问题
Does anyone have any Prolog code to non-uniformly select a random element from a list?
I want to replicate the functionality of numpy.random.choice when given the probabilities associated with each entry in the input list.
回答1:
I found nothing useful in library(random).
Here's my implementation choice(Xs, Ps, Y):
choice([X|_], [P|_], Cumul, Rand, X) :-
Rand < Cumul + P.
choice([_|Xs], [P|Ps], Cumul, Rand, Y) :-
Cumul1 is Cumul + P,
Rand >= Cumul1,
choice(Xs, Ps, Cumul1, Rand, Y).
choice([X], [P], Cumul, Rand, X) :-
Rand < Cumul + P.
choice(Xs, Ps, Y) :- random(R), choice(Xs, Ps, 0, R, Y).
It works by recursively building the cumulative probability distribution from the probabilities given in Ps, and checking if the random number R is below that.
Note: to function correctly the probabilities Ps must sum to 1, no check is made to warn you if that is not the case.
Example:
?- choice([1,2,3], [0.1,0.2,0.7], Y).
Y = 3 .
?- choice([1,2,3], [0.1,0.2,0.7], Y).
Y = 1 .
?- choice([1,2,3], [0.1,0.2,0.7], Y).
Y = 2 .
?- choice([1,2,3], [0.1,0.2,0.7], Y).
Y = 3 .
?- choice([1,2,3], [0.1,0.2,0.7], Y).
Y = 2 .
?- choice([1,2,3], [0.1,0.2,0.7], Y).
Y = 3 .
?- choice([1,2,3], [0.1,0.2,0.7], Y).
Y = 3 .
?- choice([1,2,3], [0.1,0.2,0.7], Y).
Y = 3 .
?- choice([1,2,3], [0.1,0.2,0.7], Y).
Y = 3 .
?- choice([1,2,3], [0.1,0.2,0.7], Y).
Y = 3 .
?- choice([1,2,3], [0.1,0.2,0.7], Y).
Y = 3 .
...
来源：https://stackoverflow.com/questions/50250234/prolog-how-to-non-uniformly-randomly-select-a-element-from-a-list