tools.py 文件源码

def log_p_multinomial(X, p):
    """log_p_multinomial returns the probability of drawing a vector of
    counts from a multinomial parameterized by p.

    Args:
        X: np.array of counts representing a multinomial draw.
        p: np.array of probabilities, corresponding to X

    Returns:
        the log probability of the multinomial draw.
    """
    if sum(X) == 0:
        return 0.0
    # check that input is valid
    assert len(X) == len(p)
    eps = 0.0001
    assert abs(sum(p)- 1.0) < eps

    # calculate log prob.

    log_n_choices = special.gammaln(sum(X)+1) - sum([special.gammaln(x_i+1)
                                                     for x_i in X])
    log_p_items = sum(x_i*np.log(p_i) for (x_i, p_i) in zip(X, p))
    return log_n_choices + log_p_items