More About Recursion Java Recall A Towers Problem

More About Recursion Java

Recall: A Towers Problem Source Spare Target (cone. From) (cone. Spare) (cone. To) 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

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) Two recursive calls are used in this method. base case – move 1 disk a smaller problem -recursively move the rest of the disks

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

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?

Building a bulls-eye Questions How to draw a circle? Use the Graphics method fill. Oval 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?

// method in the Picture class public void draw. Bulls. Eye() { int size = 100; int midx = 200; int midy = 200; Graphics g = this. get. Graphics(); draw. Bull(g, size, midx, midy, true); }

private void draw. Bull(Graphics g, int diam, int x, int y, Boolean on. Off) { if (on. Off) { g. set. Color(Color. black); } else { g. set. Color(Color. white); } if (diam >= 20) { g. fill. Oval(x - diam/2, y-diam/2, diam); draw. Bull(g, diam-20, x, y, !on. Off); } } // ends draw. Bull

Assignment No reading assignment from Media Computation