Sum of list elements sums| Prolog

Question

I'm trying to sum the results of list elements sum. I have this code:

adding_lists(List1, List2, Result) :- sumlist(list1, Result1), sumlist(list2, Result1), Result is Result1 + Result2.

but it returns false, what it's the way for this sum?

Answer 1

The code you posted should have given you singleton warnings when consulted on your prolog interpreter. Prolog is case sensitive. Furthermore, List1 , which starts with a upper case letter, is a variable and list1 , which starts with a lower case letter, is an atom. So in your code you should have been warned that List1 , List2 and Result2 are singleton variables (appear only once in the procedure).

Aside from that, you are issuing two calls to sumlist/2 (presumably for List1 and List2 and unifying the result with the same variable Result1 , whereas the second call should use Result2 ).

After fixing those issues you will end up with this code:

adding_lists(List1, List2, Result) :-
  sumlist(List1, Result1),
  sumlist(List2, Result2),
  Result is Result1 + Result2.

that should work fine.