# A simple example demonstrating Multinomial HMM#

The Multinomial HMM is a generalization of the Categorical HMM, with some key differences:

• a Categorical (or generalized Bernoulli/multinoulli) distribution models an outcome of a die with `n_features` possible values, i.e. it is a generalization of the Bernoulli distribution where there are `n_features` categories instead of the binary success/failure outcome; a Categorical HMM has the emission probabilities for each component parametrized by Categorical distributions.

• a Multinomial distribution models the outcome of `n_trials` independent rolls of die, each with `n_features` possible values; i.e.

• when `n_trials = 1` and `n_features = 2`, it is a Bernoulli distribution,

• when `n_trials > 1` and `n_features = 2`, it is a Binomial distribution,

• when `n_trials = 1` and `n_features > 2`, it is a Categorical distribution.

The emission probabilities for each component of a Multinomial HMM are parameterized by Multinomial distributions.

```MultinomialHMM has undergone major changes. The previous version was implementing a CategoricalHMM (a special case of MultinomialHMM). This new implementation follows the standard definition for a Multinomial distribution (e.g. as in https://en.wikipedia.org/wiki/Multinomial_distribution). See these issues for details:
https://github.com/hmmlearn/hmmlearn/issues/335
https://github.com/hmmlearn/hmmlearn/issues/340
Topics discussed:
['cat', 'cat', 'cat', 'cat', 'cat', 'dog', 'dog', 'cat', 'cat', 'dog', 'cat', 'cat', 'cat', 'cat', 'cat', 'dog', 'dog', 'cat', 'cat', 'dog', 'cat', 'cat', 'cat', 'cat', 'cat', 'dog', 'dog', 'cat', 'cat', 'dog', 'cat', 'cat', 'cat', 'cat', 'cat', 'dog', 'dog', 'cat', 'cat', 'dog', 'cat', 'cat', 'cat', 'cat', 'cat', 'dog', 'dog', 'cat', 'cat', 'dog']
Learned emission probs:
[[2.57129200e-01 2.86190571e-02 4.28541642e-01 2.85710101e-01]
[1.33352852e-01 7.33292496e-01 2.67548571e-05 1.33327897e-01]]
Learned transition matrix:
[[0.71429762 0.28570238]
[0.50007593 0.49992407]]
MultinomialHMM has undergone major changes. The previous version was implementing a CategoricalHMM (a special case of MultinomialHMM). This new implementation follows the standard definition for a Multinomial distribution (e.g. as in https://en.wikipedia.org/wiki/Multinomial_distribution). See these issues for details:
https://github.com/hmmlearn/hmmlearn/issues/335
https://github.com/hmmlearn/hmmlearn/issues/340

New Model
Topics discussed:
['dog', 'dog', 'dog', 'dog', 'dog', 'cat', 'cat', 'dog', 'dog', 'cat', 'dog', 'dog', 'dog', 'dog', 'dog', 'cat', 'cat', 'dog', 'dog', 'cat', 'dog', 'dog', 'dog', 'dog', 'dog', 'cat', 'cat', 'dog', 'dog', 'cat', 'dog', 'dog', 'dog', 'dog', 'dog', 'cat', 'cat', 'dog', 'dog', 'cat', 'dog', 'dog', 'dog', 'dog', 'dog', 'cat', 'cat', 'dog', 'dog', 'cat']
Learned emission probs:
[[1.33351722e-01 7.33294814e-01 2.58940657e-05 1.33327570e-01]
[2.57129455e-01 2.86193719e-02 4.28541215e-01 2.85709958e-01]]
Learned transition matrix:
[[0.49992086 0.50007914]
[0.28570184 0.71429816]]
```

```import numpy as np
from hmmlearn import hmm

# For this example, we will model the stages of a conversation,
# where each sentence is "generated" with an underlying topic, "cat" or "dog"
states = ["cat", "dog"]
id2topic = dict(zip(range(len(states)), states))
# we are more likely to talk about cats first
start_probs = np.array([0.6, 0.4])

# For each topic, the probability of saying certain words can be modeled by
# a distribution over vocabulary associated with the categories

vocabulary = ["tail", "fetch", "mouse", "food"]
# if the topic is "cat", we are more likely to talk about "mouse"
# if the topic is "dog", we are more likely to talk about "fetch"
emission_probs = np.array([[0.25, 0.1, 0.4, 0.25],
[0.2, 0.5, 0.1, 0.2]])

# Also assume it's more likely to stay in a state than transition to the other
trans_mat = np.array([[0.8, 0.2], [0.2, 0.8]])

# Pretend that every sentence we speak only has a total of 5 words,
# i.e. we independently utter a word from the vocabulary 5 times per sentence
# we observe the following bag of words (BoW) for 8 sentences:
observations = [["tail", "mouse", "mouse", "food", "mouse"],
["food", "mouse", "mouse", "food", "mouse"],
["tail", "mouse", "mouse", "tail", "mouse"],
["food", "mouse", "food", "food", "tail"],
["tail", "fetch", "mouse", "food", "tail"],
["tail", "fetch", "fetch", "food", "fetch"],
["fetch", "fetch", "fetch", "food", "tail"],
["food", "mouse", "food", "food", "tail"],
["tail", "mouse", "mouse", "tail", "mouse"],
["fetch", "fetch", "fetch", "fetch", "fetch"]]

# Convert "sentences" to numbers:
vocab2id = dict(zip(vocabulary, range(len(vocabulary))))
def sentence2counts(sentence):
ans = []
for word, idx in vocab2id.items():
count = sentence.count(word)
ans.append(count)
return ans

X = []
for sentence in observations:
row = sentence2counts(sentence)
X.append(row)

data = np.array(X, dtype=int)

# pretend this is repeated, so we have more data to learn from:
lengths = [len(X)]*5
sequences = np.tile(data, (5,1))

# Set up model:
model = hmm.MultinomialHMM(n_components=len(states),
n_trials=len(observations[0]),
n_iter=50,
init_params='')

model.n_features = len(vocabulary)
model.startprob_ = start_probs
model.transmat_ = trans_mat
model.emissionprob_ = emission_probs
model.fit(sequences, lengths)

print("Topics discussed:")

print("Learned emission probs:")
print(model.emissionprob_)

print("Learned transition matrix:")
print(model.transmat_)

# Try to reset and refit:
new_model = hmm.MultinomialHMM(n_components=len(states),
n_trials=len(observations[0]),
n_iter=50, init_params='ste')

new_model.fit(sequences, lengths)

print("\nNew Model")
print("Topics discussed:")