Prolog-笛卡尔积计算器 [英] Prolog - Cartesian product calculator

问题描述

我需要为Prolog创建一个笛卡尔乘积计算器.它应该像这样工作:

I need to create a Cartesian product calculator for Prolog. It should work like this:

输入:product([1,2,3], [a,b], X).

输出:X = [[1,a],[2,a],[3,a],[1,b],[2,b],[3,b]].

我知道互联网上有一些例子，但是我想自己写点东西.

I know there are examples on the Internet, but I wanted to write something myself.

这是我的代码，我认为它已经很接近了，但是由于某种原因，它并不完全有效.大家有什么想法吗?

This is my code and I think it's pretty close, but for some reason it doesn't exactly work. Any ideas, guys?

% call new with 4 parameters (so we can keep List1 in memory)
product(L1,L2,L3):- product(L1,L2,L3,L1).

% stop when both List1 and List2 are empty
product([], [], [], []).

% first list is empty, recreate it and work it again with the next element of second list (and shorten memory)
product([], [_|T2], List3, [H4|T4]):-
    product([H4|T4], T2, List3, T4).

%go through first list and always first element of second list to our answer
product([H1|T1], [H2|T2], [[H1,H2]|T3], List4):-
    product(T1, [H2|T2], T3, List4).

推荐答案

正如Coder(+1)所说，您应该将terminal子句更改为

As said by Coder (+1), you should change the terminal clause from

product([], [], [], []).

到

product(_, [], [], _).

但是还不够.

您应该更改第三个子句

product([], [_|T2], List3, [H4|T4]):-
    product([H4|T4], T2, List3, T4).

到

product([], [_|T2], List3, L4):-
    product(L4, T2, List3, L4).

我的意思是:使用已保存列表1是一个错误.

I mean: is an error to consume the saved list 1.

使用您的版本，来自

product([1,2,3,4,5], [a,b,c,d], X),

你只能得到

[[1,a],[2,a],[3,a],[4,a],[5,a],[1,b],[2,b],[3,b],[4,b],[5,b],[2,c],[3,c],[4,c],[5,c],[3,d],[4,d],[5,d]]

也就是说，您松开[1,c]，[1,d]和[2,d].

That is: you loose [1,c], [1,d] and [2,d].

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