使用Prolog中的printlist / 1谓词对列表中的所有数字求和

Question

I am trying to use the printlist/1 predicate to sum up all the numbers in a list but kind of got stuck.... tried to come up with the code for this but I keep getting false .

Here is what I've come up with:

printlist([]).
printlist([H|T], Totalsum) :- 
   print (H+Totalsum),
   nl,
   printlist(T, Totalsum).

I know it's wrong and it's probably the last part. Any help is appreciated!

I query it this way:

?- printlist([1,2,3]).
false.

Answer 1

As Paulo already said, you are defining two predicates here, which is incorrect.

Here is the solution:

printlist([], 0).
printlist([H|T], Sum) :-
   printlist(T, Subsum),
   Sum is Subsum + H.

Sample query:

?- printlist([1,2,3,5], L).
L = 11.

@ Paulo requested), a tail recursive version:

printlist(L, Sum) :-
   sumac(L, 0, Sum).

sumac([], Acc, Acc).
sumac([H|T], Acc, Sum) :-
   Nacc is Acc + H,
   sumac(T, Nacc, Sum). 

Answer 2

You're defining not one but two predicates: printlist/1 and printlist/2 . Likely a typo. The printlist/2 predicate is a recursive rule but there isn't a base case. Change the printlist/1 clause to:

printlist([], _).

But there are other errors in your code. Hint: Prolog is not a functional language.