在 Prolog 中查找每个 X 列表的程序 [英] Program to find every list of X in Prolog

问题描述

我开始学习 Prolog.该程序尝试获取给定元素的所有出现次数:

I am starting on learning Prolog. This program tries to get all occurrences of a given element:

occurences(_, [], Res):- Res is [].
occurences(X, [X|T], Res):- 
    occurences(X,T,TMP),
    Res is [X,TMP].
occurences(X, [_|T], Res):- occurences(X,T,Res).

但这里是错误:

?- occurences(a,[a,b,c,a],Res).
ERROR: is/2: Arithmetic: `[]/0' is not a function
^  Exception: (11) _G525 is [] ? creep
   Exception: (10) occurences(a, [], _G524) ? creep
   Exception: (9) occurences(a, [a], _G524) ? creep
   Exception: (8) occurences(a, [c, a], _G524) ? creep
   Exception: (7) occurences(a, [b, c, a], _G524) ? creep
   Exception: (6) occurences(a, [a, b, c, a], _G400) ? creep

推荐答案

除了其他人写的，考虑使用 diff/2 约束:

In addition to what others wrote, consider using the dif/2 constraint:

occurrences(_, [], []).
occurrences(X, [X|Ls], [X|Rest]) :-
        occurrences(X, Ls, Rest).
occurrences(X, [L|Ls], Rest) :-
        dif(X, L),
        occurrences(X, Ls, Rest).

您现在可以在所有方向上使用谓词，例如:

You can now use the predicate in all directions, for example:

?- occurrences(X, [a,a,b], Os).
X = a,
Os = [a, a] ;
X = b,
Os = [b] ;
Os = [],
dif(X, b),
dif(X, a),
dif(X, a) ;
false.

最后一个解决方案意味着如果 X 与 a 和 b 都不同，则出现列表为空.

The last solution means that the list of occurrences is empty if X is different from both a and b.

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