Learning recursion through visual patterns (ii)

Visual algorithms are a good way of learning recursion. As we have seen with recursive squares, simple is often better. What about circles? We could create a program in Processing to embed a circle within a circle, each time reducing the size of the circle. Here is the setup:

float dx, dy;
void setup() {
   size(400, 400);

void draw() {
   dx = width/2,0;
   dy = height/2.0;

The call to the recursive function recCircle(200) uses a circle diameter of 200 pixels . A circle is then drawn at position (dx,dy) with a diameter of diam. At each recursive call, the radius is reduced to 75% its previous size.  The recursion stops when diam becomes less than 5. Here is the recursive function:

void recCircle(float diam) {
  if (diam > 5) {


Here is the output:

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