Prolog findall实施 [英] Prolog findall Implementation

问题描述

我受命在Prolog中实现findall的版本，除了not和cut之外，不使用任何Prolog内置组件-因此基本上是在纯Prolog中.

我正在尝试搜索所有直系后代的树并将结果返回列表中

parent(a, b).
parent(b, c).
parent(b, d).
parent(e, d).

到目前为止，我有:

find(X, L) :- find2(X, [], L).
find2(X, Acc, L) :- parent(Y, X), find2(Y, [Y|Acc], L).
find2(_, Acc, Acc).

例如，当我输入时我想要得到什么:

find(a,X).

将是:

X = [b, c, d]

(顺序不重要)

但是我却得到了:

X = [b, c] ;
X = [b, d] ;
X = [b] ;
X = [].

我是Prolog的新手，所以对此的任何帮助将不胜感激.

谢谢

解决方案

感谢您对所有人的帮助.最后，我设法通过添加一个谓词来对它进行排序，该谓词对照当前列表检查了每个项目，但是如果该项目已经存在，则会失败:

find(X, Loa) :- find(X, [], Loa), !.
find(X, Acc, Loa) :- dec(Y, X), uList(Y, Acc, AccNew), find(X, AccNew, Loa).
find(_, Acc, Acc).

dec(X,Y) :- parent(X,Y).
dec(X,Y) :- parent(X,Z), dec(Z,Y).

uList(X, [], [X])  :- !.
uList(H, [H|_], _) :- !, fail.
uList(X, [H|T], L) :- uList(X, T, Rtn), L = [H|Rtn].

I've been tasked to implement a version of findall in Prolog without using any Prolog built-ins except for not and cut - so basically in pure Prolog.

I'm trying to search a tree for all direct descendants and return the results in a list

parent(a, b).
parent(b, c).
parent(b, d).
parent(e, d).

What I have so far is:

find(X, L) :- find2(X, [], L).
find2(X, Acc, L) :- parent(Y, X), find2(Y, [Y|Acc], L).
find2(_, Acc, Acc).

What I want to be getting when I enter for example:

find(a,X).

would be:

X = [b, c, d]

(Order not important)

However instead I am getting:

X = [b, c] ;
X = [b, d] ;
X = [b] ;
X = [].

I'm new to Prolog so any help on this would be much appreciated.

Thanks

解决方案

Thanks for you help everyone. I managed to sovle it in the end by adding a predicate which checked each item against the current list, and failed if it was already present:

find(X, Loa) :- find(X, [], Loa), !.
find(X, Acc, Loa) :- dec(Y, X), uList(Y, Acc, AccNew), find(X, AccNew, Loa).
find(_, Acc, Acc).

dec(X,Y) :- parent(X,Y).
dec(X,Y) :- parent(X,Z), dec(Z,Y).

uList(X, [], [X])  :- !.
uList(H, [H|_], _) :- !, fail.
uList(X, [H|T], L) :- uList(X, T, Rtn), L = [H|Rtn].

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