1. Java: An Eventful Approach An innovative approach to teaching Java in CS1 † Recursion Kim B. Bruce, Andrea Danyluk & Tom Murtagh Williams College † Partially supported by NSF CCLI grant DUE-0088895

2. Topic: Recursion Where does it belong? –Before or after loops or not at all? Combine traditional and recursive Doesn’t have to be boring!

3. Traditional Recursion /** @param exponent >= 0 @returns base raised to exponent power **/ public int fastPower(int base, int exponent) { if (exponent == 0) return 1 else if (exponent%2 == 1) // exponent is odd return base * fastPower(base, exponent-1) else // exponent is even return fastPower(base * base, exponent / 2) }

4. Structural Recursion More intuitive Graphics and ActiveObjects support interesting recursive drawings.

5. Recursive Pictures Structural recursion –Contain recursive instance variables –Defined by recursive constructors –Moved with recursive methods Animate concurrent growth

6. Broccoli parts are either branches or flowers: public interface BroccoliPart { /* @param x,y amount to move broccoli */ public void move( double x, double y) } Use interface for parts

7. public class BroccoliBranch implements BroccoliPart { // How much broccoli shrinks each call public final static double TOP_FRACT = 0.8; private BroccoliPart left, center, right;// branches of broccoli private AngLine stem;// stem of broccoli private FilledOval flower;// Flower of broccoli plant /* Draw broccoli by recursively drawing branches (and flower) */ public BroccoliBranch(Location startCoords, double size, double direction, DrawingCanvas canvas) /* @param x,y amount to move broccoli */ public void move( double x, double y)

8. /* Draw broccoli by recursively drawing branches (and flower) */ public BroccoliBranch(Location startCoords, double size, double direction, DrawingCanvas canvas) { stem = new AngLine(startCoords,size,direction,canvas); stem.setColor(broccoliColor); Location destCoords = stem.getEnd();// end of stem if ( size > MINSIZE ) {// Big enough to keep growing left = new BroccoliBranch (destCoords, size * TOP_FRACT, direction + Math.PI/9.0, canvas); center = new BroccoliBranch (destCoords,..., direction,...); right = new BroccoliBranch (destCoords,..., direction - Math.PI/9.0,...); } else {// draw flower when small enough left = new Flower (destCoords,..., direction + Math.PI/9.0, canvas); center = new Flower (destCoords,..., direction, canvas); right = new Flower (destCoords,..., direction - Math.PI/9.0, canvas); }

9. /* Draw broccoli by recursively drawing branches (and flower) */ public class Flower implements BroccoliPart { private AngLine stem; // stem of broccoli private FilledOval bud;// Flower of broccoli plant // Draw flower public Flower(Location startCoords, double size, double direction, DrawingCanvas canvas) { // Draw stem and color green stem = new AngLine(startCoords,size,direction,canvas); stem.setColor(broccoliColor); Location destCoords = stem.getEnd();// end of stem bud = new FilledOval(destCoords,BUD_SIZE,BUD_SIZE,canvas); bud.setColor(Color.yellow); } … }

10. Recursive data structures Natural to students –They are used to multiple instances More concrete than multiple stack frames

11. // @param x,y amount to move broccoli public void move( double x, double y) { stem.move(x,y);// move stem left.move(x,y);// move other parts center.move(x,y); right.move(x,y); } // Similar code for move in Flower, but not recursive! Recursive Methods, too