The University of Western Ontario Department of Computer Science Computer Science 1026a Midterm Exam 2 hours

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1 The University of Western Ontario Department of Computer Science Computer Science 1026a Midterm Exam 2 hours PRINT YOUR NAME: PRINT YOUR STUDENT NUMBER: Do not turn this page until instructed to do so! 1 Instructions Fill in your name and student number above immediately. On the Scantron form, fill in your name and sign in the spaces provided at the top. Enter the course number CS1026a. Encode your student number in the area indicated by STUDENT NUMBER. You have 2 hours to complete the exam. There will be no extra time given to fill in the Scantron form at the end of the exam. Part 1 of the exam consists of Multiple Choice questions. Circle your answers on this exam paper and fill in your answers on the provided Scantron form. Part 2 consists of questions for which you will provide written answers. Write your answers in the spaces provided in this exam paper. Multiple choices question are worth 1 mark, unless indicated otherwise; other than that, the marks for each individual question are given. Allow approximately 1 minute per mark on average (the exam is worth 110 marks). There are pages for rough work at the back of the exam. You may detach them if you wish, but hand them in with the rest of the exam paper. Calculators are not allowed! Mark summary for Part II total 1

2 2 Multiple choice Choose one answer for each question. Java writing style: 5 questions 1. The name of a class (a) should be all upper-case (b) should start with an upper-case letter, but not be all upper-case (c) should start with a lower-case letter (d) none of the above 2. The name of a method (a) should be all upper-case (b) should start with an upper-case letter, but not be all upper-case (c) should start with a lower-case letter (d) none of the above 3. The name of a variable (a) should be all upper-case (b) should start with an upper-case letter, but not be all upper-case (c) should start with a lower-case letter (d) none of the above 4. The name of a constant (a) should be all upper-case (b) should start with an upper-case letter, but not be all upper-case (c) should start with a lower-case letter (d) none of the above 5. The following is a valid comment /* beginning of the method */ (a) true (b) false Java concepts: 5 questions 6. What should a complete Java program contain, for you to be able to run it: (a) nothing in particular (b) one static method called main (c) one static method called run (d) one object method called main (e) one object method called run 2

3 7. A method must be written inside a class (a) true (b) false 8. A class can contain several (more than one) methods (a) true (b) false 9. A class must contain several (more than one) methods (a) true (b) false 10. A class method is the same thing as a static method (a) true (b) false Understanding a method: 4 questions 11. Consider the following method, written in a class called MyClass. Use it from question 11 to 14. public void mymethod(int i, int j) i = j; (a) this method takes no parameter and returns two integers (b) this method takes two integer parameters and returns nothing (c) this method takes two integer parameters and returns one integer 12. The method of question 11 is (a) an object method (b) a static method (c) something else 13. (2 marks) Suppose that classobj is a reference variable to an object of the class MyClass. Suppose also that we have variables int i = 1 and int j = 2. At most one of the following is valid; which one? (a) MyClass.myMethod(i, j); (b) classobj.mymethod(i, j); (c) none 14. (2 marks) After executing the statement of question 13, what is the value of i: (a) 1 (b) 2 (c) none of the above 3

4 Understanding a small Java program: 7 questions 15. Consider the following code segment, from question 15 to question 21. Remark: it uses classes and methods that may, or may not, exist in DrJava. You do not need that kind of information to answer these questions: reading the code segment is enough. class Midterm public static void main(string[] args) World worldobj = new World(); Lettuce lettuceobj = new Lettuce(worldObj); Turtle turtleobj = new Turtle(worldObj); turtleobj.eat(lettuceobj); How many variables are declared: (a) 0 (b) 1 (c) 2 (d) 3 (e) How many reference variables are declared: (a) 0 (b) 1 (c) 2 (d) 3 (e) The identifier worldobj is (a) a primitive type variable (b) a reference variable (c) a method (d) the name of a class (e) something else 18. The identifier World is (a) a primitive type variable (b) a reference variable (c) a method (d) the name of a class (e) something else 19. (2 marks) The identifier Lettuce is 4

