序言：查找偶数将它们添加到列表中

Question

Write predicate evenNumbers(L1, L2) which is true if the list L1 containing random integers and the list L2 contains even integers from L1 .

For example:

?-evenNumbers ([2,1,-3,6,8,9], L2). 

»Your program returns L2 = [2,6,8].

My code is:

    evenNumbers([],[]).
    evenNumbers([H|T],L):-
        integer(H),
        0 is H mod 2,
        append([H],L,L);
        evenNumbers(T,L).

Answer 1

Your code has multiple issues append([H],L,L); will stop recursion and give you a wrong list also your if-then-else statement isn't right .So you could write:

 evenNumbers([],[]).
 evenNumbers([H|T],L1):-
        integer(H),
        (H mod 2 =:=0 -> L1=[H|T1],evenNumbers(T,T1);
        evenNumbers(T,L1) ).

Example:

?- evenNumbers([2,1,-3,6,8,9], L2).
L2 = [2, 6, 8].

Another way writing it would be:

evenNumbers(L1,L2):-findall(X,(member(X,L1), X mod 2=:=0),L2).