Determine the answer to each of the following questions, using the available space for any necessary scratchwork.

Transcription

1 AP Computer Science Jan 2016 Semester 1 Examination Part I: Multiple Choice Suggested time: 60 minutes. Mr. Alwin Tareen BNDS Number of questions: 18. Percent of total grade: 50. Determine the answer to each of the following questions, using the available space for any necessary scratchwork. Decide which is the best of the choices given, and fill in the corresponding bubble in the provided answer booklet. Assume that the classes in the Quick Reference have been imported where needed. Assume that variables and methods are declared within the context of an enclosing class. 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) 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.

2 APCS/S1E Page 2 of 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: (c) Value is: 1 (d) Value is: 0 (e) Value is: 2 4. Consider the following method: public ArrayList<Integer> mystery(int n) ArrayList<Integer> seq = new ArrayList<Integer>(); for (int k = 1; k <= n; k++) seq.add(new Integer(k*k + 3)); return seq; Which of the following is printed as a result of executing the following statement? System.out.println(mystery(6)); (a) [3, 4, 7, 12, 19, 28] (b) [3, 4, 7, 12, 19, 28, 39] (c) [39, 28, 19, 12, 7, 4] (d) [39, 28, 19, 12, 7, 4, 3] (e) [4, 7, 12, 19, 28, 39] 5. Consider the following code segment. List<String> list = new ArrayList<String>(); list.add("p"); list.add("q"); list.add("r"); list.set(2, "s"); list.add(2, "T"); list.add("u"); What are the contents of list after executing the code segment? (a) [P,Q,R,s,T] (b) [P,Q,s,T,u] (c) [P,Q,T,s,u] (d) [P,T,Q,s,u] (e) [P,T,s,R,u]

3 APCS/S1E Page 3 of 15 Questions 6 and 7 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; 6. 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); 7. 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 APCS/S1E Page 4 of 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 9. 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) (b) (c) (d) (e) 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.

5 APCS/S1E Page 5 of 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 print all of the odd integers contained in arr but none of the even integers contained in arr? (a) (b) (c) (d) (e) for (int x : arr) if (x%2 == 1) System.out.println(x); for (int k = 1; k < arr.length; k++) if (arr[k]%2 == 1) System.out.println(arr[k]); for (int x : arr) if (x%2 == 1) System.out.println(arr[x]); for (int k = 0; k < arr.length; k++) if (arr[k]%2 == 1) System.out.println(k); for (int x : arr) if (arr[x]%2 == 1) System.out.println(arr[x]); 12. 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

6 APCS/S1E Page 6 of 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, constructors, 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 the 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"); 14. Assume that x and y are boolean variables and have been properly initialized. (x y) && x Which of the following always evaluates to the same value as the expression above? (a) x (b) y (c) x && y (d) x y (e) x!= y 15. Consider the following method, which is intended to return true if at least one of the three strings s1, s2 or s3 contains the substring "art". Otherwise, the method should return false. public static boolean containsart(string s1, String s2, String s3) String all = s1 + s2 + s3; return (all.indexof("art")!= -1);

7 APCS/S1E Page 7 of 15 Which of the following method calls demonstrates that the method does not work as intended? (a) containsart("rattrap", "similar", "today") (b) containsart("start", "article", "Bart") (c) containsart("harm", "chortle", "crowbar") (d) containsart("matriculate", "carat", "arbitrary") (e) containsart("darkroom", "cartoon", "articulate") Questions 16, 17 and 18 refer to the Time class declared below: public class Time private int myhrs; private int mymins; private int mysecs; public Time() /* implementation not shown */ public Time(int h, int m, int s) /* implementation not shown */ // Resets time to myhrs = h, mymins = m, mysecs = s. public void resettime(int h, int m, int s) /* implementation not shown */ // Advances time by one second. public void increment() /* implementation not shown */ // Returns true if this time equals t, false otherwise. public boolean equals(time t) /* implementation not shown */ // Returns true if this time is earlier than t, false otherwise. public boolean lessthan(time t) /* implementation not shown */ // Returns time as a String in the form hrs:mins:secs. public String tostring() /* implementation not shown */

8 APCS/S1E Page 8 of Which of the following is a false statement about the methods? (a) equals, lessthan, and tostring are all accessor methods. (b) increment is a mutator method. (c) Time() is the default constructor. (d) The Time class has three constructors. (e) There are no static methods in this class. 17. Which of the following represents correct implementation code for the constructor with parameters? (a) (b) myhrs = 0; mymins = 0; mysecs = 0; myhrs = h; mymins = m; mysecs = s; (c) resettime(myhrs, mymins, mysecs); (d) h = myhrs; m = mymins; s = mysecs; (e) Time = new Time(h, m, s); 18. A client class has a display method that writes the time represented by its parameter: // Outputs time t in the form hrs:mins:secs. public void display(time t) /* method body */ Which of the following are correct replacements for /* method body */? I Time T = new Time(h, m, s); System.out.println(T); II System.out.println(t.myHrs + ":" + t.mymins + ":" + t.mysecs); III System.out.println(t); (a) I only (b) II only (c) III only (d) II and III only (e) I, II and III