5 (a) a primitive type variable (b) a reference variable (c) a method (d) the name of a class (e) something else 20. The identifier main is (a) a primitive type variable (b) a reference variable (c) a method (d) the name of a class (e) something else 21. (3 marks) The identifier eat is (a) a method of the class Turtle (b) a method of the class Lettuce (c) something else Numerical expressions: 5 questions 22. What is the value of the expression 11/2 (a) 5.0 (b) 5 (c) 5.5 (d) 6.0 (e) What is the value of the expression (double) (11/2) (a) 5.0 (b) 5 (c) 5.5 (d) 6.0 (e) What is the value of the expression (int) ((double) (11/2)) (a) 5.0 (b) 5 (c) 5.5 (d) 6.0 (e) What is the value of the expression (((double) 11)/2) (a) 5.0 5

6 (b) 5 (c) 5.5 (d) 6.0 (e) What is the value of the expression (11/((double) 2)) (a) 5.0 (b) 5 (c) 5.5 (d) 6.0 (e) 6 Variables: 5 questions 27. (2 marks) What is the value of x after executing this: int x = 5; x = 2 * (x/2); (a) 4 (b) 5 (c) 4.0 (d) (2 marks) What is the value of x after executing this: int x = 5; x = (2*x) / 2; (a) 4 (b) 5 (c) 4.0 (d) What is the value of x after executing this: int x = 6; int y = x; x = 0; (a) 0 (b) (2 marks) What is the value of y after executing this: int x = 6; int y = x; x = 0; (a) 0 (b) (2 marks) What is the value of x after executing this: 6

7 int x = 0; int y = 1; y++; x = x - y; (a) -2 (b) -1 (c) 0 (d) 1 (e) 2 Arrays: 5 questions 32. Consider an array declared by int[] myarray = 1,2,3,4; We will use it in questions 32 to 36. What value is stored in myarray[3]? (a) 0 (b) 2 (c) 4 (d) 3 (e) not defined; that would create an error 33. What do you see when you execute the following: System.out.println(myArray.length); (a) 0 (b) 3 (c) 4 (d) an error 34. What do you see when you execute the following: System.out.println(myArray.Length()); (a) 0 (b) 3 (c) 4 (d) an error 35. (2 marks) What do you see when you execute the following: for (int i = 0; i < 2; i++) System.out.print(myArray[i]); (a) 1 (b) 12 (c) 1 2 (d) 123 (e) (3 marks) What is the value of t after executing the following: 7

8 int t = 0; for (int i = 0; i < 4; i++) t = myarray[i]; (a) 0 (b) 4 (c) 6 (d) 10 (e) no value, we get an error / an infinite loop Loops: 10 questions 37. (2 marks) What is the value of i after executing the following: for (int i = 0; i < 10; i++) System.out.println(i); (a) 9 (b) 10 (c) 11 (d) undefined 38. (2 marks) What is the value of i after executing the following: int i = 0; while (i < 10) System.out.println(i); (a) 9 (b) 10 (c) 11 (d) undefined 39. (2 marks) What is the value of sum after executing the following: int i = 0; int sum = 0; while (i < 5) (a) 6 (b) 10 8

9 (c) 15 (d) 20 (e) (3 marks) What is the value of sum after executing the following: int i = 0; int sum = 0; while (i < 5) (a) 6 (b) 10 (c) 15 (d) 20 (e) (2 marks) What is the value of sum after executing the following: int i = 5; int sum = 0; while (i >= 0) i--; (a) 6 (b) 10 (c) 15 (d) 20 (e) (2 marks) What is the value of sum after executing the following: int i = 0; int sum = 0; while (i < 5) i = 0; while (i < 5) 9

