int[] x = new int[0];
  int[] y = new int[0];
  
  void setup() 
  {
    size(200, 200);
  }
  
  void draw() 
  {
    background(204);
    stroke(0);
    noFill();
    beginShape();
    for (int i = 0; i < x.length; i++) {
      vertex(x[i], y[i]);
    }
    endShape();
    // Show the next segment to be added
    if (x.length >= 1) {
      stroke(255);
      line(mouseX, mouseY, x[x.length-1], y[x.length-1]);
    }
  }
  
  void mousePressed() { // Click to add a line segment
    x = append(x, mouseX);
    y = append(y, mouseY);
  }
  
  void keyPressed() { // Press a key to save the data
    String[] lines = new String[x.length];
    for (int i = 0; i < x.length; i++) {
      lines[i] = x[i] + "\t" + y[i];
    }
    saveStrings("lines.txt", lines);
    exit(); // Stop the program
  }