Factor square drawer

factorSquare.pde

final boolean monochrome = true;  // in this mode, primes are black, non-primes are white
                                  // otherwise darker colours have fewer factors, lighter colours have more

final int windowWidth= 640;
final int windowHeight= 320;
final int plotWidth = 3;
final int plotHeight = 5;

void settings() {
  size(windowWidth, windowHeight); 
}

void setup(){
  int[] factors = new int[plotWidth*plotHeight];
  
  for(int i=0; i<plotWidth*plotHeight; i++){
    factors[i] = numberOfFactors(i+1);  
    //println(factors[i]); // only if you're keen
  }
  
  int maxFactor = max(factors);
  
  noStroke();
  
  for(int i=0; i<plotWidth*plotHeight; i++){
    if(monochrome){
      fill(factors[i]==2 ? 0 : 255);
    } else {
      fill(factors[i]*255/maxFactor);
    }
    rect((windowWidth/plotWidth)*(i%plotWidth), (windowHeight/plotHeight)*(i/plotWidth), (windowWidth/plotWidth), (windowHeight/plotHeight));   
  }
}

void loop(){
  
}

int numberOfFactors(int number){
  if(number<=0) return -1; // only positive integers count
  if(number==1) return  1; // we treat 1 as a special case for ease
  int retVal = 2; // all numbers have at least 2 factors (1 and themselves)
  for(int i=2; i<number; i++){ // don't include 1 or the number
    if(number%i==0) retVal++;
  }
  return retVal;
}