10 (a) 6 (b) 10 (c) 15 (d) 20 (e) (3 marks) What is the value of sum after executing the following: int i = 0; int sum = 0; while (i < 5) while (i < 5) (a) 6 (b) 10 (c) 15 (d) 20 (e) 25 (f) no value, infinite loop 44. (3 marks) What is the value of sum after executing the following: int i = 0; int sum = 0; while (i < 5) while (i < 6) (a) 6 (b) 10 (c) 15 (d) 20 (e) (2 marks) What is the value of sum after executing the following: 10

11 int sum = 0; for (int i = 0; i < 5; i++) (a) 6 (b) 10 (c) 15 (d) 20 (e) (3 marks) What is the value of sum after executing the following: int sum = 0; for (int i = 0; i < 5; i++) (a) 6 (b) 10 (c) 15 (d) 20 (e) 25 11

12 3 Problems 1. (2 marks) Write Java statements that compute the sum (using a loop) and print its value 2. (4 marks) Write Java statements that compute the sum (using a loop) and print its value 3. (4 marks) Write Java statements that compute the product (using a loop) and print its value 4. (6 marks) Write Java statements that compute the sum (using a loop) and print its value 12

13 5. (8 marks) Describe what happens (and why) to the pixels in the picture when you execute the following statements (in the Interactions pane) String name = FileChooser.pickAFile(); Picture pictureobj = new Picture(name); Pixel[] pixelarray = pictureobj.getpixels(); int i = 0; int red, green, blue = 0; while (i < pixelarray.length) Pixel pixelobj = pixelarray[i]; red = pixelobj.getred(); pixelobj.setgreen(red); green = pixelobj.getgreen(); pixelobj.setblue(green); blue = pixelobj.getblue(); pixelobj.setred(blue); pictureobj.show(); 6. (6 marks) Complete the following, by writing Java statements that instruct a turtle to draw 2 vertical lines of length 100, separated by 10 pixels (as on the picture here). We don t ask anything special about the pen color, width,..., or the orientation of the turtle after drawing. Assume that the turtle is facing up at the beginning. 13

14 World worldobj = new World(); Turtle turtleobj = new Turtle(worldObj); 7. (8 marks) Now, write Java statements that instruct a turtle to draw 10 vertical lines as before, but of lengths 100, 90,..., 10; they should still be separated by 10 pixels. Use a loop. World worldobj = new World(); Turtle turtleobj = new Turtle(worldObj); 14

15 4 Rough work 1/4 15

16 Rough work 2/4 16

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18 Rough work 4/4 18

19 5 Method summary World: Useful constructor World() Constructor that takes no arguments Turtle: Useful constructor Turtle(World worldobj) Constructor that puts the turtle at the center of its world, facing up Turtle: Useful methods void backward() Method to go backward by 100 pixels void backward(int pixels) Method to go backward a given number of pixels void forward() Method to move the turtle foward 100 pixels void forward(int pixels) Method to move the turtle forward the given number of pixels void moveto(int x, int y) Method to move to turtle to the given x and y location void pendown() Method to set the pen down void penup() Method to lift the pen up (so the turtle doesn t draw anything when it moves) void turnleft() Method to turn left 90 degrees void turnright() Method to turn right 90 degrees Picture: Useful constructor Picture(String filename) Constructor that takes a file name and creates the picture Picture: Useful methods Pixel[] getpixels() Method to get a one-dimensional array of Pixels for this simple picture void show() Method to show the picture in a picture frame Pixel: Useful methods int getblue() Method to get the amount of blue at this pixel 19

20 int getgreen() Method to get the amount of green at this pixel int getred() Method to get the amount of red at this pixel void setblue(int value) Method to set the blue to a new blue value void setgreen(int value) Method to set the green to a new green value void setred(int value) Method to set the red to a new red value FileChooser: Useful method static String pickafile() Let the user choose a file, and return the full file name (path included) 20