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Copyright © 2014 The College Board. College Board, Advanced Placement Program, AP, and the acorn logo are registered trademarks of the College Board. All other products and services may be trademarks of their respective owners. Visit the College Board on the W eb: www.collegeboard.org . Practice Exam The questions contained in this AP ® Computer Science A Practice Exam are written to the content specifications of AP Exams for this subject. Taking this practice exam should provide students with an idea of their general areas of strengths and weaknesses i n preparing for the actual AP Exam. Because this AP Computer Science A Practice Exam has never been administered as an operational AP Exam, statistical data are not available for calculating potential raw scores or conversions into AP scores. This AP Computer Science A Practice Exam is provided by the College Board for AP Exam preparation. Teachers are permitted to download the materials and make copies to use with their students in a classroom setting only. To maintain the security of this exam, teachers should collect all materials after their administration and keep them in a secure location. Teachers may not redistribute the files electronically for any reason.
Contents Directions for Administration............................................................................................ ii Section I: Multiple- Choice Questions................................................................................ 1 Section II: Free- Response Questions ...............................................................................42 Student Answer Sheet for Multiple-Choice Section ........................................................56 Multiple -Choice Answer Key.......................................................................................... 57 Free -Response Scoring Guidelines.................................................................................. 58 L
AP ® Computer Science A Directions for Administration The AP Computer Science A Exam is three hours in length and consists of a multiple- choice section and a free- response section. • The 75-minute multiple -choice section contains 40 questions and accounts for 50 percent of the final grade. • The 105-minute free- response section contains 4 questions and accounts for 50 percent of the final grade. Students should be given a 10-minute warning prior to the end of e ach section of the exam. A 10-minute break should be provided after Section I is completed. The act ual AP Exam is administered in one session. Students will have the most realistic experience i f a complete morning or afternoon is available to administer this practice exam. If a schedule does not permit one time period for the entire practice exam administration, it would be acceptable to administer Section I one day and Section II on a subsequent day. The total score on the multiple -choice section is based only on the number of questions answered correctly. Points are not deducted for incorrect answers or unanswered questions. • The use of calculators, or any other electronic devices, is not permitted during the exam . • It is suggested that the practice exam be completed using a pencil to simulate an actual administration. • Teachers will need to provide paper for the students to write their free- response answers. Teachers should provide directions to the students indicating how they wish the responses to be labeled so the teacher will be able to associate the student ’s response with the question the stud ent intended to answer. • The AP Computer Science A Exam includes a Quick Reference to the Java Library methods that may be tested on the exam . The Java Quick Reference appears at the end of each section of the exam. • Remember that students are not allowed to remove any materials, including scratch work , from the testing site. LL
Section I Multiple-Choice Questions
