关于最大熵模型的介绍请看：http://www.cnblogs.com/hexinuaa/p/3353479.html
下面是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
运行：
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‘)]