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sklearn-MultinomialNB朴素贝叶斯分类器

Posted on 2017-08-21 20:23  AYE89  阅读(10066)  评论(0)  编辑  收藏  举报

原型

class sklearn.naive_bayes.MultinomialNB(alpha=1.0, fit_prior=True, class_prior=None)

参数

Parameters:

alpha : float, optional (default=1.0)

Additive (Laplace/Lidstone) smoothing parameter (0 for no smoothing).

fit_prior : boolean, optional (default=True)

Whether to learn class prior probabilities or not. If false, a uniform prior will be used.

class_prior : array-like, size (n_classes,), optional (default=None)

Prior probabilities of the classes. If specified the priors are not adjusted according to the data.

alpha的说明——

The parameters \theta_y is estimated by a smoothed version of maximum likelihood, i.e. relative frequency counting:

\hat{\theta}_{yi} = \frac{ N_{yi} + \alpha}{N_y + \alpha n}

where N_{yi} = \sum_{x \in T} x_i is the number of times feature i appears in a sample of class y in the training set T, and N_{y} = \sum_{i=1}^{|T|} N_{yi} is the total count of all features for class y.

The smoothing priors \alpha \ge 0 accounts for features not present in the learning samples and prevents zero probabilities in further computations. Setting \alpha = 1 is called Laplace smoothing, while \alpha < 1 is called Lidstone smoothing.

 

示例

>>> import numpy as np
>>> X = np.random.randint(5, size=(6, 100))
>>> y = np.array([1, 2, 3, 4, 5, 6])
>>> from sklearn.naive_bayes import MultinomialNB
>>> clf = MultinomialNB()
>>> clf.fit(X, y)
MultinomialNB(alpha=1.0, class_prior=None, fit_prior=True)
>>> print(clf.predict(X[2:3]))