Constraining points to a circle is actually a bit easier, because you only need a distance check from the center. Once they move past the radius of the circle, you can move them to the other side by adding the reverse difference between the center and the point, to the center. You can tweak this method a bit, but that's the general idea.

Ok, I still don't understand how to code this... I just can't think how to code a distance from a point? At the moment, this is how I've set the limits, where x and y are the location of the dot:

Please help! I thought I'd solved this but I haven't... Haaaaallpp!

Something like this...

Code Example

1. int numBoids = 50;
2. float radius = 150;
3. float centerx, centery;
4. float[] locx = new float[numBoids];
5. float[] locy = new float[numBoids];
6.  
7. void setup() {
8.   size(500, 500);
9.   centerx = width/2;
10.   centery = height/2;
11.   for (int i=0; i<numBoids; i++) {
12.     locx[i] = centerx;
13.     locy[i] = centery;
14.   }
15.   background(0);
16.   smooth();
17.   colorMode(HSB, 360, 100, 100);
18. }
19.  
20. void draw() {
21.   fill(0, 10);
22.   rect(0, 0, width, height);
23.   for (int i=0; i<numBoids; i++) {
24.     locx[i] += (noise(frameCount*0.01+i)-0.5)*15;
25.     locy[i] += (noise(frameCount*0.01+i+2)-0.5)*15;
26.     if (dist(centerx, centery, locx[i], locy[i]) > radius-10) {
27.       float difx = locx[i] - centerx;
28.       float dify = locy[i] - centery;
29.       locx[i] = centerx - difx;
30.       locy[i] = centery - dify;
31.     }
32.     noStroke();
33.     fill((i*10)%360, 100, 100);
34.     ellipse(locx[i], locy[i], 20, 20);
35.   }
36.   noFill();
37.   stroke(0, 0, 100);
38.   ellipse(centerx, centery, radius*2, radius*2);
39. }

Thanks so much, that's really helpful. I didn't realise that dist() function existed. I tried to use your code to adapt mine, making it as similar as possible, but it is still not working! I can get the balls to respawn in the centre if they go off the edge, but not at the opposite side of the circle for some reason. Perhaps you could have a look and tell me where it's going wrong?

Seems like there are a couple of problems with your code:

• xspeed and yspeed are global, but should be class variables
• the initial x and y position can be outside the circle but should be inside the circle
  
• xspeed and yspeed problems: randomly adding very high values every draw loop without maximum. Possible solutions: set speed once, add only every nth frame, add much lower values, limit the maximum range
  
• line 61: -10 should be -r