GO ON TO THE NEXT PAGE. COMPUTER SCIENCE A SECTION I Time — 1 hour and 15 minutes Number of questions — 40 Percent of total grade — 50 Directions: Determine the answer to each of the following qu estions or incomplete statements, using the available space for any necessary scratch work. Then decide which is the best of the choices given and fill in the corresponding box on the student answer sheet. No credit will be given for anything written in the examination booklet. Do not spend too much time on any one problem. Notes : • Assume that the classes listed in the Java Quic k Reference have been imported where appropriate. • Assume that declarations of variables and methods appear within the context of an enclosing class. • Assume that method calls that are not prefixed with an object or class name and are not shown within a complete class definition appear within the context of an enclosing class. • Unless otherwise noted in the question, assu me that parameters in method calls are not null and that methods are called only when their preconditions are satisfied. 2
GO ON TO THE NEXT PAGE. 1. Consider the following method. public static int mystery(int[] arr) { int x = 0; for (int k = 0; k < arr.length; k = k + 2) x = x + arr[k]; return x; } Assume that the array nums has been declared and initialized as follows. int[] nums = {3, 6, 1, 0, 1, 4, 2}; What value will be returned as a result of the call mystery(nums) ? (A) 5 (B) 6 (C) 7 (D) 10 (E) 17 3
GO ON TO THE NEXT PAGE. Questions 2-3 refer to the following information. Consider the following partial class declaration. public class SomeClass { private int myA; private int myB; private int myC; // Constructor(s) not shown public int getA() { return myA; } public void setB(int value) { myB = value; } } 2. The following declaration appears in another class. SomeClass obj = new SomeClass(); Which of the following code segments will compile without error? (A) int x = obj.getA(); (B) int x; obj.getA(x); (C) int x = obj.myA; (D) int x = SomeClass.getA(); (E) int x = getA(obj); 3. Which of the following changes to SomeClass will allow other classes to access but not modify the value of myC ? (A) Make myC public. (B) Include the method: public int getC() { return myC; } (C) Include the method: private int getC() { return myC; } (D) Include the method: public void getC(int x) { x = myC; } (E) Include the method: private void getC(int x) { x = myC; } 4
GO ON TO THE NEXT PAGE. 4. Consider the following code segment. int x = 7; int y = 3; if ((x < 10) && (y < 0)) System.out.println("Value is: " + x * y); else System.out.println("Value is: " + x / y); What is printed as a result of executing the code segment? (A) Value is: 21 (B) Value is: 2.3333333 (C) Value is: 2 (D) Value is: 0 (E) Value is: 1 5
GO ON TO THE NEXT PAGE. 5. Consider the following method. public ArrayList mystery(int n) { ArrayList seq = new ArrayList(); for (int k = 1; k
GO ON TO THE NEXT PAGE. 6. Consider the following method that is intended to determine if the double values d1 and d2 are close enough to be considered equa l. For example, given a tolerance of 0.001, the values 54.32271 and 54.32294 would be considered equal. /** @return true if d1 and d2 are within the specified tolerance, * false otherwise */ public boolean almostEqual(double d1, double d2, double tolerance) { /* missing code */ } Which of the following should replace /* missing code */ so that almostEqual will work as intended? (A) return (d1 - d2) = tolerance; (E) return Math.abs(d1 - d2)
GO ON TO THE NEXT PAGE. 7. Consider the following class declaration. public class Person { private String myName; private int myYearOfBirth; public Person(String name, int yearOfBirth) { myName = name; myYearOfBirth = yearOfBirth; } public String getName() { return myName; } public void setName(String name) { myName = name; } // There may be instance variables, constr uctors, and methods that are not shown. } Assume that the following declaration has been made. Person student = new Person("Thomas", 1995); Which of the following statements is th e most appropriate for changing the name of student from "Thomas" to "Tom" ? (A) student = new Person("Tom", 1995); (B) student.myName = "Tom"; (C) student.getName("Tom"); (D) student.setName("Tom"); (E) Person.setName("Tom"); 8
GO ON TO THE NEXT PAGE. 8. Consider the following class declaration. public class Student { private String myName; private int myAge; public Student() { /* implementation not shown */ } public Student(String name, int age) { /* implementation not shown */ } // No other constructors } Which of the following declarations will compile without error? I. Student a = new Student(); II. Student b = new Student("Juan", 15); III. Student c = new Student("Juan", "15"); (A) I only (B) II only (C) I and II only (D) I and III only (E) I, II, and III 9