9 APCS/S1E Page 9 of 15 Part II: Free Response Suggested time: 60 minutes. Number of questions: 2. Percent of total grade: 50. SHOW ALL YOUR WORK. REMEMBER THAT PROGRAM SEGMENTS ARE TO BE WRITTEN IN JAVA. Assume that the classes listed in the Quick Reference have been imported where appropriate. Unless otherwise noted in the question, assume 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 may not receive full credit. MusicDownloads question(2013 AP CompSci Free Response) 1. A music Web site keeps track of downloaded music. For each download, the site uses a DownloadInfo object to store a song s title and the number of times it has been downloaded. A partial declaration for the DownloadInfo class is shown below. p u b l i c c l a s s DownloadInfo / C r e a t e s a new i n s t a n c e with the g i v e n unique t i t l e and s e t s the number o f t i m e s downloaded to t i t l e the unique t i t l e o f the downloaded song / p u b l i c DownloadInfo ( S t r i n g t i t l e ) / i m p l e m e n t a t i o n not shown / r e t u r n the t i t l e / p u b l i c S t r i n g g e t T i t l e ( ) / i m p l e m e n t a t i o n not shown / / I n c r e m e n t the number o f t i m e s downloaded by 1 / p u b l i c void incrementtimesdownloaded ( ) / i m p l e m e n t a t i o n not shown / // There may be i n s t a n c e v a r i a b l e s, c o n s t r u c t o r s, and methods // t h a t a r e not shown.

10 APCS/S1E Page 10 of 15 The list of downloaded information is stored in a MusicDownloads object. A partial declaration for the MusicDownloads class is shown below. p u b l i c c l a s s MusicDownloads / The l i s t o f downloaded i n f o r m a t i o n. Guaranteed not to be n u l l and not to c o n t a i n d u p l i c a t e t i t l e s. / p r i v a t e L i s t <DownloadInfo> d o w n l o a d L i s t ; / C r e a t e s the l i s t o f downloaded i n f o r m a t i o n. / p u b l i c MusicDownloads ( ) d o w n l o a d L i s t = new A r r a y L i s t <DownloadInfo >(); / Returns a r e f e r e n c e to the DownloadInfo o b j e c t with the r e q u e s t e d t i t l e i f i t e x i s t t i t l e the r e q u e s t e d t i t l r e t u r n a r e f e r e n c e to the DownloadInfo o b j e c t with the t i t l e t h a t matches the parameter t i t l e i f i t e x i s t s i n the l i s t ; n u l l o t h e r w i s e. P o s t c o n d i t i o n : no changes were made to d o w n l o a d L i s t. / p u b l i c DownloadInfo getdownloadinfo ( S t r i n g t i t l e ) / to be implemented i n p a r t ( a ) / / Updates d o w n l o a d L i s t with i n f o r m a t i o n from t i t l e t i t l e s a l i s t o f song t i t l e s P o s t c o n d i t i o n : t h e r e a r e no d u p l i c a t e t i t l e s i n d o w n l o a d L i s t. no e n t r i e s were removed from d o w n l o a d L i s t. a l l songs i n t i t l e s a r e r e p r e s e n t e d i n d o w n l o a d L i s t. f o r each e x i s t i n g e n t r y i n downloadlist, the download count i s i n c r e a s e d by the number o f t i m e s i t s t i t l e appeared i n t i t l e s. the o r d e r o f the e x i s t i n g e n t r i e s i n d o w n l o a d L i s t i s not changed. the f i r s t time an o b j e c t with a t i t l e from t i t l e s i s added to downloadlist, i t i s added to the end o f the l i s t. new e n t r i e s i n d o w n l o a d L i s t appear i n the same o r d e r i n which they f i r s t appear i n t i t l e s. f o r each new e n t r y i n downloadlist, the download count i s e q u a l to the number o f t i m e s i t s t i t l e appeared i n t i t l e s. / p u b l i c void updatedownloads ( L i s t <S t r i n g > t i t l e s ) / to be implemented i n p a r t ( b ) / // There may be i n s t a n c e v a r i a b l e s, c o n s t r u c t o r s, and methods t h a t // a r e not shown.

11 APCS/S1E Page 11 of 15 (a) (9 pts) Write the MusicDownloads method getdownloadinfo, which returns a reference to a DownloadInfo object if an object with a title that matches the parameter title exists in the downloadlist. If no song in downloadlist has a title that matches the parameter title, the method returns null. For example, suppose variable webmusica refers to an instance of MusicDownloads and that the table below represents the contents of downloadlist. The list contains three DownloadInfo objects. The object at position 0 has a title of Hey Jude and a download count of 5. The object at position 1 has a title of Soul Sister and a download count of 3. The object at position 2 has a title of Aqualung and a download count of 10. The call webmusica.getdownloadinfo("aqualung") returns a reference to the object in position 2 of the list. The call webmusica.getdownloadinfo("happy Birthday") returns null because there are no DownloadInfo objects with that title in the list. Do your work in the answer booklet.

