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ASSIGNMENT 1: BELIEF PROPAGATION AND MAXIMAL PROBABILITY

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CS5340 ASSIGNMENT 1:
BELIEF PROPAGATION AND MAXIMAL PROBABILITY
1. OVERVIEW
Welcome to the course! In this first assignment, you will write code to perform inference on
simple tree-like graphs using belief propagation. You will write the code for both the sum-product
and max-product algorithm.
References: Lecture 4 and 5
2. SUBMISSION INSTRUCTIONS
Items to be submitted:
• Source code (lab1.py). This is where you fill in all your code.
• Report (report.pdf). This should describe your implementation and be no more than one
page.
Please indicate clearly your name and student number (the one that looks like A1234567X) in the
report as well as the top of your source code. Zip the two files together and name it in the
following format: A1234567X_lab1.zip (replace with your student number).
Submit your assignment by 17 September 2023, 2359HRS to Canvas. 25% of the total score will
be deducted for each day of late submission.
3. GETTING STARTED
This assignment as well as the subsequent ones require Python 3.5, or later. You need certain
python packages, which can be installed using the following command:
pip install -r requirements.txt
If you have any issues with the installation, please post them in the forum, so that other students
or the instructors can help accordingly.
4. TEST CASES
To help with your implementation, we have provided a few sample datasets and their expected
outputs to help you debug your code. They can be found in the data/ folder. You can find the code
that loads the sample data and checks your program output in test_lab1.py.
CS5340 Assignment 1 (Semester 1, AY2023/2024) 2
We will mainly work with the Bayesian network stored in graph_small.json. It is shown below
in Figure 1 for your convenience. In addition, we also provide a larger graphical model in
graph_large.json for you to further test your code.
Note that during grading, we will run your code on hidden test cases on top of the two provided
graphs.
Figure 1: The Bayesian network you will perform inference on
5. BASIC FACTOR OPERATIONS
As you recall from the lectures, we use factor tables to represent the conditional probability
distributions of a Bayesian Network. You will now implement the core functionality for
manipulating the factor tables.
• factor product(): This function should compute the product of two factors.
• factor_marginalize(): This function should sum over the indicated variable(s) and
return the resulting factor.
• observe_evidence(): This function should take in a list of factors and the observed
values of some of the variables, and modify the factors such that assignments not
consistent with the observed values are set to zero.
All the factor operations make use of a custom Factor class, which you should familiarise
yourself with. Its implementation can be found in factor.py, and a description on the factor
datatype together with some example code can be found in factor_readme.py.
X0
X1
X2 X3
X4
X0 P(X0
)
0 0.2
1 0.5
2 0.3 X0 X4 P(X4|X0
)
0 0 0.9
1 0 0.6
2 0 0.1
0 1 0.1
1 1 0.3
2 1 0.4
0 2 0.0
1 2 0.1
2 2 0.5
X1 X3 P(X3|X1
)
0 0 0.7
1 0 0.5
2 0 0.1
3 0 0.0
0 1 0.2
1 1 0.2
2 1 0.4
3 1 0.1
0 2 0.1
1 2 0.3
2 2 0.5
3 2 0.9
X1 X2 P(X2|X1
)
0 0 0.9
1 0 0.7
2 0 0.5
3 0 0.2
0 1 0.1
1 1 0.3
2 1 0.5
3 1 0.8
X0 X1 P(X1|X0
) X0 X1 P(X1|X0
)
0 0 0.6 0 2 0.1
1 0 0.3 1 2 0.2
2 0 0.1 2 2 0.45
0 1 0.2 0 3 0.1
1 1 0.4 1 3 0.1
2 1 0.2 2 3 0.25
CS5340 Assignment 1 (Semester 1, AY2023/2024) 3
6. NAÏVE SUMMATION
To motivate the need for belief propagation, we will first implement the naïve summation
algorithm. That is, you will compute the required (conditional) probabilities by computing the
full joint distribution, then marginalising out irrelevant variables.
