Prolog：试图使奇数甚至列表的特定要素

Question

我想知道列表中存在多少个偶数和奇数，直到用户输入的特定索引。

以下代码正常工作，但仅给出偶数或奇数元素。有人可以帮助我获得所需的输出吗？ 提前致谢。

:- use_module(library(clpfd)).
oddlength(L):-

    L9 is L+2,
    L6 is L9/2,
    write('List has '),
    write(L6),
    write(' Odd Element').
evenlength(L):-
    L6 is L/2,
    write('List has '),
    write(L6),
    write(' Even Element').
split(Index,List,Left,Right) :-
   length(Left,Index),       % Actually CREATES a list of fresh variables if "Left" is unbound
   append(Left,Right,List).  % Demand that Left + Right = List.
   write(List).
create(L1):-read(Elem),create(Elem,L1).
create(-1,[]):-!. create(Elem,[Elem|T]):-read(Nextel),create(Nextel,T).
chklst([H|T]):-
length(T,L),
 L>=0 ->
 (
  L1 is L+1,
  L2 is mod(L1,2),
  L2=:=0 ->
   evenlength(L1)
  ;
   oddlength(L)
 ). 
go:- write('Creating a list'),nl, write('Enter -1 to stop'),nl, create(L), nl, write('Enter index'),read(ID),split(ID,L,X1,X2),nl, chklst(X1).

Answer 1

似乎。 。 。复杂的。

首先，您需要的是测试整数是否奇怪甚至是：

even(N) :- 0 =:= N rem 2 .

一旦有了即可穿越列表的简单问题。

如果您想要的只是计数：

evens_and_odds( Ns , I, E, O ) :- evens_and_odds( Ns, 0, I, 0 , 0 , E, O ) .

evens_and_odds( []     , _ , _ , E , O , E , O ) .  % if we've exhausted the source list, we're done.
evens_and_adds( [N|Ns] , P , P , E , O , E , O ) .  % ditto, if we've hit the desired index.
evens_and_odds( [N|Ns] , P , Q , X , Y , E , O ) :- % otherwise....
  P < Q ,                                           % - we haven't yet hit the target index
  P1 is P+1,                                        % - tick the counter
  tally(N,X,Y,X1,Y1),                               % - do the tally
  evens_and_odds(Ns,P1,Q,X1,Y1,E,O)                 % - and recurse down
  .                                                 % Easy!

tally( N , E , O , E1 , O  ) :- even(N), !, E1 is E+1 .
tally( N , E , O , E  , O1 ) :-             O1 is O+1 .