).
wrote(dan_brown, angels_and_demons).
wrote(e_b_white, charlottes_web).
wrote(j_k_rowling, harry_potter).
% Find all books in your knowledge base written by one author.
wrote(dan_brown, What).
% Make a knowledge base representing musicians and instruments.
% Also represent musicians and their genre of music.
plays(bob_marley, guitar).
plays(kurt_cobain, guitar).
plays(eddie_van_halen, guitar).
plays(stevie_wonder, piano).
plays(ludwig_van_beethoven, piano).
plays(frederic_chopin, piano).
plays(franz_liszt, piano).
plays(ringo_starr, drums).
plays(travis_barker, drums).
plays(kesha, autotune).
genre(bob_marley, reggae).
genre(kurt_cobain, rock).
genre(eddie_van_halen, rock).
genre(stevie_wonder, soul).
genre(ludwig_van_beethoven, classical).
genre(frederic_chopin, classical).
genre(franz_liszt, classical).
genre(ringo_starr, rock).
genre(travis_barker, rock).
genre(kesha, pop).
% Find all musicians who play the guitar.
plays(Who, guitar).
% Bonus: Find all genres where guitar is played.
plays(X, guitar), genre(X, Genre).fib(0, 0).
fib(1, 1).
fib(F, N) :-
N1 is N - 1, % Alternatively, we could use succ(N1, N)
N2 is N - 2, % similarly succ(N2, N1); succ(A, B) means B = A + 1
fib(F1, N1),
fib(F2, N2),
F is F1 + F2. % Alternatively plus(F1,F2,F)
% Support negatives by adding following rule, and call it fib2
% Remember: F(-n) = (-1)^(n+1) * F(n)
fib2(F, N) :-
AN is abs(N), % Find AN = abs(N)
fib(PF, AN), % calculate F(abs(n)) as PF
F is PF * (sign(N) ** (N+1)). % Use sign predicate to determine 1 or -1 from N
% and then raise to (N+1)%% Recursion
fib(1, N) :- N < 3.
fib(V, N) :- N > 2, fib(V1, N - 2), fib(V2, N - 1), V is V1 + V2.
%% Tail recursion
fibTail(B, N, _, B) :- N < 3.
fibTail(V, N, A, B) :- N > 2, Sum is A + B, fibTail(V, N - 1, B, Sum).factorial(1, 0). % 0! = 1
factorial(F, N) :-
succ(N1, N),
factorial(F1, N1),
F is F1 * N.%% Recursion
factorial(1, 0).
%% factorial(V, N) :- N > 0, N2 is N - 1, factorial(V2, N2), V is N * V2.
factorial(V, N) :- N > 0, succ(N2, N), factorial(V2, N2), V is N * V2.
%% Tail Recursion
factorialTail(V, 0, V).
factorialTail(V, N, Init) :- N > 0, N2 is N - 1, Accum is N * Init, factorialTail(V, N2, Accum).A real-world community using Prolog. What problems are they solving with it today?
Reverse the elements of a list
% note that reverse/2 is already taken
revl([], []).
revl(R, [H|T]) :- revl(RT, T), append(RT, [H], R).Find the smallest element of a list
% Find the smallest element of a list, tail-recursive; note that min_list/2 is already taken
min(Min, X, Y) :- % finds the min of two values
X < Y, Min is X;
X >= Y, Min is Y.
% Beware that min_list exists already
minl(Min, [H|T]) :- minl(Min, T, H). % default min is head
minl(Min, [], Min). % Base case is empty list, return calculated min
minl(Min, [H|T], X) :- min(CurrMin, H, X), minl(Min, T, CurrMin).Alternative:
%% Find the smallest element of a list.
smallest(H, [H]).
smallest(Small, [Head|Tail]) :- smallest(Small2, Tail), Small2 < Head, Small is Small2.
smallest(Small, [Head|Tail]) :- smallest(Small2, Tail), Head =< Small2, Small is Head.Sort the elements of a list
% Let's use selection sort since we've already built minl
takeout(RetList, X, List) :- list(List), takeout(RetList, X, List, []).
takeout(RetList, H, [H|T], Past) :- append(Past, T, RetList).
takeout(RetList, X, [H|T], Past) :-
append(Past, [H], P),
takeout(RetList, X, T, P).
sortl(Sorted, List) :- list(List), sortl(Sorted, List, []).
sortl(Sorted, [], Sorted).
sortl(Sorted, List, Partial) :-
minl(Min, List),
takeout(Rest, Min, List),
append(Partial, [Min], NewList),
sortl(Sorted, Rest, NewList).
% sortl is O(n^2); it is also stableAlternative:
without([], _, []).
without(T, H, [H|T]).
without([H|T2], Item, [H|T]) :- \+(H = Item), without(T2, Item, T).
sortList([H], [H]).
sortList([H2|T2], L) :- smallest(H2, L), without(L2, H2, L), sortList(T2, L2).Prolog has some input/output features as well. Find print predicates that print out variables. Wiki Books Input/Output
Reading a File:
process(File) :-
open(File, read, In),
get_char(In, Char1),
process_stream(Char1, In),
close(In).
process_stream(end_of_file, _) :- !.
process_stream(Char, In) :-
print(Char),
get_char(In, Char2),
process_stream(Char2, In).Print variable:
| ?- X = 2, write('The value is '), write(X), write('.').
