Adding atoms to lists prolog

1
votes

I am fairly new to Prolog and I am trying to get my head around the concept of lists. An example that I am trying is:

value(a, 1).
value(b, 2).
value(a, 3).
value(a, 4).
value(c, 3).

I am trying to create a predicate which when queried find(a, List) results in: List = [1, 3, 4].

It simply goes over all the facts and adds the matching ones to the list.

I've tried something like this. but it doesn't seem right at all:

find(X, List):-
  value(X, D), append([D], [], [List|Rest]), find(X, [Head|List]).

Sorry about such a basic question. Any guidance is appreciated.

Note: I intend to extend the program by making it add all the values in the list.

1
prolog
To sum the resulting list, you need only use the sumlist( List, Sum ) built-in predicate. - lurker

1 Answers

3
votes

Take a look here: http://www.swi-prolog.org/pldoc/doc_for?object=findall/3

I believe what your trying to do already exists, try this:

?- findall(X,value(a,X),List).

I don't have prolog installed on this machine so I can't perfectly test it, but that should get you what you're looking for.

Edit: Sorry managed to get into swi and realized i had 2 arguments reversed, see updated code now. Also tested it and it works :-)

Edit 2: As per the comment above, you could gather the values which match 'a' and get the sum you could combine into this:

?- findall(X, value(a,X), List), sumlist(List, Sum).

-Ken