Question

Write two clauses in PROLOG that determines if there are three values in a list that sum up to a value of N.

The output should be a single true if there exist three values whose sum is N or a single false if there are not.

If there are multiple values whose sum is N then the program should only print true once.

Answer 1

We just need to find that are there elements present in the list that add up to our sum

If the elements are present then we check if the size of that list is 3 if yes then our answer is true else false.

Code :

1 findthree (Sum, Answer, List) 2 Answer = L,-,-], findAny(Sun, Answer, List). % This is to ensure that our final answer only % contains three elements 4 % Base Case to ensure zero sum of elements 6 findAny (Sum, [HT1], [HT2]) :- 7 Currsum is Sum-H, % Take the current element in our answer and findAny(Curr sun,T1,T2). % Make a recursive call to find furthur. 9 findAny(Sun, Answer, LH T) % This will help skipping the elements % so that a sublist can be formed :. 11 dif(Answer,[I), 12 findAny (Sum,Answer,T)

Function Call :

findthree (12,Answer, [7,6,5,8,4,2])

Result :

Answer false [6, 4, 2]

Code :

findthree(Sum,Answer,List) :-
Answer = [_,_,_], % This is to ensure that our final answer only
findAny(Sum,Answer,List). % contains three elements

findAny(0,[],_L). % Base Case to ensure Zero sum of elements
findAny(Sum,[H|T1],[H|T2]) :-   
Currsum is Sum-H, % Take the current element in our answer and
findAny(Currsum,T1,T2).   % Make a recursive call to find furthur.

findAny(Sum,Answer,[_H|T]) :- % This will help skipping the elements
dif(Answer,[]), % so that a sublist can be formed
findAny(Sum,Answer,T).