# Assignment 4 Solution

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1. (50 points) Part 1 : Programming exercise

. We want to compare the naive and Karatsuba divide-and-conquer methods to multiply two integers x and y. Download the java file multiply.java from the course web page. Here, your task is to implement a recursive version of these algorithms in the methods naive(int size, int x, int y) and karatsuba(int size, int x, int y), and to use them to compare the efficiency of each algorithm (i.e. number of arithmetic operations). The variable size is the size of the integers x and y, and is defined as the number of bits used to encode them (Note: we assume that x and y have the same size).

Each method (i.e. naive and karatsuba) will return an integer array result that stores the value of the multiplication in the first entry of the array (i.e. result), and the cost of this computation in the second entry (i.e. result). We define the cost as the number of brute force arithmetic operations of the (addition, subtraction, or multiplication) executed by the algorithm multiplied by the size (in bits) of the integers involved in this operation (Note: We ignore the multiplication by powers of 2 which can be executed using a bit shift. Of course, this is a crude approximation).

In particular, for the base case (i.e. when the size of the integers is 1 bit), this cost will be 1 (brute force multiplication of two integers of size 1). In the induction case, the naive method executes 3 arithmetic operations of integer of size m (i.e. cost is 3 m), in addition of the num-ber of operations executed by each recursive call to the function. By contrast, the Karatsuba algorithm requires 6 arithmetic operations of size m on the top of the cost of the recursion.

The output of your program will print a list of numbers such that, the first number of each row is the size of the integers that have been multiplied, the second number is the cost of the naive method, and the third number the cost of the Karatsuba method.

COMP 251 – HW4 Page 2 of 3 Fall 2018

You will test your methods on integers of size 1 to 15. Complete the provided CSV file, karatsuba.csv with your results for both methods on integers of each size. Plot those results, and write a short report describing and explaining what you observe. Include the plot and the report in a file named 260xxxxxx.pdf, editing the name to match your student ID.

Part 2 : write your computations with all the steps and justified results in a PDF file named 260xxxxxx.pdf. If this PDF is hand-written, make sure you scan it. Pictures will not be accepted.

Enter your results on the MyCourses quiz

1. (25 points) We remind the master method for determining the asymptotical behaviour of a recursive function.

Theorem 1 (Master method) Let a 1 and b 1 be two constants, and f(n) a function.

8n 2 N+ we define T (n) as:

 T (n) = aT n + f(n); where nb is interpreted as bnb c or dnb e. b

Then, we can find asymptotical bounds for T (n) such that:

1. If f(n) = O(nlogb a ) with > 0, then T (n) = (nlogb a).

2. If f(n) =(nlogb a logp n), then T (n) = (nlogb a logp+1 n).

 3. If f(n) = (nlogb a+ ) with > 0, and a f n cf(n); n > n with c < 1 and n0 > 0. Then T (n) = (f(n)). b 8 0

When possible, apply the master theorem to find the asymptotic behaviour of T (n) from the following recurrences. Show your work and justify your answer for each recurrence in the PDF file. Answer in the MyCourses quiz.

1. (5 points) T (n) = 25 T (n5 ) + n

1. (5 points) T (n) = 2 T (n3 ) + n log(n)

1. (5 points) T (n) = T (34n ) + 1

1. (5 points) T (n) = 7 T (n3 ) + n3

(e) (5 points) T (n) = T (n=2) + n(2 cos n)

1. (25 points) Let TA and TB be two function returning the running time of algorithms A and B, defined by the recusions TA(n) = 7TA(n2 ) + n2 and TB(n) = TB(n4 ) + n2. Find the largest integer value of for which algorithm B is asymptotically faster than A. Show your work and justify your answer in the PDF file. Answer in the MyCourses quiz.

You will submit multiply.java, karatsuba.csv and student_id.pdf in a single zip file in the MyCourses submission folder for Assignment 4. You will enter your solutions to question 2 and 3 on the MyCourses assignment 4 quiz. Your final answers must be entered in the quiz for you to receive a grade.

COMP 251 – HW4 Page 3 of 3 Fall 2018

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