Prolog中的递归-寻找城市之间的路径 [英] Recursion in Prolog - Finding Path Between Cities

问题描述

我正在尝试通过

I'm trying to work my way through the exercises at the bottom of this page and I find myself utterly confused on number 3.

我们将获得以下旅行信息知识库:

We are given the following knowledge base of travel information:

byCar(auckland, hamilton). 
byCar(hamilton, raglan). 
byCar(valmont, saarbruecken). 
byCar(valmont, metz). 

byTrain(metz, frankfurt). 
byTrain(saarbruecken, frankfurt). 
byTrain(metz, paris). 
byTrain(saarbruecken, paris). 

byPlane(frankfurt, bangkok). 
byPlane(frankfurt, singapore). 
byPlane(paris, losAngeles). 
byPlane(bangkok, auckland). 
byPlane(singapore, auckland). 
byPlane(losAngeles, auckland).

很容易找出是否有可能在两个城市之间旅行.我就是这样做的:

It's simple to find out if it's possible to travel between two cities. I just did this:

connected(X, Y) :- byCar(X, Y); byTrain(X, Y); byPlane(X, Y).
travel(X, Y) :- connected(X, Y).
travel(X, Z) :- connected(Y, Z), travel(X, Y).

但是，当我实际上必须使用变量统一路径时，我完全感到困惑！

However, when I have to actually unify the path with a variable, I am utterly confused!

我写了这个:

connected(X, Y) :- byCar(X, Y); byTrain(X, Y); byPlane(X, Y).
connected(Y, Z, Out) :- connected(Y, Z).
travel(X, Y, Out) :- connected(X, Y).
travel(A, Z, Out) :- connected(Y, Z),travel(A, Y, connected(Y, Z, Out)).

并称为travel(valmont, losAngeles,X).

在跟踪过程中，除了结尾处的匿名变量之外，还有一点显示正确的路径:

There is a point during the trace where the correct path shows up, aside from the anonymous variable at the end:

travel(valmont, metz, connected(metz, paris, connected(paris, losAngeles, _17)))

但是我实际上不知道如何用变量X统一它！

but I don't actually know how to unify this with the variable X!

我实在无法解决这个问题.谁能给我一个提示，将我推向正确的方向?是否只是我缺少的终止条件?

I can't really wrap my mind around this. Can anyone give me a hint just to push me in the right direction? Is there just a termination condition I'm missing or something?

现在我有

connected(X,Y) :- byCar(X,Y);byTrain(X,Y);byPlane(X,Y).

go(X,Y) :- connected(X,Y).

travel(X,Y,go(X,Y)) :- connected(X,Y).
travel(A,Z,Path) :- travel(Y,Z,Path),go(A,Y,Path).

go(A,Y,Path) :- travel(A,Y,Path).

但是它像这样被卡住了:

but it gets stuck like this:

4    4  Exit: byPlane(paris,losAngeles) ? 
3    3  Exit: connected(paris,losAngeles) ? 
2    2  Exit: travel(paris,losAngeles,go(paris,losAngeles)) ? 
5    2  Call: go(metz,paris,go(paris,losAngeles)) ? 
6    3  Call: travel(metz,paris,go(paris,losAngeles)) ? 
7    4  Call: travel(_217,paris,go(paris,losAngeles)) ? 
8    5  Call: travel(_242,paris,go(paris,losAngeles)) ? 
9    6  Call: travel(_267,paris,go(paris,losAngeles)) ? 
10    7  Call: travel(_292,paris,go(paris,losAngeles)) ? 

我已经试过了，但是我无法用它来构建整个go(a,b,go(b,c))等...

I've played around with it, but I can't get it to build the whole go(a,b,go(b,c)) etc...

推荐答案

我将为您介绍递归的基本情况:

I'll give you the base case of the recursion:

travel(X, Y, go(X, Y)) :- connected(X, Y).

递归情况看起来非常相似，除了您要构建的go/3术语的头两个参数必须是位置，第二个路径是路径(另一个go/2或go/3术语).

The recursive case looks extremely similar, except that the go/3 term you're building must have locations as its first two arguments, and a path (another go/2 or go/3 term) as its second.

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