Examples / Processing Examples / Input / Bouncing Balls

Bouncing Balls

example processing input

Note: This example uses parallel arrays. In other words, we’re storing our data across multiple arrays. This is a good way to learn about arrays, but in real life you should use classes instead of parrallel arrays. If you haven’t learned about classes yet, don’t worry about it too much.

This program takes the previous bouncing balls example and adds user input. Now a bouncing ball is added whenever the user clicks inside the window.

float[] x = new float[0];
float[] y = new float[0];
float[] xSpeed = new float[0];
float[] ySpeed = new float[0];
float[] size = new float[0];
float[] r = new float[0];
float[] g = new float[0];
float[] b = new float[0];

void setup() {
  size(400, 200);

void draw() {


  for (int i = 0; i < x.length; i++) {

    x[i] += xSpeed[i];
    if (x[i] < 0 || x[i] > width) {
      xSpeed[i] *= -1;

    y[i] += ySpeed[i];
    if (y[i] < 0 || y[i] > height) {
      ySpeed[i] *= -1;

    fill(r[i], g[i], b[i]);
    ellipse(x[i], y[i], size[i], size[i]);

void mousePressed() {
  x = append(x, mouseX);
  y = append(y, mouseY);
  xSpeed = append(xSpeed, random(-5, 5));
  ySpeed = append(ySpeed, random(-5, 5));
  size = append(size, random(5, 20));
  r = append(r, random(256));
  g = append(g, random(256));
  b = append(b, random(256));

This code uses the append() function which takes an array and a new element, and returns a new array that contains everything the first array held, plus the new element. By setting our arrays equal to these new arrays, we’re essentially adding new values to the arrays.

bouncing balls

Code Editor

See the Pen by Happy Coding (@KevinWorkman) on CodePen.

Tweak Ideas

  • What happens if you use the mouseDragged() function instead of the mousePressed() function?
  • Make it so the direction of new balls is based on the direction the mouse is going when the user clicks.

Comments and Questions

Happy Coding is a community of folks just like you learning about coding.
Do you have a comment or question? Post it here!