adding numbers

how can I add numbers define by facts?

eg.

n(1).
n(3).
n(5).
n(6).

add(N) :-
n(X),
N is N + X,
fail.


Does not seem to work...the answer I am looking for should be 1 + 3 + 5 + 6 
= 15.

Nelson Marcelino wrote:

> how can I add numbers define by facts?

Collect all answers into one list and then compute the sum of its elements:

n(1).
n(3).
n(5).
n(6).


list_sum(L,Sum) :-
list_sum(L,0,Sum).

list_sum([],S,S).
list_sum([E|Es],S1,S) :-
S2 is S1 + E,
list_sum(Es,S2,S).


thesum(Sum) :-
bagof(N,n(N),List),
list_sum(List,Sum).


Best regards,
Markus Triska.

Thanks for the reply Markus,

Just wondering is it possible to compute a summation using a recursive
predicate definition
without having to resort to the use of Lists?

Your help is greatly appreciated.

PS I'm finding prolog to be a better tool for many things than lisp,
or functional languages, such
as Ocaml, Haskell, SML, clean,etc.

Currently I am using SWI Prolog.
Can you recommend a good prolog for production grade code. SWI seems
like a very good product.
Best regards,

Nelson Marcelino

Markus Triska <triska@gmx.at> wrote in message news:<41b1cd63$0$11352$3b214f66@tunews.univi
e.ac.at>...
> Nelson Marcelino wrote:
> 
>
> Collect all answers into one list and then compute the sum of its elements
:
>
> n(1).
> n(3).
> n(5).
> n(6).
>
>
> list_sum(L,Sum) :-
> 	list_sum(L,0,Sum).
>
> list_sum([],S,S).
> list_sum([E|Es],S1,S) :-
> 	S2 is S1 + E,
> 	list_sum(Es,S2,S).
>
>
> thesum(Sum) :-
> 	bagof(N,n(N),List),
> 	list_sum(List,Sum).
>
>
> Best regards,
> Markus Triska.

Nelson Marcelino wrote:

>
> Just wondering is it possible to compute a summation using a recursive
> predicate definition
> without having to resort to the use of Lists?
>

Sure, it's only a matter of convenience. If for example you are using
trees to represent sums,:

treesum(nil,0).
treesum(tr(Val,Left,Right),Sum) :-
treesum(Left,L),
treesum(Right,R),
Sum is L + R + Val.

However, in your case I see no advantage in avoiding bagof/3. (You can
surely rewrite the predicate yourself using forced backtracking and such.)

> Currently I am using SWI Prolog.
> Can you recommend a good prolog for production grade code. SWI seems
> like a very good product.

I have no experience with commercial implementations, so I'm afraid I
can't help you with these (I heard that SICStus is very good).
Personally, I'm mainly using SWI Prolog and find it a pleasure to work with.

Best regards,
Markus.