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recursion - 如何在 PROLOG 中找到两个站点之间的路线

问题描述

抱歉，我知道这个问题出现了很多，但我做了很多研究，我就是不知道如何解决这个问题。我在 PROLOG 中表示了一个地铁地图，需要编写一个谓词来返回两个站点之间的所有路线。我知道我需要使用递归并尝试了很多不同的解决方案，但没有一个有效。我掌握的事实是：

station('AL',[metropolitan]).%Aldgate on the Metropolitan Line
station('BG',[central]).%Bethnal Green on the Central Line
station('BR',[victoria]).%Brixton on the Victoria Line
station('BS',[metropolitan]).%Baker Street on the Metropolitan Line
station('CL',[central]).%Chancery Lane on the Central Line
station('EC',[bakerloo]).%Elephant & Castle on the Bakerloo Line
station('EM',[bakerloo,northern]).%Embankment on the Bakerloo and Northern Lines
station('EU',[northern]).%Euston on the Northern Line
station('FP',[victoria]).%Finsbury Park on the Victoria Line
station('FR',[metropolitan]).%Finchley Road on the Metropolitan Line
station('KE',[northern]).%Kennington on the Northern Line
station('KX',[metropolitan,victoria]).%Kings Cross on the Metropolitan and Victoria Lines
station('LG',[central]).%Lancaster Gate on the Central Line
station('LS',[central,metropolitan]).%Liverpool Street on the Central and Metropolitan Lines
station('NH',[central]).%Notting Hill Gate on the Central Line
station('OC',[bakerloo,central,victoria]).%Oxford Circus on the Bakerloo, Central and Victoria Lines
station('PA',[bakerloo]).%Paddington on the Bakerloo Line
station('TC',[central,northern]).%Tottenham Court Road on the Central and Northern Lines
station('VI',[victoria]).%Victoria on the Victoria Line
station('WA',[bakerloo]).%Warwick Avenue on the Bakerloo Line
station('WS',[northern,victoria]).%Warren Street on the Northern and Victoria Lines
adjacent('WA','PA').%Warwick Avenue is adjacent to Paddington
adjacent('PA','OC').%Paddington is adjacent to Oxford Circus
adjacent('OC','EM').%Oxford Circus is adjacent to Embankment
adjacent('EM','EC').%Embankment is adjacent to Elephant & Castle
adjacent('NH','LG').%Notting Hill Gate is adjacent to Lancaster Gate
adjacent('LG','OC').%Lancaster Gate is adjacent to Oxford Circus
adjacent('OC','TC').%Oxford Circus is adjacent to Tottenham Court Road
adjacent('TC','CL').%Tottenham Court Road is adjacent to Chancery Lane
adjacent('CL','LS').%Chancery Lane is adjacent to Lviverpool Street
adjacent('LS','BG').%Liverpool Street is adjacent to Bethnal Green
adjacent('FR','BS').%Finchley Road is adjacent to Baker Street
adjacent('BS','KX').%Baker Street is adjacent to Kings Cross
adjacent('KX','LS').%Kings Cross is adjacent to Liverpool Street
adjacent('LS','AL').%Liverpool Street is adjacent to Algate
adjacent('EU','WS').%Euston is adjacent Warren Street
adjacent('WS','TC').%Warren Street is adjacent to Tottenham Court Road
adjacent('TC','EM').%Tottenham Court Road is adjacent to Embankment
adjacent('EM','KE').%Embankment is adjacent to Kennington
adjacent('BR','VI').%Brixton is adjacent to Victoria
adjacent('VI','OC').%Victoria is adjacent to Oxford Circus
adjacent('OC','WS').%Oxford Circus is adjacent to Warren Street
adjacent('WS','KX').%Warrent Street is adjacent to Kings Cross
adjacent('KX','FP').%Kings Cross is adjacent to Finsbury Park

我尝试过的解决方案是：

route(From,To,Route) :-
   routeattempt(From,To,[From],Route),
   reverse(Route,route).

routeattempt(From,To,Inbetween,Route) :-
   adjacent(From,To),
   \+member(From,Inbetween),
   Route = [From|Inbetween].
routeattempt(From,To,Visited,Route) :-
   adjacent(From,Inbetween),
   Inbetween \== To,
   \+member(Inbetween,Visited),
   routeattempt(Inbetween,To,Inbetween|Visited],Route).

但它只会对任何输入返回 false。如果有人可以提供帮助，那就太好了。

标签： recursionroutesprologpredicatetransitive-closure

解决方案

这很困惑。

在

route(From,To,Route) :-
   routeattempt(From,To,[From],Route),
   reverse(Route,route).

这个想法显然是routeattempt/4为了“找到一条从From到的路线，To将其存储为 中的站点列表Route。但是，这里的参数 3[From]到底是做什么的？它似乎是访问过的站点列表，但你真的需要它吗？你已经Route。

现在你必须子案例：要么From或To相邻：

routeattempt(From,To,Inbetween,Route) :-
   adjacent(From,To),
   \+member(From,Inbetween),
   Route = [From|Inbetween].

或者它们不是：

routeattempt(From,To,Visited,Route) :-
   adjacent(From,Inbetween),
   Inbetween \== To,
   \+member(Inbetween,Visited),
   routeattempt(Inbetween,To,Inbetween|Visited],Route).

在第一种情况（基本情况）中，不清楚为什么要检查是否From是Inbetween. 实际上，这将排除在两个相邻站之间找到任何路线，因为route/3已经存入From列表中Inbetween。两个相邻车站之间的路线不应该只是[From,To]代替[From|Inbetween]吗？

在第二种情况下，你从From到Toby waystation Inbetween，这也不等于To，并且还没有被访问过。好的。但是你需要Route在递归调用之后完成。

尝试在format("Current route from ~w to ~w: ~w\n", [From, To, Route])前面添加一个adjacent/2，看看会发生什么。

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