83 lines
2.7 KiB
Prolog
83 lines
2.7 KiB
Prolog
%% Given facts.
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directTrain(saarbruecken,dudweiler).
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directTrain(forbach,saarbruecken).
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directTrain(freyming,forbach).
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directTrain(stAvold,freyming).
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directTrain(fahlquemont,stAvold).
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directTrain(metz,fahlquemont).
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directTrain(nancy,metz).
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%% Rules.
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travelFromTo(X, Y) :- directTrain(X, Y).
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travelFromTo(X, Y) :- directTrain(X, Z),
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travelFromTo(Z, Y).
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%% New version from practical exercises.
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%% We are given the following knowledge base of travel information:
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byCar(auckland,hamilton).
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byCar(hamilton,raglan).
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byCar(valmont,saarbruecken).
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byCar(valmont,metz).
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byTrain(metz,frankfurt).
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byTrain(saarbruecken,frankfurt).
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byTrain(metz,paris).
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byTrain(saarbruecken,paris).
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byPlane(frankfurt,bangkok).
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byPlane(frankfurt,singapore).
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byPlane(paris,losAngeles).
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byPlane(bangkok,auckland).
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byPlane(singapore,auckland).
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byPlane(losAngeles,auckland).
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%% Write a predicate travel/2 which determines whether it is possible
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%% to travel from one place to another by chaining together car,
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%% train, and plane journeys. For example, your program should answer
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%% yes to the query travel(valmont,raglan).
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%% The base case is a direct route via car, train or plane.
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travelDirect(X, Y) :- byCar(X, Y).
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travelDirect(X, Y) :- byTrain(X, Y).
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travelDirect(X, Y) :- byPlane(X, Y).
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travel(X, Y) :- travelDirect(X, Y).
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%% The recursive case chains these together.
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travel(X, Y) :- travel(X, Z),
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travel(Z, Y).
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%% So, by using travel/2 to query the above database, you can find out
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%% that it is possible to go from Valmont to Raglan. If you are
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%% planning such a voyage, that’s already something useful to know,
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%% but you would probably prefer to have the precise route from
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%% Valmont to Raglan. Write a predicate travel/3 which tells you which
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%% route to take when travelling from one place to another. For
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%% example, the program should respond
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%% X = go(valmont,metz,
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%% go(metz,paris,
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%% go(paris,losAngeles)))
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%% to the query travel(valmont,losAngeles,X).
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%% First, we need set up the terminal scenario:
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%% travel(X, Y, go(travelDirect(X, Y))) :- travelDirect(X, Y).
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%% Next, we recurse.
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%% travel(X, Y, go(travelDirect(X, Z), W)) :-
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%% travelDirect(X, Z),
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%% travel(Z, Y, W).
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%% The next exercise says we should modify this to include the
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%% mechanism of transport. This is more tedious, but doable.
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travel(X, Y, go(byCar(X, Y))) :- byCar(X, Y).
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travel(X, Y, go(byCar(X, Z), W)) :-
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byCar(X, Z),
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travel(Z, Y, W).
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travel(X, Y, go(byPlane(X, Y))) :- byPlane(X, Y).
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travel(X, Y, go(byPlane(X, Z), W)) :-
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byPlane(X, Z),
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travel(Z, Y, W).
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travel(X, Y, go(byTrain(X, Y))) :- byTrain(X, Y).
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travel(X, Y, go(byTrain(X, Z), W)) :-
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byTrain(X, Z),
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travel(Z, Y, W).
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