# COMP-SCI 7202 Practical-05: Sorting Algorithms solution

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## Part 1 – Build Your Own Sorting Algorithms

Question 1: Set up a new project in your IDE with a Main class. Your project name should be MySortAlg and it should be saved under practical-05.

Question 2: You need to build a base class for all your sorting algorithms. You should name this base class as MySortAlg. Since we can not implement the sort method in base class, you
should declare it as an abstract class (https://docs.oracle.com/javase/tutorial/java/IandI/abstract.html) .

public abstract class MySortAlg
Methods:
abstract int[] sort(int[] array);

Question 3: Define an InsertionSort class. This class should extend the MySortAlg class. In this class, your sort() method should take a sequence of integer numbers as input and return a
sorted array in descending order.
public class InsertionSort
Methods:
public int[] sort(int[] array);

Question 4: Define a MergeSort class. This class should extend the MySortAlg class. In this class, your sort() method should take a sequence of integer numbers as input and return a sorted
array in descending order.

You can add other methods to help you to implement merge sort algorithms. Please note that for all other methods you create in this class, you can design the
signature by your self. But these methods should be declared as private.
public class MergeSort

Methods:
public int[] sort(int[] array);

Question 5: Define a QuickSort class. This class should extend the MySortAlg class. In this class, your sort() method should take a sequence of integer numbers as input and return a sorted
array in descending order. You can add other methods to help you to implement merge sort algorithms. Please note that for all other methods you create in this class should be declared
as private.
public class QuickSort
Methods:
public int[] sort(int[] array);

Question 6: In your Main class, build a test method to test your code.
public static boolean test(int[] result, int[] ans);

The test method should take two arrays as input. Pass the sorted array from your sorting function into ‘result’ and pass an array that you have sorted by hand to ‘ans’. Your test method should
check if these two arrays are exactly the same, return true when they are the same and false when they not.

You should generate at least 3 test cases and use these test cases to test all sorting
algorithms above.

For example:
int[] result = sortObject.sort( {4,2,3,1} );
int[] ans = {1,2,3,4};
test( result, ans );
Would return true if the sorting algorithm is working correctly.

Question 7: Hint: the sorting algorithm you just built will help you to solve this question. In your Main class, build a compare method.
public static boolean compare(int[] arr1, int[] arr2);

This method take two arrays as input and return true when they are equal and return false otherwise. Two arrays are said to be equal if both of them contain same set of elements, order of
elements may be different though. You can assume that in one array, one element will only appear one time.

For example,
Input: arr1[] = {2,4,1,3,5}
arr2[] = {2,4,1,3,5}
Output: True
Input: arr1[] = {1,3,5,2,4}
arr2[] = {2,4,1,3,5}
Output: True
Input: arr1[] = {1,7,2,8,5}
arr2[] = {2,4,1,3,5}
Output: False

Question 8: Hint: the sorting algorithm you just built will help you to solve this question. In your Main class, build a findSmallestSum method.

public static int findSmallestSum(int[] array);
This method will take an array as input and return the smallest sum of two elements in this array. You can assume that the length of input array is always >= 2. For example,
Input: array = {4,7,0,0,2,9,10}
Output: 0

## Part 2 – Marks of Class

Question 9: Set up a new project in your IDE with a Main class. Your project name should be MarksOfClass and it should be saved under practical-05

Question 10: You are provided a students.txt file with marks of N students. For each student, the student’s marks in Physic, Chemistry and Maths (the same order in txt file) are provided to
you. You should define a Student class to save these data.

Define a Student class.
public class Student
Attributes: name, physic, chemistry, math, average
Methods:
//average score should be calculated in Constructor
public Student(string name, int physic, int chemistry, int math);
//mutator and accessor for each attributes

Question 11: In your Main class, define a readData method, the input of this method should be a file name and the output should be an array of students.

Question 12: You want to sort the Student’s Numbers in Descending order of their average marks for three courses. In your Student class, define a sortStudents method, the input of this
method should be an array of students and the output should be an array of sorted students.

John 95 75 88
Alice 88 95 75
Johnson 95 75 88
Dennis 60 100 100
Jack 77 84 93
Tod 84 86 80
Tom 68 70 75
Dave 90 90 92
David 99 70 87
Trent 89 77 90
Bob 100 67 89
Fiona 77 89 90
Peter 80 88 82
Amy 85 95 78
Nancy 83 93 82
Richard 81 91 86
Daniel 77 78 79
James 80 90 85
Cathy 95 74 89
Paul 84 87 79
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public Student[] sortStudents(Student[] students);

Now, once this is done, only those students who have the same average marks have to be sorted in the descending order of their Physics marks.
If their Physics marks are also the same, you need to sort them in the descending order of their Chemistry marks.

If their Chemistry marks are the same, you need to sort them in the descending order of their Maths marks.
If their Math marks are the same, you need to sort them in the ascending order of their name.
Note: You have to implement the sorting algorithm by yourself. Try to add some other private methods in Student class so that you can reuse your code for sorting.

Question 13: In your Main class, define a printStudents method, the input of this method should be an array of students and it returns void.
public static void printStudents(Student[] students);

In this method, you should print the sorted students in console by the following format:
Student AverageScore Physic Chemistry Maths
John 90 88 90 92
Trent 88 87 88 89

## Part 3 – Recursive

Question 14: Set up a new project in your IDE with a Main class. Your project name should be Recursive and it should be saved under practical-05

Question 15: In your Main class, create a nextHappyNum method.
int nextHappyNum(int num)

For a given non-negative integer N, find the next smallest Happy Number. A number is called happy if it leads to 1 after a sequence of steps where in each step number is replaced by sum of
squares of its digit that is if we start with Happy Number and keep replacing it with digits square sum, we reach 1.

For example,
Input: 8
Output: 10
Input: 3
Output: 7
Input: 0
Output: 1
Input: 10
Output: 13

Question 16: In your Main class, create a decodeString method.
String decodeString(String s)
An encoded string (s) is given, the task is to decode it. The pattern in which the strings were encoded were as follows
original string: abbbababbbababbbab
encoded string : “3[a3[b]1[ab]]”.
For example:
Input: 1[b]
Output: b
Input: 3[b2[ca]1[d]]