序言.在查询中，如何在结果中不想要的变量上放置条件? [英] Prolog. In a query, how to put a condition on a variable that I do not want in the results?

问题描述

想象一下，我有以下知识库，其中给出了每个人的名字和年龄.

Imagine that I have the following knowledge base which gives for each person his first name and his age.

person(mary, 39).
person(john, 24).
person(sandy, 17).

现在，我想检索所有 20 岁以上的人.此外，我只想收集他们的名字而不是他们的年龄.在这里，我想检索 mary 和 john.

Now, I want to retrieve all the persons that are older than 20 years. Furthermore, I just want to collect their first names and not their age. Here, I want to retrieve mary and john.

一般如何在 Prolog 中，更具体地说，在 SWI-Prolog 中如何做到这一点?

How to do this generally in Prolog and more specifically in SWI-Prolog?

如果我们使用一个非匿名的变量，比如:

If we use a variable which is not anonymous, like:

?- person(X, Y), Y > 20.

Prolog 会给我 X 和 Y 的值，我不需要 Y.

Prolog will give me the values for both X and Y and I do not want Y.

我不能使用匿名变量 _ 因为 Prolog 不能链接它的两个实例.以下给出了一个错误:

I cannot use the anonymous variable _ because Prolog cannot link its two instantiations. The following gives an error:

?- person(X, _), _ > 20.

那么，如何做到这一点?

So, how to do this?

推荐答案

为什么不定义谓词

ofintrest(X):- person(X,Y),Y>20.

查询

ofintrest(X).

如果你不想定义谓词，你也可以使用双重否定

If you don't want to define a predicate you could also use double negation

person(X,_) ,\+(\+ (person(X,Y), Y>20))

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