Simple ArrayIndex problem

Processing Beginners
9

Keep in mind that, if you are safely converting each index with %, you can access more than the next point – you can access three points, or every tenth point, and at any offset.

Here is an example that draws a series of triangles based on a ring of points, constantly changing its starting points and offsets. As frameCount and step both increase, code is eventually using i values like:

1270 1292 1314

but instead of getting index errors, with modulo they access safe indices in the ArrayList of size 24 – points:

22, 20, 18

/**
 * Accessing points like a circular array
 * 2020-04-14 Processing 3.4 
 */
ArrayList<PVector> points;
int num = 24;
int step = 1;

void setup() {
  size(600,600);
  points = new ArrayList<PVector>();
  // circle of points
  for (int i=0; i<num; i++) {
    float x = width * 4/9.0 * sin(TWO_PI*i/(float)num);
    float y = height * 4/9.0 * cos(TWO_PI*i/(float)num);
    points.add(new PVector(x, y));
  }
}

void draw(){
  background(128);
  translate(width/2,height/2);
  PVector p1, p2, p3;
  // count from anywhere
  for (int i=frameCount; i<frameCount+points.size(); i++) {
    // access points 
    p1 = points.get(i%points.size());
    p2 = points.get((i+step)%points.size());
    p3 = points.get((i+2*step)%points.size());
    triangle(p1.x, p1.y, p2.x, p2.y, p3.x, p3.y);
  }
  println(frameCount, frameCount+step, frameCount+2*step);
  if(frameCount%60==0) step++;
}

CircularArrayAccess--screenshot--small

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