CS 132 Midterm Exam Fall 2004

Transcription

1 Date:1-Nov-2004 Time:7:00-9:00p.m. Permitted Aids:None CS 132 Midterm Exam Fall 2004 Please complete this page in ink. Last Name: Signature: First Name: ID: Please check off your Practicum Section: Dana Mohaplova 101, W 2:30 102, Th 10:30 unsure Instructions: (Read carefully before the exam begins): Question Max. Marks 1. Before you begin, make certain that you have one booklet with pages numbered 1-18 printed double-sided and a 1 page Reference Page of Selected Documentation. 2. The marks assigned to each question are shown at the beginning of the question; use this information to organize your time effectively. 3. Place all your answers in the spaces provided on these pages. Use the spaces as an indication of the length of answer expected. Use the last page for extra space if you need it, but clearly indicate this if you do so. Part I Marks Given Grader Initials 4. You do not need to document your code unless it is specifically required by the question. 5. Questions will not be interpreted. Proctors will only confirm or deny errors in the questions. If you consider the wording of a question to be ambiguous, state your assumptions clearly and proceed to answer the question to the best of your ability. You may not trivialize the problem in your assumptions. 6. Cheating is an academic offense. Your signature on this exam indicates that you understand and agree to the University s policies regarding cheating on exams Total 75

2 Part I: Multiple Choice Questions Answer the questions in Part I on the provided bubble card. For each question, select the single best answer. 1. Testing can only start after the implementation is complete. a) True b) False 2. Design Architecture and Integration Testing are parts of the Software Life Cycle. a) True b) False 3. An array is a sequence of values of the same type. a) True b) False 4. To store primitive type values in an array, you must use wrapper classes. a) True b) False 5. Two- dimensional arrays always have the same numbers of rows as columns. a) True b) False 6. When you throw an Error, the current method terminates immediately. a) True b) False Page 2 of 18

3 Name: 7. Which of the object diagrams illustrates the effect of the following statements? int[] grades = new int[5]; double lowest = 36; grades[0] = lowest; for(int i=1; i<5; i++) { grades[i] = 36; grades grades lowest lowest a) b) grades grades lowest lowest 36 c) d) e) None of the above pictures illustrates the statements. Page 3 of 18

4 8. Which set of statements represents the given object diagram? String name value Joe all String value Joe a) b) c) d) String name = new String( Joe ); String[] all = new String[3]; all[0] = name; all[1] = name; all[2] = name; String name = Joe ; String[] all; all[0] = Joe ; all[1] = Joe ; all[2] = Joe ; String name = Joe ; String[] all; all[0] = Joe ; all[1] = name; all[2] = all[0]; String name = new String( Joe ); String[] all = new String[3]; all[0] = new String( Joe ); all[1] = name; all[2] = all[0]; e) Both c) and d) are correct. Page 4 of 18

5 9. What is the output of the following program? Name: public class Exam extends Object { private String course = "noname"; private static int count = 0; private int courseid = 100; a) b) c) d) public Exam(String name) { super(); this.course = name; Exam.count++; this.courseid++; public void printexam() { System.out.println(courseID + ";" + course + ";" + count); public static void main(string[] args) { int numofstudents = 3; for(int i=0; i<numofstudents; i++) { Exam e = new Exam ("cs132_" + i); e.printexam(); 101;cs132_0;1 101;cs132_0;1 101;cs132_0;1 101;cs132_0;1 101;cs132_1;1 101;cs132_2;1 101;cs132_0;1 101;cs132_1;2 101;cs132_2;3 101;cs132_0;1 102;cs132_1;2 103;cs132_2;3 e) None of the above. Page 5 of 18

6 10. Consider the following Boolean expression where done is a boolean variable and x, y and z are integers (assume all variables are appropriately initialized): result =!((x<y && done) (x>0)) Which of the following expressions is/are equivalent to result? i. (!(x<y)!done) (x<=0) ii.!(x<y && done) &&!(x>0) iii.!(x<y)!done &&!(x>0) a) i b) ii c) iii d) Both b) and c) are correct. e) None of the above. 11. What is the output of the following code fragment? char[][] arrchar = new char[6][3]; for(int i=0; i<6; i++) { for(int j=0; j<3; j++) { arrchar[i][j] = 'A'; for(int k=0; k<arrchar.length; k++) { System.out.print(arrChar[k].length + " "); System.out.print("\n"); a) b) c) A A A A A A d) A A A e) None of the above. Page 6 of 18

7 Name: 12. Assume that the 2D array of integers mystery has been initialized with the following values: mystery What is the output of the following code fragment? a) b) c) d) e) for(int m=0; m<3; m++) { System.out.print(mystery[m][m] + " "); System.out.print("\n"); Page 7 of 18

8 Part II: A Telephone Directory Program 1. [13] You have a client named Anne, who wants to store a simple phone directory in her computer. She wants to use it for storage and retrieval of names and numbers. A data file that contains the names and numbers of her friends already exists. Anne wants to be able to insert new numbers, change the number for an entry, and retrieve selected telephone numbers. She also wants to save any changes to her data file. a. [3] Identify and write three use cases for the telephone directory program. b. [2] Identify two classes (besides a UserInterface) that should be a part of the solution. Page 8 of 18

9 Name: c. [8] Draw a class diagram for the two classes you identified in part b. Identify a constructor, at least one instance variable and at least two services, including the return values and needed parameters, for each class. You do not need to show the relationship of these classes. Page 9 of 18