GO ON TO THE NEXT PAGE. 9. Consider the following method that is intended to return the sum of the elements in the array key. public static int sumArray(int[] key) { int sum = 0; for (int i = 1; i
GO ON TO THE NEXT PAGE. Questions 10-11 refer to the following information. Consider the following instance vari able and methods. You may assume that data has been initialized with length > 0. The methods are intended to retu rn the index of an array element equal to target, or -1 if no such element exists. private int[] data; public int seqSearchRec(int target) { return seqSearchRecHelper(target, data.length - 1); } private int seqSearchRecHelper(int target, int last) { // Line 1 if (data[last] == target) return last; else return seqSearchRecHelper(target, last - 1); } 10. For which of the following test cases will the call seqSearchRec(5) always result in an error? I. data contains only one element. II. data does not contain the value 5. III. data contains the value 5 multiple times. (A) I only (B) II only (C) III only (D) I and II only (E) I, II, and III 11. Which of the following should be used to replace // Line 1 in seqSearchRecHelper so that seqSearchRec will work as intended? (A) if (last = 0) 11
GO ON TO THE NEXT PAGE. 12. Consider the following method. public String mystery(String input) { String output = ""; for (int k = 1; k < input.length(); k = k + 2) { output += input.substring(k, k + 1); } return output; } What is returned as a result of the call mystery("computer") ? (A) "computer" (B) "cmue" (C) "optr" (D) "ompute" (E) Nothing is returned because an IndexOutOfBoundsException is thrown. 12
GO ON TO THE NEXT PAGE. 13. Consider the following code segment. int[] arr = {7, 2, 5, 3, 0, 10}; for (int k = 0; k < arr.length - 1; k++) { if (arr[k] > arr[k + 1]) System.out.print(k + " " + arr[k] + " "); } What will be printed as a result of executing the code segment? (A) 0 2 2 3 3 0 (B) 0 7 2 5 3 3 (C) 0 7 2 5 5 10 (D) 1 7 3 5 4 3 (E) 7 2 5 3 3 0 13
GO ON TO THE NEXT PAGE. 14. Consider the following interface and class declarations. public interface Vehicle { /** @return the mileage traveled by this Vehicle */ double getMileage(); } public class Fleet { private ArrayList myVehicles; /** @return the mileage traveled by all vehicles in this Fleet */ public double getTotalMileage() { double sum = 0.0; for (Vehicle v : myVehicles) { sum += /* expression */ ; } return sum; } // There may be instance variables, constr uctors, and methods that are not shown. } Which of the following can be used to replace /* expression */ so that getTotalMileage returns the total of the miles traveled for all vehicles in the fleet? (A) getMileage(v) (B) myVehicles[v].getMileage() (C) Vehicle.get(v).getMileage() (D) myVehicles.get(v).getMileage() (E) v.getMileage() 14
GO ON TO THE NEXT PAGE. 15. Consider the following method, isSorted, which is intended to return true if an array of integers is sorted in nondecreasing order and to return false otherwise. /** @param data an array of integers * @return true if the values in the array appear in sorted (nondecreasing) order */ public static boolean isSorted(int[] data) { /* missing code */ } Which of the following can be used to replace /* missing code */ so that isSorted will work as intended? I. for (int k = 1; k < data.length; k++) { if (data[k - 1] > data[k]) return false; } return true; II. for (int k = 0; k < data.length; k++) { if (data[k] > data[k + 1]) return false; } return true; III. for (int k = 0; k < data.length - 1; k++) { if (data[k] > data[k + 1]) return false; else return true; } return true; (A) I only (B) II only (C) III only (D) I and II only (E) I and III only 15
GO ON TO THE NEXT PAGE. 16. Consider the following incomplete method that is intended to return an array that contains the contents of its first array parameter followed by the contents of its second array parameter. public static int[] append(int[] a1, int[] a2) { int[] result = new int[a1.length + a2.length]; for (int j = 0; j < a1.length; j++) result[j] = a1[j]; for (int k = 0; k < a2.length; k++) result[ /* index */ ] = a2[k]; return result; } Which of the following expressions can be used to replace /* index */ so that append will work as intended? (A) j (B) k (C) k + a1.length - 1 (D) k + a1.length (E) k + a1.length + 1 16
GO ON TO THE NEXT PAGE. 17. Consider the following code segment. int[] arr = {1, 2, 3, 4, 5, 6, 7}; for (int k = 3; k < arr.length - 1; k++) arr[k] = arr[k + 1]; Which of the following represents the contents of arr as a result of executing the code segment? (A) {1, 2, 3, 4, 5, 6, 7} (B) {1, 2, 3, 5, 6, 7} (C) {1, 2, 3, 5, 6, 7, 7} (D) {1, 2, 3, 5, 6, 7, 8} (E) {2, 3, 4, 5, 6, 7, 7} 17
