事实列表中所有实例的序言总和

Question

I have a list of facts. Each fact defines a relationship between two subjects and the number of projects they've completed. They're defined like this:

label(allGroups,[group(a,b,10),group(b,c,3),group(c,d,12)]).

I'm trying to write a function that will make a list of all the projects completed by an individual. For example, 'b' has completed a total of 13 projects while c has completed a total of 15 projects.

This is the function I've got going right now.

individualSum([],_,0).
individualSum([group(Name,_,Projects)|Tail],Name,Sum) :-
    individualSum(Tail,Name,Tailsum),
    Sum is Projects + Tailsum.
individualSum([group(_,Name,Projects)|Tail],Name,Sum) :-
    individualSum(Tail,Name,Tailsum),
    Sum is Projects + Tailsum.

I keep getting false and can't figure out if that's due to an incomplete basecase for the recursion or something else entirely. Here's what I'm running:

?- [groupSum].
?- label(allGroups,L),Groups=L).

(spits out allGroups, then:)

?- individualSum($Groups,b,Total).
false.

Any idea where I'm going wrong? I appreciate any help I can get.

Answer 1

You need to add another clause to skip groups here neither of the subjects is the one you are looking for:

individualSum([group(Name1,Name2,Projects)|Tail],Name,Sum):-
  Name \= Name1,
  Name \= Name2,
  individualSum(Tail,Name,Sum).

Answer 2

Gusbro already spotted the problem (+1). Since you're using SWI-Prolog, you could do with an aggregation builtin.

individualSum(L,Name,Sum) :-
    aggregate_all(sum(Projects),
       ( member(group(A,B,Projects), L), ( A = Name ; B = Name )), Sum).