% code_2009_04_15.pro
% Glenn G. Chappell
% 15 Apr 2009
%
% For CS 331 Spring 2009
% Sample Prolog code
% From the Wednesday, April 15, 2009 class meeting


% father(A, B) means A is the father of B.
father(glenn, nathaniel).
father(glenn, theresa).

% mother(A, B) means A is the mother of B.
mother(joanne, nathaniel).
mother(joanne, theresa).
mother(mary, joanne).

% grandmother(A, B) means A is the grandmother of B.
grandmother(A, B) :-
    mother(A, X),
    mother(X, B).
grandmother(A, B) :-
    mother(A, X),
    father(X, B).


% fact(A, B) means A! = B.
% fact(A, B) can find B, given A.
fact(0, 1).
fact(A, B) :-
    A > 0,
    X is A-1,
    fact(X, Y),
    B is A*Y.


% fibo(A, B) means F(A) = B, that is, the Ath Fibonacci number is B.
% fibo(A, B) can find B, given A.
fibo(0, 0).
fibo(1, 1).
fibo(A, B) :-
    A > 1,
    S is A-1,
    fibo(S, Sf),
    T is A-2,
    fibo(T, Tf),
    B is Sf + Tf.


% len(X, A) means X is a list, and its length is A.
% len(X, A) can find A, given X.
len([], 0).
len([_|T], A) :-
    len(T, B),
    A is B+1.