Python入门练习（二）Viterbi算法

资料见：http://en.wikipedia.org/wiki/Viterbi_algorithm

代码如下

 

C:Python26MyViterbiMyViterbi.py

#  -*- coding: cp936 -*-

def  forward_viterbi(obs, states, start_p, trans_p, emit_p):
   T  =  {}
    for  state  in  states:
        # #          prob.           V. path  V. prob.
       T[state]  =  (start_p[state], [state], start_p[state])
    for  output  in  obs:
       U  =  {}
        for  next_state  in  states:
           total  =  0
           argmax  =  None
           valmax  =  0
            for  source_state  in  states:
               (prob, v_path, v_prob)  =  T[source_state]
               p  =  emit_p[source_state][output]  *  trans_p[source_state][next_state]
               prob  *=  p
               v_prob  *=  p
               total  +=  prob
                if  v_prob  >  valmax:
                   argmax  =  v_path  +  [next_state]
                   valmax  =  v_prob
           U[next_state]  =  (total, argmax, valmax)
       T  =  U
    # # apply sum/max to the final states:
   total  =  0
   argmax  =  None
   valmax  =  0
    for  state  in  states:
       (prob, v_path, v_prob)  =  T[state]
       total  +=  prob
        if  v_prob  >  valmax:
           argmax  =  v_path
           valmax  =  v_prob
    return  (total, argmax, valmax)

 

 

C:Python26MyViterbimyTestViterbi.py

#  -*- coding: cp936 -*-
from  MyViterbi  import  forward_viterbi
states = ( ' Rainy ' , ' Sunny ' ) # 元组
observations = ( ' walk ' , ' shop ' , ' clean ' ) # 元组
start_probability = { ' Rainy ' : 0.6 , ' Sunny ' : 0.4 }
transition_probility = {
     ' Rainy ' :{ ' Rainy ' : 0.7 , ' Sunny ' : 0.3 },
     ' Sunny ' :{ ' Rainy ' : 0.4 , ' Sunny ' : 0.6 }
    }
emission_probability = {
     ' Rainy ' :{ ' walk ' : 0.1 , ' shop ' : 0.4 , ' clean ' : 0.5 },
     ' Sunny ' :{ ' walk ' : 0.6 , ' shop ' : 0.3 , ' clean ' : 0.1 }
    }
observation = ( ' walk ' , ' clean ' , ' walk ' )


c = forward_viterbi(observation,states,start_probability,transition_probility,emission_probability)
print  c

 

别忘了在MyViterbi文件夹下加init.py

直接运行myTestViterbi.py

结果如下：

(0.023729999999999994, ['Sunny', 'Rainy', 'Sunny', 'Sunny'], 0.0051839999999999994)