1. Discussion 9 CS1101X Group 5

2. Lab7 Minor note that default value of int[][] are all 0 default value of boolean[][] are all false

3. Discussion Plan General notes on Recursion Recursion practice PE Problem solving

4. Recursion Base case Recursive case n! = n * (n-1) * (n-2) * … * 2 * 1 for n > 0 (recursive case) 0! = 1 (base case) int fac(int n){ if (n == 0) return 1; else{ return n * fac(n-1); } }

5. Recursion General approaches Think recursive ◦ Split the problem into simple cases ◦ Work in the opposite way  Work from a base case  See what you need to do for the 2 nd last case  etc

6. Recursion Practice Can you write a “while” method that does what the while loop is doing

7. Recursion Practice Can you write a “while” method that does what the while loop is doing public static void While (int i){ if (cond){ //code of while body While(i); }

8. Recursion Practice while(i > 0){ System.out.println(i); i--; } public static void While (int i){ if (i > 0 ){ System.out.println(i); i--; While(i); }

9. Recursion Practice Download Practical Test http://www.comp.nus.edu.sg/~cs1101x/3_ ca/labs.html http://www.comp.nus.edu.sg/~cs1101x/3_ ca/labs.html Can you write a recursive solution for both questions?

10. PE Problem Solving 2006/07 Houses in a town Binary image and pattern detection

11. Houses in a town Need a current_maxDistance and current_maxPoint Method to find the distance from (0,0) to (x,y) Using the final current_maxDistance as radius, find area

12. Binary image and pattern detection Basically the problem seems very difficult but the general skeleton has been given to you Stuff you might want to try out: rotate90CW()

13. PE Problem Solving 2004/05 Plurals Candles Teams

14. Plurals Check out String api public boolean endsWith(String suffix)String Just a series of if-else and endsWith and substring

15. Teams Method 1: ◦ ArrayList and count and remove Method 2: ◦ Keep a String[] and each time you allocate a player, set the index to null and go to the next index (index + 1) % n ◦ Do until allocate n players ◦ Only increment the “count” when you are at non-null String[location]

16. Other past year questions… Only got the final sourcecode, lost the questions

17. Snake

18. “Board” class ◦ void move(char direction) ◦ boolean isBlank Main ◦ create a “menu”

19. Snake “Board” class ◦ void move(char direction) ◦ boolean isBlank Main ◦ create a “menu”

20. Crossword Puzzle

21. Boggle

22. Kings Can’t really remember what it does But basically chess game

23. Do you know… toString getClass Array – 1D, 2D ArrayList – contains, indexOf, get, set, add, remove, removeFirst, removeLast,… String – indexOf, lastIndexOf, startsWith, endsWith, substring, charAt, split Math – min, max, floor, ceil Classes – declare and use

24. Do you know… Recursion – good to know in case it can be easily solve with Point class Scanner – how to get tokens new Scanner(“some_string”) Regular Expression