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);

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);

Functions Used


Sets the color used to draw lines and borders around shapes

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Defines the dimension of the display window width and height in units of pixels

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Draws a line (a direct path between two points) to the screen

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The background() function sets the color used for the background of the Processing window

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