CP122 CS I. Chapter 13: Recursion

Transcription

1 CP122 CS I Chapter 13: Recursion

2 Russian law banning VPNs comes into effect today Tech News!

3 Tech News! Russian law banning VPNs comes into effect today Shelley: AI to write horror story openings

4 THE PATH Tech News! The path becomes longer and longer and sharper until it seems like she's coming towards me. I am running through the shadows and the paintings begin to become more distorted. Russian law banning VPNs comes into effect today Shelley: AI to write horror story openings

5 Chapter 13: Recursion Some problems can be solved elegantly by first solving a smaller, related problem We can model this technique using recursive methods: methods that call themselves

6 Calling a Method public void scanjunk() { Scanner myscanner = new Scanner(System.in); String line = myscanner.nextline();... }

7 Calling a Method public void scanjunk() { Scanner myscanner = new Scanner(System.in); String line = myscanner.nextline();... } This nextline method call is within the scanjunk method.

8 public String nextline() {... line += buffer.charat(i); } Calling a Method

9 Calling a Method public String nextline() {... line += buffer.charat(i); } Probably somewhere in the nextline method there is another call to charat (or something similar)

10 Execution Flow scanjunk nextline charat

11 Return Execution Flow scanjunk nextline charat

12 How does the computer know which statement to execute next?

13 Sequential with Jumps Rule 1: Execute the next statement in sequential order (the next statement down)

14 Sequential with Jumps Rule 1: Execute the next statement in sequential order (the next statement down) Rule 2: Don't follow Rule 1 in certain cases: Last statement in the loop body: jump to top of loop if statements: jump based on condition Method call: jump to that method's body

15 How does the computer know what to execute once a method finishes?

16 Method Call Stack public static void main(string[] args) { System.out.println( HELLO! ); System.out.println( WORLD! ); }

17 Method Call Stack public static void main(string[] args) { System.out.println( HELLO! ); System.out.println( WORLD! ); } println is a method call. When it is called, the memory address of where to execute within main after it finishes must be stored

18 Method Call Stack public static void main(string[] args) { System.out.println( HELLO! ); System.out.println( WORLD! ); } When the first println finishes, execution returns here.

19 Method Call Stack 1 public static void main(string[] args) { 2 System.out.println( HELLO! ); 3 System.out.println( WORLD! ); 4 } When the first println is called, the value of 3 is added to the call stack. This value is used and removed when the method returns.

20 Method Call Stack : Example 1 public static void main(string[] args) { 2 MyObject obj = new MyObject(); 3 obj.scanjunk(); 4 } 5 public void scanjunk() { 6 Scanner myscanner = new Scanner(System.in); 7 String line = myscanner.nextline(); 8 } 9 public String nextline() { line = buffer.charat(i); 16 }

21 What if a method calls itself?

22 Method Calling Itself public void sayhello() { System.out.println( HELLO! ); sayhello(); }

23 Method Calling Itself public void sayhello() { System.out.println( HELLO! ); sayhello(); } Every single time sayhello is called...it calls itself again!

24 Helloer.java

25 Recursion A method that calls itself is called a recursive method Recursion often provides an elegant way to solve some problems

26 The Two Rules of Recursion Rule 1: The recursion has to stop somewhere Infinite recursion is not helpful

27 The Two Rules of Recursion Rule 1: The recursion has to stop somewhere Infinite recursion is not helpful Rule 2: Each recursive call should get the program closer to the stopping point Recursive calls solve a smaller version of the original problem

28 The Two Rules of Recursion Rule 1: Base case Rule 2: Recursive case

29 Recursive Example: Factorial What is 7 factorial?

30 Recursive Example: Factorial What is n factorial?

31 Factorial Base case (n == 1) Recursive case (n > 1)

32 Factorial Base case (n == 1) : result = 1 Recursive case (n > 1) : result = n * factorial(n -1)

33 Factorial.java

34 The way to understand recursion is to understand recursion.

35 Google recursion

36 Practical Recursion: Counting Files Pictures Vacations Birthdays Friends

37 Counting Files Given a directory name, compute how many files are stored in that directory or any of its subdirectories.

38 Counting Files Java File objects can be used for files and directories

39 Counting Files Java File objects can be used for files and directories We can use myfile.isfile() to determine if a File object is a file or a directory.

40 Counting Files Java File objects can be used for files and directories We can use myfile.isfile() to determine if a File object is a file or a directory. If a File object is a directory, then it has a listfiles() method

41 Counting Files Base case: The directory doesn't have any subdirectories. Just return the number of files.

42 Counting Files Base case: The directory doesn't have any subdirectories. Just return the number of files. Recursive case: Call countfiles for each subdirectory Return sum of all the subdirectories

43 FileCounter.java