Problem Set2

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Georgia Institute Of Technology *

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7641

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Computer Science

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Feb 20, 2024

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Problem Set 2 1 ． To design an appropriate encoding for the language with symbols A, B, and C, we can use variable-length codes based on the probabilities of each symbol. In this case, the probability distribution is as follows: P(A)=0.5 P(B)=0.25 P(C)=0.25 A common approach is to assign shorter codes to more probable symbols. One widely used method is Huffman coding. Let's construct a Huffman tree to determine the codes: Create nodes for each symbol (A, B, C) with their probabilities. Repeatedly combine the nodes with the lowest probabilities until there's only one node left, which represents the entire code. Huffman Tree: Copy code (0.5) / \ (A:0.5) \ (0.5) / \ (B:0.25) (C:0.25) Codes: A: 0 B: 10 C: 11 So, the Huffman codes for the symbols are A: 0, B: 10, and C: 11. Now, let's calculate the entropy of this signal. The entropy (H) is given by: H=−∑Pi ⋅ log2(Pi) where Pi is the probability of symbol i. Using the given probabilities: H=−(0.5 ⋅ log 2(0.5)+0.25 ⋅ log 2 (0.25)+0.25 ⋅ log 2 (0.25)) Now, compute the values and sum them up to get the entropy in bits. The logarithms are typically base 2 in information theory. H=−(0.5 ⋅ (−1)+0.25 ⋅ (−2)+0.25 ⋅ (−2))

H=0.5+0.5+0.5 H=1.5bits So, the entropy of this signal is 1.5 bits. 2. Show that the K-means procedure can be viewed as a special case of the EM algorithm applied to an appropriate mixture of Gaussian densities model. Mixture of Gaussian Densities Model In the EM algorithm, one common application is to fit a mixture of Gaussian densities model to the data. The model assumes that the data is generated from a mixture of several Gaussian distributions, each characterized by a mean and a covariance matrix. E-step (Expectation Step) In the E-step of the EM algorithm, the algorithm computes the probability that each data point belongs to each of the Gaussian components. These probabilities are often referred to as "responsibilities." M-step (Maximization Step) In the M-step, the algorithm updates the parameters (means and covariances) of the Gaussian components based on the responsibilities computed in the E-step. K-means as a Special Case Now, let's consider the special case where we have a mixture of spherical Gaussians (i.e., each component has a diagonal covariance matrix with equal variances along the dimensions). In this case, the EM algorithm simplifies, and the responsibilities become binary (either 0 or 1). Connection to K-means In the specific case of a mixture of spherical Gaussians, the responsibilities in the E-step effectively become "hard" assignments, indicating which cluster each data point belongs to with certainty. This is exactly what happens in the K-means algorithm, where each data point is assigned to the cluster with the nearest centroid. In other words, the K-means algorithm can be seen as a special case of the EM algorithm for a mixture of spherical Gaussians, where the responsibilities are "hard" assignments. 7. Consider the following simple grid-world problem. (Actions are N, S, E, W and are deterministic.) Our goal is to maximize the following reward: ● 10 for the transition from state 6 to G ● 10 for the transition from state 8 to G ● 0 for all other transitions S 2 3 4 5 6 7 8 G

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