12 APCS/S1E Page 12 of 15 (b) (9 pts) Write the MusicDownloads method updatedownloads, which takes a list of song titles as a parameter. For each title in the list, the method updates downloadlist, either by incrementing the download count if a DownloadInfo object with the same title exists, or by adding a new DownloadInfo object with that title and a download count of 1 to the end of the list. When a new DownloadInfo object is added to the end of the list, the order of the already existing entries in downloadlist remains unchanged. For example, suppose variable webmusicb refers to an instance of MusicDownloads and that the table below represents the contents of the instance variable downloadlist. Assume that the variable List<String> songtitles has been defined and contains the following entries. "Lights", "Aqualung", "Soul Sister", "Go Now", "Lights", "Soul Sister" The call webmusicb.updatedownloads(songtitles) results in the following downloadlist with incremented download counts for the objects with titles of Soul Sister and Aqualung. It also has a new DownloadInfo object with a title of Lights and a download count of 2, and another DownloadInfo object with a title of Go Now and a download count of 1. The order of the already existing entries remains unchanged. In writing your solution, you must use the getdownloadinfo method. Assume that getdownloadinfo works as specified, regardless of what you wrote for part (a). Do your work in the answer booklet.

13 APCS/S1E Page 13 of 15 Trail question(2010 AP CompSci Free Response) 2. A hiking trail has elevation markers posted at regular intervals along the trail. Elevation information about a trail can be stored in an array, where each element in the array represents the elevation at a marker. The elevation at the first marker will be stored at array index 0, the elevation at the second marker will be stored at array index 1, and so forth. Elevations between markers are ignored in this question. The graph below shows an example of trail elevations.

14 APCS/S1E Page 14 of 15 The declaration of the Trail class is shown below. You will write two unrelated methods of the Trail class. p u b l i c c l a s s T r a i l / R e p r e s e n t a t i o n o f the t r a i l. The number o f markers on the t r a i l i s markers. l e n g t h. / p r i v a t e i n t [ ] markers ; / Determines i f a t r a i l segment i s l e v e l. A t r a i l segment i s d e f i n e d by a s t a r t i n g marker, an ending marker, and a l l markers between t h o s e two markers. A t r a i l segment i s l e v e l i f i t has a d i f f e r e n c e between the max e l e v a t i o n and min e l e v a t i o n t h a t i s l e s s than or e q u a l to s t a r t the i n d e x o f the s t a r t i n g end the i n d e x o f the ending marker P r e c o n d i t i o n : 0 <= s t a r t < end <= markers. l e n g t h r e t u r n t r u e i f the d i f f e r e n c e between the maximum and minimum e l e v a t i o n on t h i s segment o f the t r a i l i s l e s s than or e q u a l to 10 meters ; f a l s e o t h e r w i s e. / p u b l i c boolean i s L e v e l T r a i l S e g m e n t ( i n t s t a r t, i n t end ) / to be implemented i n p a r t ( a ) / / Determines i f t h i s t r a i l i s r a t e d d i f f i c u l t. A t r a i l i s r a t e d by c o u n t i n g the number o f changes i n e l e v a t i o n t h a t a r e at l e a s t 30 meters ( up or down ) between two c o n s e c u t i v e markers. A t r a i l with 3 or more such changes i s r a t e d d i f f i c u l r e t u r n t r u e i f the t r a i l i s r a t e d d i f f i c u l t ; f a l s e o t h e r w i s e. / p u b l i c boolean i s D i f f i c u l t ( ) / to be implemented i n p a r t ( b ) / // There may be i n s t a n c e v a r i a b l e s, c o n s t r u c t o r s, and methods // t h a t a r e not shown.

15 APCS/S1E Page 15 of 15 (a) (9 pts) Write the Trail method isleveltrailsegment. A trail segment is defined by a starting marker, an ending marker, and all markers between those two markers. The parameters of the method are the index of the starting marker and the index of the ending marker. The method will return true if the difference between the maximum elevation and the minimum elevation in the trail segment is less than or equal to 10 meters. For the trail shown at the beginning of the question, the trail segment starting at marker 7 and ending at marker 10 has elevations ranging between 70 and 80 meters. Because the difference between 80 and 70 is equal to 10, the trail segment is considered level. The trail segment starting at marker 2 and ending at marker 12 has elevations ranging between 50 and 120 meters. Because the difference between 120 and 50 is greater than 10, this trail segment is not considered level. Do your work in the answer booklet. (b) (9 pts) Write the Trail method isdifficult. A trail is rated by counting the number of changes in elevation that are at least 30 meters (up or down) between two consecutive markers. A trail with 3 or more such changes is rated difficult. The following table shows trail elevation data and the elevation changes between consecutive trail markers. This trail is rated difficult because it has 4 changes in elevation that are 30 meters or more (between markers 0 and 1, between markers 1 and 2, between markers 3 and 4, and between markers 5 and 6). Do your work in the answer booklet.