GO ON TO THE NEXT PAGE. 18. Assume that myList is an ArrayList that has been correctly constructed and populated with objects. Which of the following expressions produces a valid random index for myList ? (A) (int)( Math.random() * myList.size() ) - 1 (B) (int)( Math.random() * myList.size() ) (C) (int)( Math.random() * myList.size() ) + 1 (D) (int)( Math.random() * (myList.size() + 1) ) (E) Math.random(myList.size()) 18
GO ON TO THE NEXT PAGE. 19. Assume that a and b have been defined and initialized as int values. The expression !(!(a != b ) && (b > 7)) is equivalent to which of the following? (A) (a != b) || (b < 7) (B) (a != b) || (b
GO ON TO THE NEXT PAGE. 20. Consider the following method. public static void arrayMethod(int nums[]) { int j = 0; int k = nums.length - 1; while (j < k) { int x = nums[j]; nums[j] = nums[k]; nums[k] = x; j++; k--; } } Which of the following describes what the method arrayMethod() does to the array nums? (A) The array nums is unchanged. (B) The first value in nums is copied to every location in the array. (C) The last value in nums is copied to every location in the array. (D) The method generates an ArrayIndexOutOfBoundsException . (E) The contents of the array nums are reversed. 20
GO ON TO THE NEXT PAGE. 21. Consider the following method, which is intended to return the element of a 2-dimensional array that is closest in value to a specified number, val. /** @return the element of 2-dimensional array mat whose value is closest to val */ public double findClosest(double[][] mat, double val) { double answer = mat[0][0]; double minDiff = Math.abs(answer - val); for (double[] row : mat) { for (double num : row) { if ( /* missing code */ ) { answer = num; minDiff = Math.abs(num - val); } } } return answer; } Which of the following could be used to replace /* missing code */ so that findClosest will work as intended? (A) val - row[num] < minDiff (B) Math.abs(num - minDiff) < minDiff (C) val - num < 0.0 (D) Math.abs(num - val) < minDiff (E) Math.abs(row[num] - val) < minDiff 21
GO ON TO THE NEXT PAGE. 22. Consider the following Book and AudioBook classes. public class Book { private int numPages; private String bookTitle; public Book(int pages, String title) { numPages = pages; bookTitle = title; } public String toString() { return bookTitle + " " + numPages; } public int length() { return numPages; } } public class AudioBook extends Book { private int numMinutes; public AudioBook(int minutes, int pages, String title) { super(pages, title); numMinutes = minutes; } public int length() { return numMinutes; } public double pagesPerMinute() { return ((double) super.length()) / numMinutes; } } 22
GO ON TO THE NEXT PAGE. Consider the following code segmen t that appears in a class other than Book or AudioBook. Line 1: Book[] books = new Book[2]; Line 2: books[0] = new AudioBook(100, 300, "The Jungle"); Line 3: books[1] = new Book(400, "Captains Courageous"); Line 4: System.out.println(books[0].pagesPerMinute()); Line 5: System.out.println(books[0].toString()); Line 6: System.out.println(books[0].length()); Line 7: System.out.println(books[1].toString()); Which of the following best explains why the code segment will not compile? (A) Line 2 will not compile because variables of type Book may not refer to variables of type AudioBook. (B) Line 4 will not compile because variables of type Book may only call methods in the Book class. (C) Line 5 will not compile because the AudioBook class does not have a method named toString declared or implemented. (D) Line 6 will not compile because the statemen t is ambiguous. The compiler cannot determine which length method should be called. (E) Line 7 will not compile because the element at index 1 in the array named books may not have been initialized. 23
GO ON TO THE NEXT PAGE. 23. Consider the following instance variable and method. private List animals; public void manipulate() { for (int k = animals.size() - 1; k > 0; k--) { if (animals.get(k).substring(0, 1).equals("b")) { animals.add(animals.size() - k, animals.remove(k)); } } } Assume that animals has been instantiated and initialized with the following contents. ["bear", "zebra", "bass", "cat", "koala", "baboon"] What will the contents of animals be as a result of calling manipulate ? (A) ["baboon", "zebra", "bass", "cat", "bear", "koala"] (B) ["bear", "zebra", "bass", "cat", "koala", "baboon"] (C) ["baboon", "bear", "zebra", "bass", "cat", "koala"] (D) ["bear", "baboon", "zebra", "bass", "cat", "koala"] (E) ["zebra", "cat", "koala", "baboon", "bass", "bear"] 24
