Some list operation implemented in Prolog.

average.pl

/*This to call the outside function .. the inside function "average"
 calculate the result and give it back to the outside function
 "avg"
*/
avg(List,Ans):-
	Count is 0,
	Sum is 0 ,
	average(List,Count,Sum,Ans).

/* This is the most important line you need to handle base case well use trace command in SWI-PROLOG
to see what is happening in your calls */

average([],Count,Sum,Ans):-
	Ans is Sum/Count .


average([X|List],Count,Sum,Ans):-
	C is Count+1,
	S is Sum+X ,
	average(List,C,S,Ans).

intersection.pl

inter(List1,List2,In):-intersection(List1,List2,[],In).

intersection([],_,X,X).

intersection([H|List1],List2,X,In):-
	member(H,List2),
	intersection(List1,List2,[H|X],In).

intersection([H|List1],List2,X,In):-
	not(member(H,List2)),
	intersection(List1,List2,X,In).

maximum.pl

maxi([H|List],Ans):-max(List,H,Ans).

max([],H,H).

max([H|List],X,Ans):-
	H > X ,
	max(List,H,Ans).

max([H|List],X,Ans):-
	H < X,
	max(List,X,Ans).

reverse.pl

reverse([],X,X).
reverse(List,NList):-
	reverse(List,[],NList).

reverse([X|List],NList,Reversed):-
	reverse(List,[X|NList],Reversed).

union.pl

union(X,Y,Ans):-
	unique(X,List1),
	unique(Y,List2),
	unify(List1,List2,[],Ans).

unify([],List2,X,Ans):-
	append(List2,X,Ans).

unify([H|List1],List2,X,Ans):-
	member(H,List2),
	unify(List1,List2,X,Ans).

unify([H|List1],List2,X,Ans):-
	not(member(H,List2)),
	unify(List1,List2,[H|X],Ans).

/* another solution easy one */
/* union(X,Y,Ans) :-
	append(X,Y,L),
	unique(L,Ans).
*/



unique(List1,U):- uni(List1,[],U).

/*base case the remove from stack in recursion will empty the temp list so i need to save the output in something else .. here in this code is RET ->
the line says that Temp = Ret and empty list will be a true statement*/
uni([],X,X).

uni([H|Tail],Temp,Ret):-
	member(H,Tail),
	uni(Tail,Temp,Ret).
/*the procedure goes like this if the head of this list , is not in the tail
therefor this is a non duplicate element , add it with , */
uni([H|Tail],Temp,Ret):-
	not(member(H,Tail)),
	uni(Tail,[H|Temp],Ret).