1. CS1101: Programming Methodology http://www.comp.nus.edu.sg/~cs1101/ http://www.comp.nus.edu.sg/~cs1101/

2. Week 5: Selection Statements (cont’d) and Repetition Statements  Last week:  Chapter 4: Defining Your Own Classes – Part 1 (cont’d)  Chapter 5: Selection Statements  This week:  Chapter 5: Selection Statements (cont’d)  Chapter 6: Repetition Statements  Next week:  Chapter 6: Repetition Statements (cont’d)  Testing and Debugging (not in textbook) © CS1101 (AY2009-2010 Semester 1)Week5 - 2

3. Reminder Have you read the relevant chapters and PowerPoint files before coming to this lecture? Have you been filling your Progress Chart? Have you been asking questions in class/IVLE forums? Have you done the Quick Check questions in the textbook? Have you been writing programs and exploring on your own? © CS1101 (AY2009-2010 Semester 1)Week5 - 3

4. Last week’s Exercise #4: Matric Check Code (1/3) Algorithm for Matriculation check code  Matriculation number consists of 6 digits (ignore the ‘U’ in front)  Eg: U097325  Step 1: Multiply the 2 nd through 6 th digits with their corresponding weights 2, 6, 2, 4, 1 and sum them up.  Eg: 9  2 + 7  6 + 3  2 + 2  4 + 5  1 = 18+42+6+8+5 = 79  Step 2: Divide step 1 result by 13 to obtain the remainder.  Eg: 79 % 13 = 1 © CS1101 (AY2009-2010 Semester 1)Week5 - 4

5. Last week’s Exercise #4: Matric Check Code (2/3) Algorithm for Matriculation check code (cont’d)  Step 3: Subtract step 2 result from 13.  Eg: 13 – 1 = 12  Step 4: Match step 3 result in this table for the check code.  Eg: The check code corresponding to 12 is ‘L’.  Therefore, the check code for U097325 is ‘L’. Write a program CheckMatric.java © CS1101 (AY2009-2010 Semester 1)Week5 - 5 12345678910111213 MBNAREUHWJXLY

6. Last week’s Exercise #4: Matric Check Code (3/3) Show me your program! How did you develop your program? How did you test your program? How did you debug your program? © CS1101 (AY2009-2010 Semester 1)Week5 - 6

7. Comparing Objects Refer to Thomas Wu’s slides  What is the difference between == and.equals()? © CS1101 (AY2009-2010 Semester 1)Week5 - 7

8. Comparing Real Numbers Real numbers, especially computed ones, are not represented accurately in computers. (Why?) Test out the program ExploreRealNumbers.java on some data.  What are the inputs where value2 is different from value1 ? To cater to such inaccuracy, we sometimes allow some ‘tolerance’ in the difference.  Instead of using == to compare, we may conclude that 2 values are “equal” if they are close enough  See ExploreRealNumbersV2.java © CS1101 (AY2009-2010 Semester 1)Week5 - 8

9. Naming Boolean Variables Good Boolean variable names make the code easier to understand Are these good names of Boolean variables?  boolean1  flag  isNotInvisible Are these good names of Boolean variables?  isValid  isPrime  toContinue © CS1101 (AY2009-2010 Semester 1)Week5 - 9

10. Exercise #5: Leap Year A year is a leap year if  It is divisible by 4 but not by 100; or  It is divisible by 400 Examples of leap year:  2004, 1980, 2000, 2400 Examples of non-leap year:  1997, 2001, 2006, 2100, 2200, 2300 Download LeapYear.java and correct it © CS1101 (AY2009-2010 Semester 1)Week5 - 10

11. Chapter 6: Repetition Statements Three syntactic flavours  while, do-while, for Nested loops Using break and continue in loop (Note that recursion – Section 6.10 Recursive Methods – is not in the CS1101 syllabus. We will cover it at the end of the course as a non-examinable topic.) © CS1101 (AY2009-2010 Semester 1)Week5 - 11

12. while, do-while, for Example: Print positive odd numbers < 100. © CS1101 (AY2009-2010 Semester 1)Week5 - 12 int i = 1; while (i < 100) { System.out.println(i); i += 2; } int i = 1; do { System.out.println(i); i += 2; } while (i < 100); for (int i=1; i<100; i+=2) System.out.println(i);

