Prolog：找出所有列表元素的总和是否等于N.

Question

I want to check if all values in a list add up to some value. So far, I have written the following:

list_sum([Head|Tail], Sum) :-
    list_sum(Tail, Sum1),
    Sum is Head + Sum1.

However, when I try list_sum([1,2,3,4], 10) Prolog returns false . Could someone help me out? I don't know what I'm doing wrong.

Answer 1

Use clpfd !

:- use_module(library(clpfd)).

For calculating the sum of a list of integers simply use clpfd library predicate sum/3 :

?- sum([1,2,3,4], #=, S).                 % Q: What is 1+2+3+4?
S = 10.                                   % A: Sum is 10

?- sum([1,2,3,4], #=, 10).                % Q: Does 1+2+3+4 equal 10?
true.                                     % A: yes, it does

?- sum([1,2,3,4], #=, 11).                % Q: Does 1+2+3+4 equal 11?
false.                                    % A: no, it doesn't

A use case showing the benefits of using clpfd instead of (is)/2 , suggested by @CapelliC:

?- [A,B,C] ins 1..sup, sum([A,B,C,A], #=, 12), labeling([], [A,B,C]).
   A = B, B = 1, C = 9
;  A = 1, B = 2, C = 8
;  A = 1, B = 3, C = 7
;  A = 1, B = 4, C = 6
;  A = 1, B = C, C = 5
;  A = 1, B = 6, C = 4
;  A = 1, B = 7, C = 3
;  A = 1, B = 8, C = 2
;  A = C, B = 9, C = 1
;  A = 2, B = 1, C = 7
;  A = B, B = 2, C = 6
;  A = 2, B = 3, C = 5
;  A = 2, B = C, C = 4
;  A = 2, B = 5, C = 3
;  A = C, B = 6, C = 2
;  A = 2, B = 7, C = 1
;  A = 3, B = 1, C = 5
;  A = 3, B = 2, C = 4
;  A = B, B = C, C = 3
;  A = 3, B = 4, C = 2
;  A = 3, B = 5, C = 1
;  A = 4, B = 1, C = 3
;  A = 4, B = C, C = 2
;  A = 4, B = 3, C = 1
;  A = 5, B = C, C = 1.

Answer 2

It seems to me that this is rather simple.

Try this:

list_sum([], 0).
list_sum([Head|Tail], Sum):-
    list_sum(Tail, Sum1),
    Sum is Head + Sum1.