GO ON TO THE NEXT PAGE. 24. Consider the following code segment. int[] oldArray = {1, 2, 3, 4, 5, 6, 7, 8, 9}; int[][] newArray = new int[3][3]; int row = 0; int col = 0; for (int value : oldArray) { newArray[row][col] = value; row++; if ((row % 3) == 0) { col++; row = 0; } } System.out.println(newArray[0][2]); What is printed as a result of executing the code segment? (A) 3 (B) 4 (C) 5 (D) 7 (E) 8 25
GO ON TO THE NEXT PAGE. 25. A rectangular box fits inside another rectangular box if and only if the height, width, and depth of the smaller box are each less than the corresponding values of th e larger box. Consider the following three interface declarations that are intended to represent information necessary for rectangular boxes. I. public interface RBox { /** @return the height of this RBox */ double getHeight(); /** @return the width of this RBox */ double getWidth(); /** @return the depth of this RBox */ double getDepth(); } II. public interface RBox { /** @return true if the height of this RBox is less than the height of other; * false otherwise */ boolean smallerHeight(RBox other); /** @return true if the width of this RBox is less than the width of other; * false otherwise */ boolean smallerWidth(RBox other); /** @return true if the depth of this RBox is less than the depth of other; * false otherwise */ boolean smallerDepth(RBox other); } III. public interface RBox { /** @return the surface area of this RBox */ double getSurfaceArea(); /** @return the volume of this RBox */ double getVolume(); } Which of the interfaces, if correctly implemented by a Box class, would be sufficient functionality for a user of the Box class to determine if one Box can fit inside another? (A) I only (B) II only (C) III only (D) I and II only (E) I, II, and III 26
GO ON TO THE NEXT PAGE. 26. Assume that the array arr has been defined and initialized as follows. int[] arr = /* initial values for the array */ ; Which of the following will correctly pr int all of the odd integers contained in arr but none of the even integers contained in arr ? (A) for (int x : arr) if (x % 2 == 1) System.out.println(x); (B) for (int k = 1; k < arr.length; k++) if (arr[k] % 2 == 1) System.out.println(arr[k]); (C) for (int x : arr) if (x % 2 == 1) System.out.println(arr[x]); (D) for (int k = 0; k < arr.length; k++) if (arr[k] % 2 == 1) System.out.println(k); (E) for (int x : arr) if (arr[x] % 2 == 1) System.out.println(arr[x]); 27
GO ON TO THE NEXT PAGE. Questions 27-28 refer to the following method. public static int mystery(int n) { int x = 1; int y = 1; // Point A while (n > 2) { x = x + y; // Point B y = x – y; n--; } // Point C return x; } 27. What value is returned as a result of the call mystery(6) ? (A) 1 (B) 5 (C) 6 (D) 8 (E) 13 28. Which of the following is true of method mystery ? (A) x will sometimes be 1 at // Point B. (B) x will never be 1 at // Point C. (C) n will never be greater than 2 at // Point A. (D) n will sometimes be greater than 2 at // Point C. (E) n will always be greater than 2 at // Point B. 28
GO ON TO THE NEXT PAGE. 29. Consider the following code segment. for (int k = 1; k
GO ON TO THE NEXT PAGE. 30. Consider the following method. public static String scramble(String word, int howFar) { return word.substring(howFar + 1, word.length()) + word.substring(0, howFar); } What value is returned as a result of the call scramble("compiler", 3) ? (A) "compiler" (B) "pilercom" (C) "ilercom" (D) "ilercomp" (E) No value is returned because an IndexOutOfBoundsException will be thrown. 30
GO ON TO THE NEXT PAGE. 31. Consider the following method. public void mystery(int[] data) { for (int k = 0; k < data.length - 1; k++) data[k + 1] = data[k] + data[k + 1]; } The following code segment appears in another method in the same class. int[] values = {5, 2, 1, 3, 8}; mystery(values); for (int v : values) System.out.print(v + " "); System.out.println(); What is printed as a result of executing the code segment? (A) 5 2 1 3 8 (B) 5 7 3 4 11 (C) 5 7 8 11 19 (D) 7 3 4 11 8 (E) Nothing is printed because an ArrayIndexOutOfBoundsException is thrown during the execution of method mystery. 31
