61 lines
1.5 KiB
Perl
61 lines
1.5 KiB
Perl
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%% Imagine that the following knowledge base describes a maze. The
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%% facts determine which points are connected, that is, from which
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%% points you can get to which other points in one step. Furthermore,
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%% imagine that all paths are one-way streets, so that you can only
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%% walk them in one direction. So, you can get from point 1 to point
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%% 2, but not the other way round.
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connected(1,2).
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connected(3,4).
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connected(5,6).
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connected(7,8).
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connected(9,10).
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connected(12,13).
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connected(13,14).
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connected(15,16).
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connected(17,18).
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connected(19,20).
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connected(4,1).
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connected(6,3).
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connected(4,7).
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connected(6,11).
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connected(14,9).
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connected(11,15).
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connected(16,12).
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connected(14,17).
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connected(16,19).
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%% Write a predicate path/2 that tells you from which points in the
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%% maze you can get to which other points when chaining together
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%% connections given in the above knowledge base. Can you get from
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%% point 5 to point 10? Which other point can you get to when starting
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%% at point 1? And which points can be reached from point 13?
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path(X, Y) :- connected(X, Y).
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path(X, Y) :- connected(X, Z),
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path(Z, Y).
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%% Questions:
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%% 1. Can you get from point 5 to point 10? Yes:
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%%
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%% ?- path(5, 10).
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%% true .
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%%
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%% 2. Which other point can you get when starting at point 1? You can't
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%% get to any other points:
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%%
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%% ?- path(1, P).
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%% P = 2 ;
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%% false.
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%%
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%% 3. Which points can be reached from point 13? Points 9, 10, 14, 17,
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%% 18.
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%%
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%% ?- path(13, P).
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%% P = 14 ;
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%% P = 9 ;
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%% P = 17 ;
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%% P = 10 ;
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%% P = 18 ;
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%% false.
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