How to Apply Viterbi Algorithm in Python

Question

I am a beginner to Python. Currently I am learning the Viterbi algorithm. I found the code in Wiki, and I would like to implement it in Python.

I am using online Python to execute the algorithm. However, I encounter a problem.

After I copy the code into the online Python site, it shows

'sh-4.3$ python main.py'

Does this mean I don't have any output? How can I insert data into online Python site?

Ruzihm · Accepted Answer · 2018-11-02 16:04:14Z

2

To try Viterbi algorithm online you should paste following code

def viterbi(obs, states, start_p, trans_p, emit_p):
V=[{}]
    for i in states:
        V[0][i]=start_p[i]*emit_p[i][obs[0]]
    # Run Viterbi when t > 0
    for t in range(1, len(obs)):
        V.append({})
        for y in states:
            (prob, state) = max((V[t-1][y0] * trans_p[y0][y] * emit_p[y][obs[t]], y0) for y0 in states)
            V[t][y] = prob
        for i in dptable(V):
            print (i)
        opt=[]
        for j in V:
            for x,y in j.items():
                if j[x]==max(j.values()):
                    opt.append(x)
    #the highest probability
    h=max(V[-1].values())
    print ('The steps of states are '+' '.join(opt)+' with highest probability of %s'%h)
    #it prints a table of steps from dictionary

def dptable(V):
    yield " ".join(("%10d" % i) for i in range(len(V)))
    for y in V[0]:
        yield "%.7s: " % y+" ".join("%.7s" % ("%f" % v[y]) for v in V)

states = ('Healthy', 'Fever')
observations = ('normal', 'cold', 'dizzy')
start_probability = {'Healthy': 0.6, 'Fever': 0.4}
transition_probability = {
'Healthy' : {'Healthy': 0.7, 'Fever': 0.3},
'Fever' : {'Healthy': 0.4, 'Fever': 0.6}
}
emission_probability = {
'Healthy' : {'normal': 0.5, 'cold': 0.4, 'dizzy': 0.1},
'Fever' : {'normal': 0.1, 'cold': 0.3, 'dizzy': 0.6}
}

viterbi(observations,
                   states,
                   start_probability,
                   transition_probability,
                   emission_probability)

to online ide and print in command line python main.py (if your file named main.py). Or press Execute button above editor.

edited Nov 2, 2018 at 16:04

Ruzihm

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answered Mar 1, 2016 at 6:47

kvorobiev

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3 Comments

Idonknow Over a year ago

It works! Thanks. By the way, I am new to Python. May I know why after the last line, which is Viterbi (observations, states, start_probability, transition_probability, emission_probability), the code outputs somethings?

Idonknow Over a year ago

I understand now. It works because we are calling the function viterbi, right?

kvorobiev Over a year ago

@Idonknow Yes. And there are two print statements inside function.

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