三重门问题.py

__author__ = 'xilin'
from thinkbayes import Pmf

class Monty(Pmf):
    """Map from string location of car to probability"""

    def __init__(self, hypos):
        """Initialize the distribution.
        hypos: sequence of hypotheses
        """
        Pmf.__init__(self)
        for hypo in hypos:
            self.Set(hypo, 1)
        self.Normalize()

    def Update(self, data):
        """Updates each hypothesis based on the data.
        data: any representation of the data
        """
        for hypo in self.Values():
            like = self.Likelihood(data, hypo)
            self.Mult(hypo, like)
        self.Normalize()

    def Likelihood(self, data, hypo):
        """Compute the likelihood of the data under the hypothesis
        hypo: string name of the door where the prize is
        data: string name of the door Monty opened
        """
        if hypo == data:
            return 0
        elif hypo == 'A':
            return 0.5
        else:
            return 1


def main():
    hypos = 'ABC'
    pmf = Monty(hypos)

    data = 'B'
    pmf.Update(data)

    for hypo, prob in sorted(pmf.Items()):
        print(hypo, prob)


if __name__ == '__main__':
    main()