/**
 * True/False. 
 * 
 * A Boolean variable has only two possible values: true or false. 
 * It is common to use Booleans with control statements to 
 * determine the flow of a program. In this example, when the
 * boolean value "x" is true, vertical black lines are drawn and when
 * the boolean value "x" is false, horizontal gray lines are drawn. 
 */
 
boolean b = false;

size(640, 360);
background(0);
stroke(255);

int d = 20;
int middle = width/2;

for (int i = d; i <= width; i += d) {
  
  if (i < middle) {
    b = true;
  } else {
    b = false;
  }
  
  if (b == true) {
    // Vertical line
    line(i, d, i, height-d);
  }
  
  if (b == false) {
    // Horizontal line
    line(middle, i - middle + d, width-d, i - middle + d);
  }
}