Using Timers

Timers are a key part of creating animations. As such, they are often used to create fun and interesting graphical programs. Let's take a look at one such program, called "Random Circles."

 * The Random Circles Program

var MAX_RADIUS = 100;
var MAX_CIRCLES = 1000;
var counter = 0;

function start(){
    setTimer(draw, 50);

function draw(){
    drawCircle(Randomizer.nextInt(0, MAX_RADIUS),
               Randomizer.nextInt(0, getWidth()),
               Randomizer.nextInt(0, getHeight()));


    if(counter == MAX_CIRCLES){

function drawCircle(radius, color, x, y){
    var circle = new Circle(radius);
    circle.setPosition(x, y);

Using Timers

Let's start by examining the timer. The timer is created at the very beginning of the program in the start function.


What function is the timer calling?

  • ( ) makeBall
  • (x) draw
  • ( ) drawCircle
  • ( ) start
  • ( ) timer

There is no makeBall function in the code. Correct! The drawCircle function is called by the draw function, not the timer. The start function is not called by the timer. The timer cannot call itself.

How often does the timer call this function?

  • ( ) Every minute
  • ( ) Every 50 seconds
  • ( ) Every 5 milliseconds
  • ( ) Every 5 seconds
  • (x) Every 50 milliseconds

Incorrect. The delay is given in milliseconds, not seconds. Note that it is 50 and not 5.



Stopping a Timer

Timers will continue to run until they are given the command to stop. A timer can be stopped by issuing the stopTimer(functionName) command. The functionName should be the name of the function that was originally passed to the timer when creating it with startTimer().

For example, Karel could create a timer that called a giveTreat function every 1000 milliseconds:

startTimer(giveTreat, 1000);

Karel could then stop the timer by issuing the following command:



In the Random Circles program above, what command will stop the timer from drawing new circles?

  • ( ) stopTime(draw)
  • ( ) stop(draw)
  • (x) stopTimer(draw)
  • ( ) stopTimer(draw, 50)
  • ( ) stopTimer(drawCircle)

Close, but the command is stopTimer, not stopTime. The proper command is stopTimer. Correct! You should only pass the name of the function being called, not a delay. You'll need to give the proper function name that the timer is calling.

When does the timer stop in the Random Circles program?

  • (x) When the counter reaches 1000.
  • ( ) When the screen is filled with circles.
  • ( ) It does not stop until there are 990 circles.
  • ( ) The timer stops when there are no more circles.
  • ( ) We can't stop, we won't stop.

Correct! Notice the counter variable is checked each time draw is called. The value of MAX_CIRCLES is 1000, not 990. There is not a built in number of circles that can be drawn. We run things, things don't run we.


Using Multiple Functions with a Timer

Programs often need to call multiple functions every tick of the timer. However, the call to startTimer can only take one single function. This code would not work for Karel:

startTimer(giveTreat, giveToy, takeKarelForWalk, 1000);

Instead of passing multiple functions when calling startTimer, Karel will need to create one function that then makes calls to the other functions:

startTimer(makeKarelHappy, 1000);


function makeKarelHappy(){

With this code, the makeKarelHappy function will be called every 1000 milliseconds. That function then calls each of Karel's other functions. Notice that in the Random Circles program, the timer calls the draw function, which then calls drawCircle.