Prolog计数列表元素高于n

Question

我是Prolog的新手,所以我在某项任务中遇到了一些问题.任务是编写尾递归谓词count_elems(List,N,Count)条件List_Element > N, Count1 is Count+1.

我的方法:

count_elems( L, N, Count ) :-
   count_elems(L,N,0).
count_elems( [H|T], N, Count ) :-
   H > N ,
   Count1 is Count+1 ,
   count_elems(T,N,Count1).
count_elems( [H|T], N, Count ) :-
   count_elems(T,N,Count).

错误信息:

ERROR: toplevel: Undefined procedure: count_elems/3 (DWIM could not correct goal)

我不太清楚问题出在哪里.thx任何帮助:)

Answer 1

如果你想编写一个尾递归版本的代码,你需要(如CapelliC指出的那样)一个额外的参数来充当累加器.您可以在第一个子句中看到问题:

count_elems(L, N, Count) :- count_elems(L,N,0).

这里Count是一个单例变量,没有在任何地方实例化.您对start count_elems开始的递归调用计数0,但不再需要使用total实例化变量.所以,你需要:

count_elems(L, N, Count) :-
    count_elems(L, N, 0, Count).

然后声明count_elem/4条款:

count_elems([H|T], N, Acc, Count) :-
    H > N,                            % count this element if it's > N
    Acc1 is Acc + 1,                  % increment the accumulator
    count_elems(T, N, Acc1, Count).   % check the rest of the list
count_elems([H|T], N, Acc, Count) :-
    H =< N,                           % don't count this element if it's <= N
    count_elems(T, N, Acc, Count).    % check rest of list (w/out incrementing acc)
count_elems([], _, Count, Count).     % At the end, instantiate total with accumulator

您还可以使用"if-else"结构count_elems/4:

count_elems([H|T], N, Acc, Count) :-
    (H > N
    ->  Acc1 is Acc + 1
    ;   Acc1 = Acc
    ),
    count_elems(T, N, Acc1, Count).
count_elems([], _, Count, Count).

同样正如CapelliC指出的那样,您声明的错误消息可能是因为您没有阅读prolog源文件.