谓词解压缩列表 [英] Predicate to unzip a list

问题描述

List1=[(x,1),(y,1),(z,1)]

我正在尝试拆分此列表:

I'm attempting to split this list:

分为两个列表:

List3=[x,y,z]
List4=[1,1,1]

所以我写了这个谓词来尝试做到这一点:

So I have written this predicate to try to do it:

splt([], [], []).
splt([X|Xs], [Y|Ys], [X,Y|Zs]) :-
    splt(Xs,Ys,Zs).

但是，谓词返回的不是预期的结果:

However instead of the desired result, the predicate returns:

1 ?- splt([(x,1),(y,2),(z,3)],L3,L4).
L3 = [_G1760, _G1769, _G1778],
L4 = [ (z, 1), _G1760, (y, 2), _G1769, (z, 3), _G1778].

推荐答案

首先，您选择的术语.这:(a, b)，绝对不是，您通常会在Prolog中表示元组".您几乎总是将a-b用作对"，并且在整个标准库中都使用了对.

First, the term you have chosen. This: (a, b), is most definitely not how you would usually represent a "tuple" in Prolog. You almost always use a-b for a "pair", and pairs are used throughout the standard libraries.

因此您的初始列表如下所示:[x-1, y-1, z-1].

So your initial list would look like this: [x-1, y-1, z-1].

这也应该解释为什么您遇到问题.您编写了(a, b)，但是您的谓词为a, b，并且当您希望获得一个,(a,b)术语时会消耗两个元素.因此，要修正您当前的谓词，您可以编写:

This should also explain why you are having your problem. You write (a, b), but your predicate says a, b, and you consume two elements when you expect to get one ,(a,b) term. So, to fix your current predicate you would write:

split([], [], []).
split([X|Xs], [Y|Ys], [(X,Y)|XYs]) :-
    split(Xs, Ys, XYs).

?- split(Xs, Ys, [(x,1), (y,1), (z,1)]).
Xs = [x, y, z],
Ys = [1, 1, 1].

但是，使用更常规的名称，术语顺序和Prolog对:

But instead, using a more conventional name, term order, and Prolog pairs:

zip([], [], []).
zip([X-Y|XYs], [X|Xs], [Y|Ys]) :-
    zip(XYs, Xs, Ys).

?- zip([x-1, y-1, z-1], Xs, Ys).
Xs = [x, y, z],
Ys = [1, 1, 1].

当然，SWI-Prolog至少具有库(对)，并且它带有pairs_keys_values/3:

And of course, SWI-Prolog at least has a library(pairs), and it comes with a pairs_keys_values/3:

?- pairs_keys_values([x-1, y-1, z-1], Xs, Ys).
Xs = [x, y, z],
Ys = [1, 1, 1].

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