primes.java

import java.util.ArrayList;
// Stuff to remember:
// * ellipseMode -> can take CENTER to do what you expect
final int width = 600;
final int height = 600;
boolean[] primality;
final int scale = 5;
void setup() {
  size(600, 600);
  primality = sieve();
  int x = 0;
  int y = 0;
  fill(0, 0, 0);
  for(int i = 0; i < primality.length; i++) {
    if(x > width / scale) {
      x = 0;
      y++;
    }
    x++;
    if(primality[i]) {
      rect(x * scale, y * scale, scale, scale);
    }
  }
}
boolean[] sieve() {
  // Everyone loves the sieve of erastosthenes
  boolean[] primality = new boolean[10000];
  for(int i = 0; i < primality.length; i++) {
    primality[i] = true;
  }
  for(int i = 2; i < primality.length / 2; i++) {
    if(primality[i] == false) {
      continue;
    }
    for(int j = 2 * i; j < primality.length; j += i) {
      primality[j] = false;
    }
  }
  return primality;
}