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Random Items in Prolog

I know what I can do X is random(10).to get a random number from 0 to 10, but is there a similar command to get a random matching element?

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2 answers

You can implement it. Here is the version:

%% choose(List, Elt) - chooses a random element
%% in List and unifies it with Elt.
choose([], []).
choose(List, Elt) :-
        length(List, Length),
        random(0, Length, Index),
        nth0(Index, List, Elt).

From http://ozone.wordpress.com/2006/02/22/little-prolog-challenge/

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SWI-Prolog v6 has random_member/2, defined as follows:

?- listing(random_member).
random:random_member(D, A) :-
    length(A, B),
    C is random(B),
    nth0(C, A, D).

Usage example:

?- random_member(a(N), [a(1), a(2), b(3)]).
N = 1.

?- random_member(a(N), [a(1), a(2), b(3)]).
N = 1.

?- random_member(a(N), [a(1), a(2), b(3)]).
N = 2.

?- random_member(a(N), [a(1), a(2), b(3)]).
false.

?- random_member(a(N), [a(1), a(2), b(3)]).
false.

?- random_member(a(N), [a(1), a(2), b(3)]).
N = 2.

You might want to use it in mode (-,+).

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