GO ON TO THE NEXT PAGE. 32. Consider the following method. public int compute(int n, int k) { int answer = 1; for (int i = 1; i
GO ON TO THE NEXT PAGE. 33. Consider the following code segment. int sum = 0; int k = 1; while (sum < 12 || k < 4) sum += k; System.out.println(sum); What is printed as a result of executing the code segment? (A) 6 (B) 10 (C) 12 (D) 15 (E) Nothing is printed due to an infinite loop. 33
GO ON TO THE NEXT PAGE. 34. Consider the following class declarations. public class Point { private double x; // x-coordinate private double y; // y-coordinate public Point() { x = 0; y = 0; } public Point(double a, double b) { x = a; y = b; } // There may be instance variables, constr uctors, and methods that are not shown. } public class Circle { private Point center; private double radius; /** Constructs a circle where (a, b) is the center and r is the radius. */ public Circle(double a, double b, double r) { /* missing code */ } } Which of the following replacements for /* missing code */ will correctly implement the Circle constructor? I. center = new Point(); radius = r; II. center = new Point(a, b); radius = r; III. center = new Point(); center.x = a; center.y = b; radius = r; (A) I only (B) II only (C) III only (D) II and III only (E) I, II, and III 34
GO ON TO THE NEXT PAGE. 35. Consider the following code segment. int num = 2574; int result = 0; while (num > 0) { result = result * 10 + num % 10; num /= 10; } System.out.println(result); What is printed as a result of executing the code segment? (A) 2 (B) 4 (C) 18 (D) 2574 (E) 4752 35
GO ON TO THE NEXT PAGE. 36. Consider the following method. public void test(int x) { int y; if (x % 2 == 0) y = 3; else if (x > 9) y = 5; else y = 1; System.out.println("y = " + y); } Which of the following test data sets would test each possible output for the method? (A) 8, 9, 12 (B) 7, 9, 11 (C) 8, 9, 11 (D) 8, 11, 13 (E) 7, 9, 10 36
GO ON TO THE NEXT PAGE. 37. Consider the following code segment. int x = 1; while ( /* missing code */ ) { System.out.print(x + " "); x = x + 2; } Consider the following possible replacements for /* missing code */. I. x < 6 II. x != 6 III. x < 7 Which of the proposed replacements for /* missing code */ will cause the code segment to print only the values 1 3 5 ? (A) I only (B) II only (C) I and II only (D) I and III only (E) I, II, and III 37
GO ON TO THE NEXT PAGE. 38. Assume that x and y have been declared and initialized with int values. Consider the following Java expression. (y > 10000) || (x > 1000 && x < 1500) Which of the following is equiva lent to the expression given above? (A) (y > 10000 || x > 1000) && (y > 10000 || x < 1500) (B) (y > 10000 || x > 1000) || (y > 10000 || x < 1500) (C) (y > 10000) && (x > 1000 || x < 1500) (D) (y > 10000 && x > 1000) || (y > 10000 && x < 1500) (E) (y > 10000 && x > 1000) && (y > 10000 && x < 1500) 38
GO ON TO THE NEXT PAGE. 39. Consider the following recursive method. public int recur(int n) { if (n
40. Consider the following recursive method. public static void whatsItDo(String str) { int len = str.length(); if (len > 1) { String temp = str.substring(0, len – 1); whatsItDo(temp); System.out.println(temp); } } What is printed as a result of the call whatsItDo("WATCH") ? (A) WATC WAT WA W (B) WATCH WATC WAT WA (C) W WA WAT WATC (D) W WA WAT WATC WATCH (E) WATCH WATC WAT WA W WA WAT WATC WATCH END OF SECTION I IF YOU FINISH BEFORE TIME IS CALLED, YOU MAY CHECK YOUR WORK ON THIS SECTION. DO NOT GO ON TO SECTION II UNTIL YOU ARE TOLD TO DO SO. 40
Java Quick Reference Accessible Methods from the Java Library That May Be Included on the Exam class java.lang.Object • boolean equals(Object other) • String toString() class java.lang.Integer • Integer(int value) • int intValue() • Integer.MIN_VALUE // minimum value represented by an int or Integer • Integer.MAX_VALUE // maximum value represented by an int or Integer class java.lang.Double • Double(double value) • double doubleValue() class java.lang.String • int length() • String substring(int from, int to) // returns the substring beginning at from // and ending at to-1 • String substring(int from) // returns substring(from, length()) • int indexOf(String str) // returns the index of the first occurrence of str; // returns -1 if not found • int compareTo(String other) // returns a value < 0 if this is less than other // returns a value = 0 if this is equal to other // returns a value > 0 if this is greater than other class java.lang.Math • static int abs(int x) • static double abs(double x) • static double pow(double base, double exponent) • static double sqrt(double x) • static double random() // returns a double in the range [0.0, 1.0) interface java.util.List • int size() • boolean add(E obj) // appends obj to end of list; returns true • void add(int index, E obj) // inserts obj at position index (0size) ££ index , // moving elements at position index and higher // to the right (adds 1 to their indices) and adjusts size • E get(int index) • E set(int index, E obj) // replaces the element at position index with obj // returns the element formerly at the specified position • E remove(int index) // removes element from position index, moving elements // at position index + 1 and higher to the left // (subtracts 1 from their indices) and adjusts size // returns the element formerly at the specified position class java.util.ArrayList implements java.util.List
Section II Free-Response Questions
GO ON TO THE NEXT PAGE. COMPUTER SCIENCE A SECTION II Time — 1 hour and 45 minutes Number of questions — 4 Percent of total grade — 50 Directions: SHOW ALL YOUR WORK. REMEMBER THAT PROGRAM SEGMENTS ARE TO BE WRITTEN IN JAVA. Notes : • Assume that the classes listed in the Java Quic k Reference have been imported where appropriate. • Unless otherwise noted in the question, assu me that parameters in method calls are not null and that methods are called only when their preconditions are satisfied. • In writing solutions for each question, you may use any of the accessible methods that are listed in classes defined in that question. Writing significant amounts of code that can be replaced by a call to one of these methods will not receive full credit. 1. Consider the following partial declaration for a WordScrambler class. The constructor for the WordScrambler class takes an even-length array of String objects and initializes the instance variable scrambledWords. public class WordScrambler { private String[] scrambledWords; /** @param wordArr an array of String objects * Precondition : wordArr.length is even */ public WordScrambler(String[] wordArr) { scrambledWords = mixedWords(wordArr); } /** @param word1 a String of characters * @param word2 a String of characters * @return a String that contains the first half of word1 and the second half of word2 */ private String recombine(String word1, String word2) { /* to be implemented in part (a) */ } /** @param words an array of String objects * Precondition : words.length is even * @return an array of String objects created by recombining pairs of strings in array words * Postcondition : the length of the returned array is words.length */ private String[] mixedWords(String[] words) { /* to be implemented in part (b) */ } // There may be instance variables, constr uctors, and methods that are not shown. } 43
GO ON TO THE NEXT PAGE. (a) Write the WordScrambler method recombine. This method returns a String created from its two String parameters as follows. • take the first half of word1 • take the second half of word2 • concatenate the two halves and return the new string. For example, the following table shows some results of calling recombine. Note that if a word has an odd number of letters, the second half of the word contains the extra letter. word1 word2 recombine(word1, word2) "apple" "pear" "apar" "pear" "apple" "peple" Complete method recombine below. /** @param word1 a String of characters * @param word2 a String of characters * @return a String that contains the first half of word1 and the second half of word2 */ private String recombine(String word1, String word2) (b) Write the WordScrambler method mixedWords. This method creates and returns a new array of String objects as follows. It takes the first pair of strings in words and combines them to produce a pair of strings to be included in the array returned by the method. If this pair of strings consists of w1 and w2, the method should include the result of calling recombine with w1 and w2 as arguments and should also include the result of calling recombine with w2 and w1 as arguments. The next two strings, if they exist, would form the next pair to be processed by this method. The method should continue until all the strings in words have been processed in this way and the new array has been filled. For example, if the array words contains the following elements: {"apple", "pear", "this", "cat"} then the call mixedWords(words) should return the following array. {"apar", "peple", "that", "cis"} In writing mixedWords, you may call recombine. Assume that recombine works as specified, regardless of what you wrote in part (a). Complete method mixedWords below. /** @param words an array of String objects * Precon dition: words.length is even * @return an array of String objects created by recombining pairs of strings in array words * Postcondition : the length of the returned array is words.length */ private String[] mixedWords(String[] words) 44
