Teaching Loops

Facilitated by introducing simple threads• Few interesting loop examples before arrays

– Boring calculations– Interaction– Drawing– Animation

• Bad idea to postpone loops until arrays

Simple While Loops

• Repetition in drawing– Grass, bricks– Laundry basket

grassX = 0;while ( grassX < SCREEN_WIDTH ){

new Line(grassX, GR_BASE, grassX, GR_TOP,

canvas).setColor(GRASSGREEN);grassX = grassX + GR_SPACING;

}

Relate Loops to Familiar Concepts

int grassX = 0;

public void onMousePress (Location pt){ if ( grassX < SCREEN_WIDTH ){

new Line(grassX, GR_BASE, grassX, GR_TOP,

canvas).setColor(GRASSGREEN);grassX = grassX + GR_SPACING;

}}

More Simple While Loops

Animation• Balls (leaves, snow) fall down• Cars move left to right (or vice versa)• Balls bounce

Exercise: other examples that don’t require arrays?

Animations

• Natural way to introduce looping

• Interesting to students

• Requires introduction of threads– Introduction of threads at this stage sounds

crazy!– But a natural way to get sophisticated behaviors

with fairly simple programs.

Active Objects

• Students extend ActiveObject

• Syntax simplified

• ActiveObjects (Threads) registered for appropriate clean-up.

ActiveObject

• Methods– public void run() - overridden by students– public void start() - called by students– public void pause(double) - exceptionless

• “start()” at end of constructor

A Simple ActiveObject

public FallingBall(DrawingCanvas canvas) {ball = new FilledOval(SCREENWIDTH/2 ,TOP,

BALLSIZE, BALLSIZE, canvas);start();

}public void run() {

while (ball.getY() < BOTTOM ) {ball.move(0, Y_SPEED);pause(DELAY_TIME);

}ball.hide();

}

Interactions

Interactions between ActiveObjects and other objects yield more interesting loop conditions.

• With single non-active object

• Between different active objects– Active objects as generators of others– More complex interactions

Interactions

• Pong

• Falling snow (or leaves)

• Hungry ball

Boxball: a Simple First Exercise

• Falling ball is familiar

• Interaction with a single non-active object (the box)

• Opportunity for thinking about parameters

Frogger: a Second Exercise

Purpose of lab: to consider a variety of simple interactions that appear quite sophisticated.

• Lanes create cars

• Cars know about frog

• Frog knows whether its dead or alive

• Most difficult component of the exercise is management of parameters.

Additional topics

• Smooth animation– Interesting– Necessary for Frogger

• Visible Images– Many more interesting examples– Necessary for Frogger– Good time with respect to our syllabus, but could be done earlier

A Gentle Reminder

• Don’t compare threads to “the old way”

• Many simple but interesting examples… but some simple examples aren’t so simple