Prolog，带有条件子句的构建列表 [英] Prolog, building list with conditional clauses

问题描述

我需要使用 prolog(SWI 风格)来完成这项家庭作业，并且无法理解某些事情.

I need to do this homework assignment using prolog (SWI-flavor) and cant get my head around some things.

例如，如果我想遍历一个列表并将其元素添加到另一个列表中，但前提是它们满足特定条件，我将如何处理它?我可以将它们全部添加，也可以不添加，但是如果我添加检查此条件的子句，则整个递归结果为假".我明白为什么会这样，但不知道如何解决它.基本上我想要的是:

For example, if i want to iterate through a list and add its elements to another, but ONLY if they meet certain condition, how would I go about it? I can add them all, or none, but if I add clause that checks this condition, the whole recursion turns out as "false". I understand why this is, but have no idea how to fix it. Basically what i want is:

goal(Stuff) :- do_something(X),
               only_do_this_if_something(Y),
               always_do_this(Z).

目前，如果 only_do_this_if_something(Y) 失败，always_do_this(Z) 也不会发生，因为整个目标都变为 false...

Currently, if only_do_this_if_something(Y) fails, also always_do_this(Z) doesnt happen as the whole goal turns false...

推荐答案

可以使用if结构:

<condition> -> (do_something) ; (do_something else)

在这种情况下:

goal(Stuff):-
  do_something(X),
  if_something(Y)-> do_this(Y) ; true,
  always_do_this(Z).

或者您只需编写两个子句，例如:

or you simply write two clauses like:

goal(Stuff):-
  do_something(X),
  conditional_stuff(Y),
  always_do_this(Z).

conditional_stuff(Y):-
  condition(Y),
  do_this(Y).

conditional_stuff(_).

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