%--------
% some more recursive examples...
% last modified: 03-29-11
%--------

%--------
% from the clicker question...

% (note, though -- sumlist is a predicate for which the first
%    parameter must be instantiated at the time of the call...)

sumlist([], 0).
sumlist([H|T], Sum) :- sumlist(T, TSum), 
                       Sum is H + TSum.

%--------
/* rev_countlist(A, B) is true if B is the list of A .. 1) */

rev_countlist(0, []).
rev_countlist(Num, [Num|Rest]) :- 
    Num > 0,                                 
    Next is Num-1,                                
    rev_countlist(Next, Rest).

%--------
% adapted from Prolog tutorial posted at
% http://www.csupomona.edu/~jrfisher/www/prolog_tutorial/pt_framer.html

factorial(0,1).
  
factorial(Num,NumFactorial) :-  
           Num > 0, 
           PrevNum is Num-1,
           factorial(PrevNum, PrevNumFactorial),
           NumFactorial is Num*PrevNumFactorial. 

%--------
% adapted from:
% http://www.cse.unsw.edu.au/~billw/cs9414/notes/write-recursive-proc.html

% lastitem(List, Last)
% Binds Last to the item at the end of List.
% List must be instantiated at the time of call, and must not be empty.
% Example of use:
% ?- lastitem([a,b,c,d], X).
% X = d

%% base case: list with just one item.

lastitem([OnlyOne], OnlyOne).

%% recursive case: ignore first item, seek last item of rest of list

lastitem([First|Rest], Last) :-
    lastitem(Rest, Last).