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yap-6.3/packages/prism/exs/direction.psm
2011-11-10 12:24:47 +00:00

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%%%%
%%%% Decision of the direction by a coin tossing -- direction.psm
%%%%
%%%% This program has just one random switch named `coin'.
%%%%
%%%% Copyright (C) 2004,2006,2008
%%%% Sato Laboratory, Dept. of Computer Science,
%%%% Tokyo Institute of Technology
%%-------------------------------------
%% Sample session
%%
%% (1) Load this program:
%% ?- prism(direction).
%%
%% (2) Get a sample:
%% ?- sample(direction(D)).
%%
%% (3) Display the information about the switch `coin':
%% ?- show_sw.
%%
%% (4) Set the probability distribution to the switch `coin':
%% ?- set_sw(coin,[0.7,0.3]).
%%
%% (5) Display the switch information again with the distribution set
%% at step 4:
%% ?- show_sw.
%%
%% (6) Get a sample again with the distribution set at step 4:
%% ?- sample(direction(D)).
%%
%% [Note1]
%% Since 1.9, without any extra settings, the probability distribution
%% of every switch is set to a uniform distribution.
%%
%% [Note2]
%% If you go (3) with skipping (2), nothing should be displayed. This
%% is because any random switch will not be registered by the system until
%% it is explicitly used or referred to.
values(coin,[head,tail]). % The switch `coin' takes `head' or `tail' as its value
direction(D):-
msw(coin,Face), % Make a coin tossing
( Face==head -> D=left ; D=right). % Decide the direction according to
% the result of coin tossing