The value is 2.
X = 2 ?
yesFind a way to use the print predicates to print only successful solutions. How do they work?
- Modify the Sudoku solver to work on six-by-six puzzles (squares are 3x2) and 9x9 puzzles.
- Make the Sudoku solver print prettier solutions.
%% Replace _ variable so they can be printed properly
replaceUnderscores([], []).
replaceUnderscores([H|L], [H2|L2]) :-
(\+integer(H) -> H2 = (-) ; H2 = H),
replaceUnderscores(L, L2).
%% Format output
formatSudoku([]).
formatSudoku(Puzzle) :- formatSudoku(0, Puzzle).
formatSudoku(0, S) :-
write('┌───────┬───────┬───────┐'), nl,
formatSudoku(1, S).
formatSudoku(4, S) :-
write('│───────┼───────┼───────│'), nl,
formatSudoku(5, S).
formatSudoku(8, S) :-
write('│───────┼───────┼───────│'), nl,
formatSudoku(9, S).
formatSudoku(12, _) :-
write('└───────┴───────┴───────┘'), nl.
formatSudoku(N, [C1, C2, C3, C4, C5, C6, C7, C8, C9 | S]) :-
\+ member(N, [0, 4, 8, 12]),
format(
'│ ~k ~k ~k │ ~k ~k ~k │ ~k ~k ~k │~n',
[C1, C2, C3, C4, C5, C6, C7, C8, C9]
),
succ(N, N1),
formatSudoku(N1, S).
%% Validator for each list
valid([]).
valid([Head|Tail]) :-
fd_all_different(Head),
valid(Tail).
sudoku(Puzzle) :-
write('Input:'), nl,
replaceUnderscores(Puzzle, FormattedInput),
formatSudoku(FormattedInput),
Solution = Puzzle,
Puzzle = [
S11, S12, S13, S14, S15, S16, S17, S18, S19,
S21, S22, S23, S24, S25, S26, S27, S28, S29,
S31, S32, S33, S34, S35, S36, S37, S38, S39,
S41, S42, S43, S44, S45, S46, S47, S48, S49,
S51, S52, S53, S54, S55, S56, S57, S58, S59,
S61, S62, S63, S64, S65, S66, S67, S68, S69,
S71, S72, S73, S74, S75, S76, S77, S78, S79,
S81, S82, S83, S84, S85, S86, S87, S88, S89,
S91, S92, S93, S94, S95, S96, S97, S98, S99
],
fd_domain(Solution, 1, 9),
R1 = [S11, S12, S13, S14, S15, S16, S17, S18, S19],
R2 = [S21, S22, S23, S24, S25, S26, S27, S28, S29],
R3 = [S31, S32, S33, S34, S35, S36, S37, S38, S39],
R4 = [S41, S42, S43, S44, S45, S46, S47, S48, S49],
R5 = [S51, S52, S53, S54, S55, S56, S57, S58, S59],
R6 = [S61, S62, S63, S64, S65, S66, S67, S68, S69],
R7 = [S71, S72, S73, S74, S75, S76, S77, S78, S79],
R8 = [S81, S82, S83, S84, S85, S86, S87, S88, S89],
R9 = [S91, S92, S93, S94, S95, S96, S97, S98, S99],
C1 = [S11, S21, S31, S41, S51, S61, S71, S81, S91],
C2 = [S12, S22, S32, S42, S52, S62, S72, S82, S92],
C3 = [S13, S23, S33, S43, S53, S63, S73, S83, S93],
C4 = [S14, S24, S34, S44, S54, S64, S74, S84, S94],
C5 = [S15, S25, S35, S45, S55, S65, S75, S85, S95],
C6 = [S16, S26, S36, S46, S56, S66, S76, S86, S96],
C7 = [S17, S27, S37, S47, S57, S67, S77, S87, S97],
C8 = [S18, S28, S38, S48, S58, S68, S78, S88, S98],
C9 = [S19, S29, S39, S49, S59, S69, S79, S89, S99],
Sq1 = [S11, S12, S13, S21, S22, S23, S31, S32, S33],
Sq2 = [S14, S15, S16, S24, S25, S26, S34, S35, S36],
Sq3 = [S17, S18, S19, S27, S28, S29, S37, S38, S39],
Sq4 = [S41, S42, S43, S51, S52, S53, S61, S62, S63],
Sq5 = [S44, S45, S46, S54, S55, S56, S64, S65, S66],
Sq6 = [S47, S48, S49, S57, S58, S59, S67, S68, S69],
Sq7 = [S71, S72, S73, S81, S82, S83, S91, S92, S93],
Sq8 = [S74, S75, S76, S84, S85, S86, S94, S95, S96],
Sq9 = [S77, S78, S79, S87, S88, S89, S97, S98, S99],
valid([
R1, R2, R3, R4, R5, R6, R7, R8, R9,
C1, C2, C3, C4, C5, C6, C7, C8, C9,
Sq1, Sq2, Sq3, Sq4, Sq5, Sq6, Sq7, Sq8, Sq9
]),
write('Solution:'), nl,
formatSudoku(Solution),
fail.