6.1 COMPUTING THE JOINT DISTRIBUTION
Complete the function compute_joint_distribution(), which computes the joint
distribution over a Bayesian Network. Recall that for a Bayesian network over variables
𝑋1, … , 𝑋𝑛 , the joint distribution can be computed as1:
𝑃(𝑋1, … , 𝑋𝑛
) = ∏𝑃(𝑋𝑖
|𝑃𝑎𝑟𝑒𝑛𝑡(𝑋𝑖
))
𝑖
6.2 COMPUTING MARGINALS
Having computed the joint distribution, we can compute the marginal probabilities for a variable
by marginalising out irrelevant variables from the joint distribution. In the event we have
observed evidence, we will also need to reduce the joint distribution by setting the assignments
inconsistent with the evidence to zero. So, complete compute_marginals_naive() with your
implementation of the full naïve summation algorithm. In summary, there are three steps to
perform:
• Compute the joint distribution over all variables
• Reduce the joint distribution by the evidence
• Marginalizing out irrelevant variables
After you perform all these operations, you will notice that the factor obtained is un-normalized,
so be sure to normalize the final probability distribution.
If you implemented it correctly, for the small Bayesian network in Figure 1, you will get the
following marginals for 𝑃(𝑋0|𝑋3 = 1).
𝑿𝟎 𝑷(𝑿𝟎|𝑿𝟑 = 𝟏)
0 0.178
1 0.486
2 0.336
1 Generally, each node can have multiple parents, and the conditional probability should consider all parents. However,
for the tree-like graphs considered in this assignment, nodes have at most one parent.
CS5340 Assignment 1 (Semester 1, AY2023/2024) 4
7. SUM-PRODUCT ALGORITHM
Notice that even for our small graph in Figure 1, the joint distribution has 3 × 4 × 2 × 3 × 3 =
216 different assignments. This is very large and not scalable2! The standard variable elimination
reduces this computational complexity but requires a separate run for every variable we want to
compute the marginals for.
An efficient solution is to use the belief propagation (or sum-product) algorithm, which allows us
to compute all single-node marginals for tree-like graphs in a single pass by “reusing” messages.
Recall that the sum-product algorithm consists of the following phases:
• Apply evidence
• Send messages inward towards the root
• Send messages outward towards the leaves
• Compute the marginal of the relevant variables
The belief propagation algorithm will require you to traverse through the graph. For this purpose,
we use a NetworkX graph. We have provided a function generate_graph_from_factors()
that converts a list of factors into a NetworkX graph.
NetworkX provides useful functions for easy graph traversal. For example, neighbours of variable
𝑖 can be accessed through:
>>> graph.neighbors(i)
For convenience, unary and pairwise factors will also be stored as node and edge attributes. To
access the unary factor of variable 𝑖,
>>> graph.nodes[i][‘factor’]
Similarly, the pairwise factor between 𝑖 and 𝑗 can be accessed as,
>>> graph.edges[i, j][‘factor’]
Note that if a node does not have a unary factor, it will not contain the ‘factor’ field. You will have
to handle the various scenarios yourself.
Complete the compute_marignals_bp() function, which computes the marginal probabilities
of multiple variables using belief propagation. If implemented correctly, your inference should
agree with compute_marginals_naive().
Hint: It might be useful to create additional functions for this part. You can place those functions
anywhere in the file.
2 At your own risk, you can try performing using your naive summation code on the graph in graph_large.json.
CS5340 Assignment 1 (Semester 1, AY2023/2024) 5
8. MAP INFERENCE USING MAX-PRODUCT
In the final part you will write code to compute the Maximum a Posterior (MAP) configuration
and its probability. To avoid underflow, we will be working in log-space for this assignment;
otherwise, the probability of any single assignment in a large enough network is typically low
enough to cause numerical issues. You should therefore take logarithm of all the probabilities
before message passing, and perform all operations in log-space. That is, max-product is actually
implemented using max-sum.
To facilitate this, first implement the relevant factor operations for log-space:
• factor_sum(): Analogous to factor_product(), but for log space.
• factor_max_marginalize(): This function should max over the indicated variable(s)
and return the resulting factor. You should also keep track of the maximising values in the
field factor.val_argmax.
Now, you are ready to complete the code for map_eliminate(). The function should take in a
list of factors and optionally evidence, and compute the most probable joint assignment and its
log probability.
If implemented correctly, the MAP conditioned on 𝑋3 = 1 should be:
argmax𝑋0,𝑋1,𝑋2,𝑋4
𝑃(𝑋0,𝑋1, 𝑋2,𝑋4|𝑋3 = 1) = {𝑋0 = 0, 𝑋1 = 0, 𝑋2 = 0, 𝑋4 = 0},
with log probability log(𝑃(𝑋0 = 0, 𝑋1 = 0, 𝑋2 = 0, 𝑋4 = 0, 𝑋3 = 1)) = −3.940.
Notice that 𝑋0 = 0 in the MAP assignment. This is despite 𝑋0 = 1 maximising the conditional
marginal probability, as you have computed in the previous sections.

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