10 Part III: The DoubleList Class 2. [8] Consider a class DoubleList that stores doubles. The partially filled array elements is used to store the data. The array can be resized as necessary. DoubleList provides the following services: Service Purpose Usage Example get Returns the element at the specified index. int val = dlist.get(3); size Returns the number of elements in the list. if (dlist.size() > 10) isempty Returns true if the list has zero elements, if (dlist.isempty()) false otherwise. clear Removes all elements from the list. dlist.clear(); indexof Returns the first index where the given value int p = this. indexof(10.5); can be found, -1 if it isn t present. add Add the given element to the end of the list. dlist.add(9.8); delete Delete the data at the given position. The order of the array need not be preserved. dlist.delete(2); public class DoubleList extends Object { private static final int INITIAL_SIZE = 30; private double[] elements = new double[initial_size]; private int numelements = 0; public DoubleList() { super(); //all other methods are implemented here a. [2] Write a signature for a new method of the DoubleList class called allsmallerthan which addresses the following use case: Determine and return all numbers that are smaller than a given number. Page 10 of 18

11 Name: b. [6] Implement a helper method called resizelist (see the signature below) which increases or decreases the size of the array to a given size and keeps all its elements unchanged. If the number of valid elements in the array is bigger than newlength, the method throws an Error with the error message "Cannot resize the list". private void resizelist(int newlength) { Page 11 of 18

12 3. [11] John, a first year student, was asked to familiarize himself with the services of the DoubleList class. Inside the main method of DoubleList, he was instructed to write some code that prints out every second item in the list. public class DoubleList extends Object { // instance variables, constructor, and methods here public static void main(string[]args) { /* a */ DoubleList testlist = new DoubleList(); for (int i=0; i<10; i++) { testlist.add(i*10); /* b */ // John will put his code here a. [5] Draw an object diagram with the content of the testlist after the code between the comments /* a */ and /* b */ is executed. Page 12 of 18

13 Name: b. [6] The following two code fragments represent John s attempts in accomplishing the task outlined above. For each code fragment, state what the output is. If an error arises or the output does not match the expected output (10.0; 30.0; 50.0; 70.0; 90.0;), briefly explain how to fix the code fragment. Code Fragment 1 for (int i=0; i<testlist.numelements; i++) { int pos = i*2+1; System.out.print(testList.elements[pos] + "; "); Output of Code Fragment 1. Include errors and how to fix them, if any. Code Fragment 2 boolean done = false; int pos = 0; int i = 0; while(!done) { pos = i*2 + 1; if (pos >= testlist.numelements) { done = true; else { System.out.print(testList.elements[pos] + "; "); i++; Output of Code Fragment 2. Include errors and how to fix them, if any. Page 13 of 18

14 Part IV: Using DoubleList 4. [12] Assume your teacher uses DoubleList to store all marks for this exam. She uses it only for computing some statistics, so storing information about the students (other than their marks) is not necessary. She designed the following class: public class MyClass extends Object { DoubleList cs132 = new DoubleList(); public MyClass(String filename) { // constructor implementation here // other methods Input files containing exam marks are of the following format: each line contains a mark, followed by a space, followed by the student s userid. A sample file is shown below abraun 34.5 phsmith 98.1 c2morgan 0 dwhite 66.4 w7lu... One possible file. <mark> <userid> <mark> <userid> <mark> <userid> <mark> <userid> <mark> <userid>... File Format for all files. a. [8] Write the MyClass constructor. It reads all numbers from an input file as described above and stores them in the DoubleList object. Your solution must read the file once only. Assume that the file is properly formatted according to the format given above. public MyClass(String filename) { Page 14 of 18

15 Name: b. [2] Assume for a moment that the DoubleList class is not available. Can the teacher use ArrayList available in the Java package java.util instead? If yes, briefly explain i) if and how the instance variables in MyClass must change and ii) if and how the constructor must change. If no, explain why not. c. [2] If both ArrayList and DoubleList are not available, can she still implement MyClass? If yes, briefly explain i) if and how the instance variables in MyClass must change and ii) if and how the constructor must change. If no, explain why not. Page 15 of 18

16 5. [9] Let us suppose that the teacher wants to remove all zeros from the marks list, because they represent students who did not write the exam. Hence, she designed a method in MyClass called public int didnotwrite() that removes all zeros from the list and returns the number of zeros removed. The method preserves the existing order of the array. This method would be called as in the following (where m is a MyClass object): int numzeros = m.didnotwrite(); List three situations (different than the example below) which should be tested (and passed) to have reasonable evidence that an implemented version of didnotwrite works properly. Also include the type of the situation (i.e. boundary, typical, error) being tested in each call: Values in the list (description of the test) Example: (testing more than one 0 in the list) Expected Result Type of Test typical Page 16 of 18

17 Name: 6. [10] The teacher needs to implement a method of MyClass that will delete a specific item from the list of marks. The method returns true if the item was removed from the list and it returns false otherwise (i.e. the item to delete was not found in the list). The order of items in the list does not need to be preserved. a. [5] Write the pseudo-code for the method body. Use Stepwise Refinement as required. b. [5] Implement the delete method. public class MyClass extends Ojbect { // instance variables, constructor, and other methods here public boolean delete(double item) { Page 17 of 18

18 Extra Space for Rough Work Page 18 of 18