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95行代码实现最大熵模型训练

时间:2015-03-12 13:02:27      阅读:211      评论:0      收藏:0      [点我收藏+]

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关于最大熵模型的介绍请看:http://www.cnblogs.com/hexinuaa/p/3353479.html

以下是GIS训练算法的python实现,代码不到100行。


from collections import defaultdict

import math


class MaxEnt(object):

    def __init__(self):

        self.feats = defaultdict(int)

        self.trainset = []

        self.labels = set()  

      

    def load_data(self,file):

        for line in open(file):

            fields = line.strip().split()

            # at least two columns

            if len(fields) < 2: continue

            # the first column is label

            label = fields[0]

            self.labels.add(label)

            for f in set(fields[1:]):

                # (label,f) tuple is feature 

                self.feats[(label,f)] += 1

            self.trainset.append(fields)

            

    def _initparams(self):

        self.size = len(self.trainset)

        # M param for GIS training algorithm

        self.M = max([len(record)-1 for record in self.trainset])

        self.ep_ = [0.0]*len(self.feats)

        for i,f in enumerate(self.feats):

            # calculate feature expectation on empirical distribution

            self.ep_[i] = float(self.feats[f])/float(self.size)

            # each feature function correspond to id

            self.feats[f] = i

        # init weight for each feature

        self.w = [0.0]*len(self.feats)

        self.lastw = self.w

        

    def probwgt(self,features,label):

        wgt = 0.0

        for f in features:

            if (label,f) in self.feats:

                wgt += self.w[self.feats[(label,f)]]

        return math.exp(wgt)

            

    """

    calculate feature expectation on model distribution

    """        

    def Ep(self):

        ep = [0.0]*len(self.feats)

        for record in self.trainset:

            features = record[1:]

            # calculate p(y|x)

            prob = self.calprob(features)

            for f in features:

                for w,l in prob:

                    # only focus on features from training data.

                    if (l,f) in self.feats:

                        # get feature id

                        idx = self.feats[(l,f)]

                        # sum(1/N * f(y,x)*p(y|x)), p(x) = 1/N

                        ep[idx] += w * (1.0/self.size)

        return ep

    

    def _convergence(self,lastw,w):

        for w1,w2 in zip(lastw,w):

            if abs(w1-w2) >= 0.01:

                return False

        return True

                

    def train(self, max_iter =1000):

        self._initparams()

        for i in range(max_iter):

            print iter %d ...‘%(i+1)

            # calculate feature expectation on model distribution

            self.ep = self.Ep()           

            self.lastw = self.w[:]  

            for i,win enumerate(self.w):

                delta = 1.0/self.M * math.log(self.ep_[i]/self.ep[i])

                # update w

                self.w[i] += delta

            print self.w

            # test if the algorithm is convergence

            if self._convergence(self.lastw,self.w):

                break

    

    def calprob(self,features):

        wgts = [(self.probwgt(features, l),l) for l in self.labels]

        Z = sum([ w for w,l in wgts])

        prob = [ (w/Z,l) for w,l in wgts]

        return prob 

            

    def predict(self,input):

        features = input.strip().split()

        prob = self.calprob(features)

        prob.sort(reverse=True)

        return prob   


执行:

prepare training data:

Outdoor Sunny Happy

Outdoor Sunny Happy Dry

Outdoor Sunny Happy Humid

Outdoor Sunny Sad Dry

Outdoor Sunny Sad Humid

Outdoor Cloudy Happy Humid

Outdoor Cloudy Happy Humid

Outdoor Cloudy Sad Humid

Outdoor Cloudy Sad Humid

Indoor Rainy Happy Humid

Indoor Rainy Happy Dry

Indoor Rainy Sad Dry

Indoor Rainy Sad Humid

Indoor Cloudy Sad Humid

Indoor Cloudy Sad Humid


open ipython to run the following commands:

In [11]: import maxent


In [12]: model = maxent.MaxEnt()


In [13]: model.load_data(‘data/gameLocation.dat‘)


In [14]: model.train()

In [11]: import maxent


In [12]: model = maxent.MaxEnt()


In [13]: model.load_data(‘data/gameLocation.dat‘)


In [14]: model.train()

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In [16]: model.predict(‘Sunny‘)

Out[16]: [(0.9763203118841158, ‘Outdoor‘), (0.02367968811588421, ‘Indoor‘)]

In [18]: model.predict(‘Cloudy‘)

Out[18]: [(0.7136730549489295, ‘Outdoor‘), (0.28632694505107054, ‘Indoor‘)]


95行代码实现最大熵模型训练

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原文地址:http://www.cnblogs.com/gcczhongduan/p/4331996.html

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