Here is a suggestion for the sum of the even numbers from a list:
even(X) :-
Y is mod(X,2), % using "is" to evaluate to number
Y =:= 0.
odd(X) :- % using even
Y is X + 1,
even(Y).
sum_even(0, []). % empty list has zero sum
sum_even(X, [H|T]) :-
even(H),
sum_even(Y, T),
X is Y+H.
sum_even(X, [H|T]) :-
odd(H),
sum_even(X, T). % ignore the odd numbers
Note: My Prolog has oxidized, so there might be better solutions. :-)
Note: Holy cow! There seems to be no Prolog support for syntax highlighting (see here), so I used Erlang syntax.
Ha, it really works. :-)
Running some queries in GNU Prolog, I get:
| ?- sum_even(X,[]).
X = 0 ?
yes
| ?- sum_even(X,[2]).
X = 2 ?
yes
| ?- sum_even(X,[3]).
X = 0 ?
yes
| ?- sum_even(X,[5,4,3,2,1,0]).
X = 6 ?
yes
The ideas applied here should enable you to come up with the needed product.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…