CS 2102 Exam 2 B-Term 2013

Transcription

1 NAME: CS 2102 Exam 2 B-Term 2013 Question 1: (25) Question 2: (55) Question 3: (20) TOTAL: (100) 1

2 1. A method that computes n m is correctly coded recursively as follows: class Power{ Power(){ // computes n to the m power, for any positive ints n and m int power(int n, int m){ if (m==0) return 1; else if (m == 1) return n; else return n * power(n, m-1); It is desired to memoize the power method. (a) (10 points) The memoized results are to be stored in a Java HashMap. As a reminder, the Java documentation for HashMap contains this information: class HashMap<K,V> Type Parameters: K - the type of keys maintained by this map V - the type of mapped values A class called Key is developed to provide an appropriate key type for the power method. The Key class defines two fields. Answer these questions about the Key class: i. What should the two fields in the Key class represent? ii. Besides the constructor, how many methods does the Key class need to provide to make it usable as a key for a HashMap? iii. Write the signatures for each of the methods you counted in part (ii) 2

3 (b) (15 points) Suppose that an instance of an appropriate HashMap, called memtable, is available in the Power class for storing the memoized results. Rewrite the given power method so that it uses memtable. Here are the relevant method signatures from the HashMap class: public boolean containskey(object key); public V get(object key); public V put(k key, V value); // Note: put can be called as a void method // (i.e. you may ignore the return value) 3

4 2. Refer to the tear-off handout at the end of the exam, which provides code for a LotteryTicket class. The idea behind this class is that we want to be able to check if a lottery ticket meets some specification for winning; for example, if the ticket contains at least one occurrence of the number 8, or if the ticket s numbers are all the same. Here s an example of how we could activate the iswinningticket method: // in Examples class... LotteryTicket ticket = new LotteryTicket(); boolean testwinner (Tester t){ // check if the ticket contains at least one occurrence of the number 8 return t.checkexpect (ticket.iswinningticket(new HasAnEight()), false); (a) (5 points) Which of the following classes provides the correct definition for HasAnEight? i. class HasAnEight implements ISelect{ HasAnEight(){ public boolean select (ISelect s){ return s.select(this.contains(8)); ii. class HasAnEight implements ISelect{ HasAnEight(){ public boolean select (LotteryTicket t){ return this.select(t.numbers.contains(8)); iii. class HasAnEight implements ISelect{ HasAnEight(){ public boolean select (LotteryTicket t){ return t.iswinningticket(this); iv. class HasAnEight implements ISelect{ HasAnEight(){ public boolean select (LotteryTicket t){ return t.numbers.contains(8); 4

5 Currently, a linked list is used to hold the numbers in a lottery ticket. In parts (b) and (c) of this problem, you ll encapsulate the data structure used to hold the numbers, so that we re not restricted to using a linked list. Your answers to parts (b) and (c) should be Java code. As you write each piece of code, use appropriate access modifiers on fields and methods. (b) (15 points) An interface to represent the numbers in a lottery ticket is defined as follows: interface INumbers{ Rewrite the entire LotteryTicket class so that it uses INumbers instead of LinkedList<Integer> to hold the numbers. (Your class should contain a constructor for LotteryTicket, and an iswinningticket() method. You don t have to write any comments.) 5

6 (c) (30 points) You should now provide the complete Java definitions for two classes: one that implements the INumbers interface, and one that implements the ISelect interface. The class that implements INumbers is called Numbers. It stores the lottery ticket numbers as three separate Integers (as opposed to putting the three numbers in a linked list, as was done in the original code). As was done in the original code, specific numbers may be used instead of generating random numbers. The class that implements ISelect is called HasASeven. It selects a winning ticket if that ticket contains at least one occurrence of the number 7. As you re writing these classes, you may find the need to add new methods to the LotteryTicket class, and/or new method signatures to the INumbers interface. If so, go back to part (b) and add those methods/method signatures in the appropriate places. Put your Numbers class definition here (the definition for HasASeven should go on the next page): 6

7 Put your HasASeven class definition here: (d) (5 points) Write a statement for the Examples class that creates an instance of a LotteryTicket that uses the Numbers class to represent the numbers. 7

8 3. (20 points) The Node class we developed in lecture used a parameter of type LinkedList<Node> in the hasroute() method to keep track of visited nodes: class Node{ private String cityname; private LinkedList<Node> getsto; // list of adjacent nodes public Node (String cityname){ this.cityname = cityname; this.getsto = new LinkedList<Node>(); public boolean hasroute(node to, LinkedList<Node> visited){ if (this.equals(to)) return true; else if (visited.contains(this)) return false; else{ visited.add(this); for (Node n: getsto){ if (n.hasroute(to, visited)) return true; return false; // rest of class omitted 8

9 Suppose that instead of using a linked list to keep track of visited nodes, we use a boolean field visited in the Node class, as follows: class Node{ private String cityname; private LinkedList<Node> getsto; // list of adjacent nodes private boolean visited; // true if this node has been visited public Node(String cityname){ this.cityname = cityname; this.getsto = new LinkedList<Node>(); this.visited = false; // public method to reset this node s visited field public void reset(){ this.visited = false; //...rest of class omitted... (a) (10 points) Using a for-each loop, write a void method called resetvisited() in the Graph class that sets the value of the visited field to false for each node in the graph. Here s the relevant code from the Graph class: class Graph{ private LinkedList<Node> nodes; public boolean hasroute(node from, Node to){ return from.hasroute(to); // the set of nodes in the graph //...rest of class omitted... (b) (10 points) Indicate the changes that should be made to the hasroute() method (on page 8) to implement this new way of tracking visited nodes. (You may cross out and/or add code to the hasroute() method on page 8, or you may rewrite the entire method.) Assume that the Graph method resetvisited() from part (a) has been called prior to calling the Graph method hasroute(). 9

10 HANDOUT FOR USE IN PROBLEM class LotteryTicket{ LinkedList<Integer> numbers; LotteryTicket (){ // in a real application, random numbers would be used // to initialize a lottery ticket. We ll use specific // numbers here to simplify testing. this.numbers.add(9); this.numbers.add(3); this.numbers.add(1); boolean iswinningticket (ISelect selector){ return selector.select(this); 10