GO ON TO THE NEXT PAGE. 2. An array of positive integer values has the mountain property if the elements are ordered such that successive values increase until a maximum value (the peak of the mountain) is reached and then the successive values decrease. The Mountain class declaration shown below contains methods that can be used to determine if an array has the mountain property. You will implement two methods in the Mountain class. public class Mountain { /** @param array an array of positive integer values * @param stop the last index to check * Precondition: 0 £stop < array.length * @return true if for each j such that 0 £j < stop, array[j] < array[j + 1] ; * false otherwise */ public static boolean isIncreasing(int[] array, int stop) { /* implementation not shown */ } /** @param array an array of positive integer values * @param start the first index to check * Precondition: 0 £start < array.length - 1 * @return true if for each j such that start £j < array.length - 1 , * array[j] > array[j + 1] ; * false otherwise */ public static boolean isDecreasing(int[] array, int start) { /* implementation not shown */ } /** @param array an array of positive integer values * Precondition: array.length > 0 * @return the index of the first peak (local ma ximum) in the array, if it exists; * -1 otherwise */ public static int getPeakIndex(int[] array) { /* to be implemented in part (a) */ } /** @param array an array of positive integer values * Precondition: array.length > 0 * @return true if array contains values ordered as a mountain; * false otherwise */ public static boolean isMountain(int[] array) { /* to be implemented in part (b) */ } // There may be instance variables, constr uctors, and methods that are not shown. } 45
GO ON TO THE NEXT PAGE. (a) Write the Mountain method getPeakIndex. Method getPeakIndex returns the index of the first peak found in the parameter array, if one exists. A peak is defined as an element whose value is greater than the value of the element immediately before it and is also greater than the value of the element immediately after it. Method getPeakIndex starts at the beginning of the array and returns the index of the first peak that is found or -1 if no peak is found. For example, the following table illust rates the results of several calls to getPeakIndex. arr getPeakIndex(arr) {11, 22, 33, 22, 11} 2 {11, 22, 11, 22, 11} 1 {11, 22, 33, 55, 77} -1 {99, 33, 55, 77, 120} -1 {99, 33, 55, 77, 55} 3 {33, 22, 11} -1 Complete method getPeakIndex below. /** @param array an array of positive integer values * Precondition: array.length > 0 * @return the index of the first peak (local ma ximum) in the array, if it exists; * -1 otherwise */ public static int getPeakIndex(int[] array) 46
GO ON TO THE NEXT PAGE. (b) Write the Mountain method isMountain. Method isMountain returns true if the values in the parameter array are ordered as a mountain; otherwise, it returns false. The values in array are ordered as a mountain if all thr ee of the following conditions hold. • There must be a peak. • The array elements with an index smaller than the peak’s index must appear in increasing order. • The array elements with an index larger than th e peak’s index must appear in decreasing order. For example, the following table illust rates the results of several calls to isMountain. arr isMountain(arr) {1, 2, 3, 2, 1} true {1, 2, 1, 2, 1} false {1, 2, 3, 1, 5} false {1, 4, 2, 1, 0} true {9, 3, 5, 7, 5} false {3, 2, 1} false In writing isMountain, assume that getPeakIndex works as specified, regardless of what you wrote in part (a). Complete method isMountain below. /** @param array an array of positive integer values * Precondition: array.length > 0 * @return true if array contains values ordered as a mountain; * false otherwise */ public static boolean isMountain(int[] array) 47
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