COMP9313 Exam solved

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Question 1. Concepts (4 marks)

(a) (2 marks) In what kind of problems a combiner class and a reducer class can be used
interchangeably? Please use an example to explain your answer.
(b) (2 marks) In one project, a student complained that her approach took a lot of time at the step when
using the reduce() function, but all the previous operations including reading the data by textFile(),
filtering the data by filter(), and transform the data by map() and flatmap(). Could you please explain
the reason to her?

Question 2. MapReduce Programming (14 marks)

Requirement: You should explain how the input is mapped into (key, value) pairs by the map stage, i.e.,
specify what is the key and what is the associated value in each pair, and how the key(s) and value(s) are

Then you should explain how the (key, value) pairs produced by the map stage are processed
by the reduce stage to get the final answer(s). You only need to provide the pseudo code for the classes
including Mapper and Reducer (optionally Combiner etc.if necessary, and the efficiency of your method
will be considered).

(a) (4 marks) Given a table shown as below, find out the person(s) with the maximum salary in each
department (employees could have the same salary).
EmployeeID Name DepartmentID Salary
001 Emma 1 100,000
002 Helen 2 85,000
003 Jack 3 85,000
004 James 1 110,000

(b) (10 marks) Problem Background: Given an undirected graph G, its “line graph” is another graph L(G)
that represents the adjacencies between edges of G, such that:
• each vertex of L(G) represents an edge of G; and
• two vertices of L(G) are adjacent if and only if their corresponding edges share a common
endpoint (“are incident”) in G.

The following figures show a graph (left) and its line graph (right). Each vertex of the line graph is
shown labelled with the pair of endpoints of the corresponding edge in the original graph. For instance,
the vertex on the right labelled (1,3) corresponds to the edge on the left between the vertices 1 and 3.

Vertex (1,3) is adjacent to three other vertices: (1,2) and (1,4) (corresponding to edges sharing the
endpoint 1 in G) and (3,4) (corresponding to an edge sharing the endpoint 3 in G). Note that the vertex
(4, 3) in the below example should be (3, 4) in the output.

Problem: Given you the adjacency list of an undirected graph G, use MapReduce to generate the
adjacency list of its line graph L(G). Note that each edge connecting two nodes i and j is represented by (i,
j) in L(G) (if i<j). In the output, the edges in each list should be ranked in ascending order by comparing
the first node and then the second node.

The adjacency lists should be ranked by the keys according to
the same order as well. Take the above figure as an example, sample input and output are as below:
Input: Output:
1: 2, 3, 4
2: 1, 5
3: 1, 4
4: 1, 3, 5
5: 2, 4

(1, 2): (1, 3), (1, 4), (2, 5)
(1, 3): (1, 2), (1, 4), (3, 4)
(1, 4): (1, 2), (1, 3), (3, 4), (4, 5)
(2, 5): (1, 2), (4, 5)
(3, 4): (1, 3), (1, 4), (4, 5)
(4, 5): (1, 4), (2, 5), (3, 4)

Question 3. Spark Programming (14 marks)

Provide the PySpark code for the given problems (minor errors are acceptable).
(a) (7 marks) RDD programming: Given a set of marks from different courses (the input format is as
shown in the left column), the task is to: For each student, get his/her ranking in different courses. The
output format is <student_id: course_name, rank>.

Sort the output by student_id first and then by
course_name (the format is as shown in the right column).
Input: Output:
student1 course1 90
student1 course2 92
student1 course3 80
student1 course4 79

student1 course5 93
student2 course1 92
student2 course2 77
student2 course5 85
student3 course3 64
student3 course4 97
student3 course5 82
student1: course1,2
student1: course2,1

student1: course3,2
student1: course4,2
student1: course5,1
student2: course1,1
student2: course2,2
student2: course5,2
student3: course3,2
student3: course4,1
student3: course5,3

(b) (7 marks) DataFrame programming (RDD APIs not allowed): Given the same input (but different
format!) as in problem (a), compute average marks for every course and sort the result by course_name
in alphabetical order.
Input: Output:


Question 4. Finding Similar Items (6 marks)

(a) (2 marks) Given two documents A = (“the sky is dark the moon is bright”) and B = (“the moon in the
sky is bright”), using the words as tokens, compute the 2-shingles for A and B, and then compute their
Jaccard similarity based on their 2-shingles.

(b) (3 marks) We want to compute min-hash signature for two columns, C1 and C2 using two pseudorandom permutations of columns using the following function:
h1(n) = (5n + 2) mod 7
h2(n) = (3n + 1) mod 7

Here, n is the row number in original ordering. Instead of explicitly reordering the columns for each hash
function, we use the implementation discussed in class, in which we read each data in a column once in
a sequential order, and update the min hash signatures as we pass through them.
Complete the steps of the algorithm and give the resulting signatures for C1 and C2.

(c) (1 marks) Suppose we wish to find similar sets, and we do so by minhashing the sets 10 times and
then applying locality-sensitive hashing using 5 bands of 2 rows (minhash values) each. If two sets had
Jaccard similarity 0.6, what is the probability that they will be identified in the locality-sensitive hashing
as candidates (i.e. they hash at least once to the same bucket)?

You may assume that there are no
coincidences, where two unequal values hash to the same bucket. A correct expression is sufficient: you
need not give the actual number.

Question 5. Mining Data Streams (6 marks)

(a) (3 marks) Counting Bloom Filter
Consider a Counting Bloom filter of size m = 7 and 2 hash functions that both take a string (lowercase) as
h1(str) = ∑𝑐 𝑖𝑛 𝑠𝑡𝑟(𝑐 − ′𝑎′) mod 7
h2(str) = (str.length * 2) mod 7
Here, c – ‘a’ is used to compute the position of the letter c in the 26 alphabetical letters, e.g., h1(“bd”) =
(1 + 3) mod 7 = 4.

(i) (2 marks) Given a set of string S = {“hi”, “big”, “data”, “spark”}, show the update of the
Bloom filter
(ii) (1 mark) Delete “hi” from S, and then use the bloom filter to check if “sql” is contained in S.
(b) (3 marks) CM-Sketch
Assume that we have 5 buckets and three hash functions:
h0(str) = (str.length * 2) mod 5

h1(str) = (str.length + 3) mod 5
h2(str) = (str[0]-‘a’) mod 5
Given you a stream of terms: “big”, “data”, “data”, “hadoop”, “data”, “spark”, show the steps of building
the CM-Sketch. Then, use the built CM-sketch to get the count for word “data”.

Question 6. Link Analysis (6 marks)

Given a directed graph G with the set of nodes {1,2,3,4,5,6} and the edges arranged as below:
Using the MapReduce PageRank algorithm (lecture slides 9.50 and 9.51), show the computation process
in the first two rounds (including the mapper input, mapper output, reducer input, and reducer output).
End Of Paper