13. Another Very Common Mistake Besides the pitfalls mentioned in Thomas Wu’s book, here is another very common mistake. The following codes to display “Hello” 5 times compile and run, but give wrong result. Describe what will happen and correct the errors. © CS1101 (AY2009-2010 Semester 1)Week5 - 13 for (int x = 0; x < 5; x++); System.out.println("Hello"); int x = 0; while (x < 5); { System.out.println("Hello"); x++; }

14. Exercise #1 Complete OddIntegers.java to print odd integers between 1 and 39 inclusive. In your program, include 3 versions, using ‘for’, ‘while’ and ‘do-while’. Sample run: © CS1101 (AY2009-2010 Semester 1)Week5 - 14 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39

15. Exercise #2 Complete AsterisksV1.java to read in an integer n and print n asterisks on a single line. (If n is non-positive, then no asterisk will appear.) Sample runs: © CS1101 (AY2009-2010 Semester 1)Week5 - 15 Enter n: 7 ******* Done! Enter n: -2 Done!

16. Exercise #3: Nested Loops Download NestedLoopEx1.java, NestedLoopEx2.java and NestedLoopEx3.java. Hand trace the programs and write out the outputs without running the programs. Verify your answers by running the programs. © CS1101 (AY2009-2010 Semester 1)Week5 - 16

17. Using ‘break’ in loop (1/3) We have used ‘break’ in ‘switch’ statement. ‘break’ can also be used in a loop. Download BreakInLoop.java and run it. What do you understand about ‘break’? © CS1101 (AY2009-2010 Semester 1)Week5 - 17

18. Using ‘break’ in loop (2/3) Use ‘break’ sparingly, because using it violates the one-entry-one-exit control flow. A loop with ‘break’ can be rewritten into another without ‘break’. (See next slides.) Use ‘break’ only when it does not obfuscate readers. Example: Loop-and-half repetition control in Thomas Wu’s Chapter 6 slide 21. © CS1101 (AY2009-2010 Semester 1)Week5 - 18

19. Using ‘break’ in loop (3/3) Codes with and without ‘break’ – to add 10 integers entered by the user, or until a negative integer is encountered. © CS1101 (AY2009-2010 Semester 1)Week5 - 19 // with ‘break’ Scanner sc = new Scanner(System.in); int n, i, sum; sum = 0; i = 1; while (i <= 10) { n = sc.nextInt(); if (n < 0) break; sum += n; i++; } // without ‘break’ Scanner sc = new Scanner(System.in); int n, i, sum; sum = 0; i = 1; n = sc.nextInt(); while ((i = 0)) { sum += n; i++; n = sc.nextInt()); }

20. Using ‘continue’ in loop Download ContinueInLoop.java and run it. What do you understand about ‘continue’? ‘continue’ is even less used than ‘break’. © CS1101 (AY2009-2010 Semester 1)Week5 - 20

21. Strange Behaviour #2 Some students encountered this and wondered why. Download Strange2.java and test it. What happens?  You notice that you have no chance to enter the name! Why?  When you hit ‘enter’ after typing in the integer for the nextInt() method, the newline character is picked up by the nextLine() method. How to solve it?  One simple solution: add a nextLine() call to ‘trap’ this newline character. © CS1101 (AY2009-2010 Semester 1)Week5 - 21

22. Strange Behaviour #3 Download Strange3.java and test it. This is almost the same as the previous program, except that here you want the user to repeatedly enter name and height. When the user enters an empty name, the loop terminates. What happens when you run the program? Why? How do you correct it? © CS1101 (AY2009-2010 Semester 1)Week5 - 22

23. Control Structures You have learned the 3 control structures:  Sequence  Selection  Repetition With these, you are able to solve any general problem! However, writing good programs is more than just learning the syntax:  Logic should be clear  Variables (especially Boolean variables) should be descriptive  Algorithm should be efficient (see Thomas Wu’s chapter 6 slide 8 on Finding GCD) © CS1101 (AY2009-2010 Semester 1)Week5 - 23

24. Exercise #4: Prime-Number (Take-home) Primality test is a classic programming problem  Given a positive integer, determine whether it is a prime number or not  A prime number has two distinct factors (divisors): 1 and itself. Examples: 2, 3, 5, 7, 11, … Write a program PrimeTest.java. Discuss this at this week’s discussion session. Sample runs: © CS1101 (AY2009-2010 Semester 1)Week5 - 24 Enter a positive integer: 131 131 is a prime. Enter a positive integer: 713 713 is not a prime.

