Increment Decrement

/**
 * Increment Decrement. 
 * 
 * Writing "a++" is equivalent to "a = a + 1".  
 * Writing "a--" is equivalent to "a = a - 1".   
 */
 
int a;
int b;
bool direction;

void setup() {
    size(640, 360);
    colorMode(RGB, width);

    a = 0;
    b = width;
    direction = true;

    frameRate(30);
}

void draw() {
    a++;

    if (a > width) {
        a = 0;
        direction = !direction;
    }

    if (direction == true) {
        stroke(a);
    } else {
        stroke(width - a);
    }

    line(a, 0, a, height / 2);

    b--;

    if (b < 0) {
        b = width;
    }

    if (direction == true) {
        stroke(width - b);
    } else {
        stroke(b);
    }

    line(b, height / 2 + 1, b, height);
}