1. More About Recursion
Java

2. Recall: A Towers Problem
Source Spare Target
(coneFrom) (coneSpare) (coneTo)
The challenge is to move all the disks from the source cone to the target cone.
Move 1 disk at a time
A larger disk may never be on top of a smaller disk

3. The solution
To solve the towers problem, we need to know howmany disks we have and which cone is the source, the target, and the spare:
towers
Parameters: howmany, source, target, spare
If howmany is equal to 1
move it (the smallest disk) from the source to the target
Else
Do in order
call towers to move howmany-1 disks from source to spare
(using target as spare)
move it (disk # howmany) from the source to the target
call towers to move howmany-1 disks from the spare to the target
(using the source as the spare)
base case –
move 1 disk
a smaller problem -- recursively move the rest of the disks
Two recursive calls are used in this method.

4. Looking at recursion in Java
The idea is identical
A recursive call
One or more base cases

5. A sample problem Building a bulls-eye on a picture
A bulls-eye is a series of concentric circles of alternating color
Question:
Should we build it from outside in, or from inside out?

6. Building a bulls-eye Questions How to draw a circle?
Use the Graphics method fillOval
We need to specify the upper left corner, and the width and height of the circle
What is the base case?
What is the recursive case?
How can we alternate colors?

7. // method in the Picture class
public void drawBullsEye() { int size = 100; int midx = 200; int midy = 200; Graphics g = this.getGraphics(); drawBull(g, size, midx, midy, true); }

8. private void drawBull(Graphics g, int diam, int x, int y, Boolean onOff)
{ if (onOff) { g.setColor(Color.black); } else { g.setColor(Color.white); }
if (diam >= 20) { g.fillOval(x - diam/2, y-diam/2, diam, diam); drawBull(g,diam-20,x,y,!onOff); }
} // ends drawBull

9. Assignment
No reading assignment from Media Computation