25. Exercise #5: BallV2 (1/2) Do you still remember the Ball class we created? As promised, we will now enhance it to BallV2 with the following additional items:  Two additional integer data members xCoord and yCoord: the x- and y-coordinates of the centre of a BallV2 instance.  A method moveVertical (int dist) that moves a ball object vertically by dist value. This will update the yCoord of the ball object.  A method moveHorizontal (int dist) that moves a ball object horizontally by dist value. This will update the xCoord of the ball object. Write a program BallV2.java and an application program TestBallV2.java. © CS1101 (AY2009-2010 Semester 1)Week5 - 25

26. Exercise #5: BallV2 (2/2) A sample run of TestBallV2: © CS1101 (AY2009-2010 Semester 1)Week5 - 26 Enter colour: red Enter radius: 1.2 Enter centre’s x- and y- coordinates: 3 4 Colour is red Radius is 1.2 Centre is (3,4) Distance to move horizontally: -8 Distance to move vertically : 2 Centre is (-5,6) Your program should create an object of BallV2 with the user’s supplied values, then use the accessor methods on the object to retrieve the values for display.

27. Exercise #6: BallV2GUI (1/3) Let’s have some fun. You have seen an GUI version of Lab #2 Ex2 Elevator, now we shall have an GUI version for Ball. Let’s see how it works. You are given the following:  Canvas.class (if it doesn’t work, ask for the source code)  BallV2GUI.java  TestBallV2GUI.java You are to study the Java programs given, and complete the two methods in BallV2GUI.java  slowMoveHorizontal(int dist)  slowMoveVertical(int dist) © CS1101 (AY2009-2010 Semester 1)Week5 - 27

28. Exercise #6: BallV2GUI (2/3) Note that the display is 300 pixels by 300 pixels. In graphics, the larger the y-coordinate value, the further down. Sample run: © CS1101 (AY2009-2010 Semester 1)Week5 - 28

29. Exercise #6: BallV2GUI (3/3) Possible exercises (to try out on your own at home):  Currently the ball object is still displayed (in part) if portion of it goes outside the boundary of the 300x300 display window.  Modify the code TestBallV2GUI.java such if you enter a position of the centre that causes the ball to be displayed outside (partially or fully) the screen, you ask for the user to enter a new position. Repeat this until the position is valid (that is, the ball can be displayed in its entirety.)  Modify the methods moveHorizontal(int) and moveVertical() in BallV2GUI.java such that if the new position causes the ball to be outside (partially or fully) the screen, then it rejects the move and keeps the ball in its current position.  Modify the methods slowMoveHorizontal(int) and slowMoveVertical() in BallV2GUI.java such that once the ball object hits the edge of the window, it stops moving and that will be its final position.  Any other changes/enhancements that you can think of. © CS1101 (AY2009-2010 Semester 1)Week5 - 29

30. Next week Do you know how to play Master Mind? Please find out before you come for next week’s lecture! © CS1101 (AY2009-2010 Semester 1)Week5 - 30

31. Summary for Today Selection statements (cont’d) Repetition statements  ‘while’  ‘do-while’  ‘for’  break  continue © CS1101 (AY2009-2010 Semester 1)Week5 - 31

32. Announcements/Things-to-do (1/2) Complete the following  Do Quick Check questions in Chapter 6. Check out the answers (on CS1101 website) yourself.  Exercise 4: PrimeTest.java  Exercise 5: BallV2GUI.java To prepare for next week’s lecture:  Read Week6.ppt (topic “Testing and Debugging” is not found in your book)  Find out about the game Master Mind © CS1101 (AY2009-2010 Semester 1)Week5 - 32

33. Announcements/Things-to-do (2/2) Survey  We will conduct an online survey during lecture next week (no need to prepare) To prepare for this week’s discussion session:  Download “week5_discussion_qns.pdf” from module website, “CA – Discussion”.  Do the questions before you attend your discussion session. © CS1101 (AY2009-2010 Semester 1)Week5 - 33

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