% % Append lists: app(X,Y,Z) means that X append Y yields Z. The base case says % that anything appended to the empty list gives back the anything. The % second one says that, if T1 append X yeilds T2, then I can append A to the % front of T1 and T2 and maintain the relationship. More precisely, if T1 % append X yeilds T2, then [A|T1] append X yeilds [A|T2]. % app([],X,X). app([A|T1],X,[A|T2]) :- app(T1,X,T2). % % The length of the list: len(N, L) means N is the length of list L. The % basis rule says that the length of the empty list is 0. The recursive % rule says that, if N2 is the length of Tail, then N2 + 1 is the length of % Tail with one additional member added to the front. (Recall that _ is a % variable. The name _ is used when the value of a variable is not used % elsewhere.) % len(0,[]). len(N, [_ | Tail]) :- len(N2, Tail), N is N2 + 1. % % Take the sum of the members of a list of integers: sum_list(N, L) means that % N is the sum of the members of L. If SubTot is the sum of the members of % Tail, then Head + SubTot is the sum of the members of [Head | Tail]. sum_list(0, []). sum_list(Sum, [Head | Tail]) :- sum_list(SubTot, Tail), Sum is Head + SubTot. % % Reverse the list. rev([],[]). rev([H|T], R) :- rev(T, TR), app(TR, [H], R). % Tell if the list (second argument) contains the first argument. contains(X, [X|_]). contains(X, [_|T]) :- contains(X,T).