Lecture 12 or President’s Day Holiday

• Chapter 7.2 While Loops– Finish up our race of the penguins

Hint to getting while loops right

• Humans naturally think “until”– I’ll keep spending until I run out of money

(balance <= 0)– I’ll keep dancing until I fall asleep

• Computers use “while” loops – the opposite– I’ll keep spending while I still have money

(balance > 0)– I’ll keep dancing while I am not asleep– While I have a dirty dish, I’ll keep washing dishes

Let’s have a race…

• A – Wind up penguin (he just goes)• While loop with “walk and spin” inside it

– Jet-pack penguin2 (controlled by <- event)• Moves forward .5 meters

• Race to a stop sign (within 2 meters)• Whenever someone gets within 2 meters– Stop looping (going)

When should we keep going?e.g. while loop expression true

(P1: wind up, P2: jet pack)

Cases A B C DP1, P2 outside T T T TP1 inside, P2 outside T T T FP2 inside, P1 outside T T F FP1, P2 inside T F F F

Which while loop header (tile) would you use to control the “going”?

This would STOP the game (evaluate to false) when

A. Both penguins must be close to the stop signB. Either penguin is close to the stop signC. Neither penguin is close to the stop signD. I don’t know

What does the other one do?

Cases E1 E2 Evaluates To (keep playing while true)

P1, P2 outside T TP1 inside, P2 outside

F T

P2 inside, P1 outside

T F

P1, P2 inside F F

Truth Table for OR logical operator

Cases E1 E2 Evaluates To(keep playing while true)

P1, P2 outside T TP1 inside, P2 outside

F T

P2 inside, P1 outside

T F

P1, P2 inside F F

Truth Table for AND logical operator

Let’s look at the code I wrote:

The jet pack penguin (P2) can move forward on a <- event when

A. Neither penguin is close to the stop signB. The windup penguin is close, but the jet pack penguin

isn’tC. The jet pack penguin is close, but the windup penguin

isn’tD. Any time (any possible situation of TT, TF, FT, FF)

Both penguins stop moving when someone “close”

To fix this we’d need to create how many of the following?

1. Method for <- event handler2. Method to be called by the windUpAndGo

method3. If statement in <- event handler4. If statement in penguin move method5. If statement in windUpAndGo method

Which if statement would you want and why?Allow to move when…

C) Both A and BD) Neither A nor BE) I don’t know

Let’s Build This…

2 meters from the stop sign? That seems far!

(3-D object representation trickiness)

• Stop Sign center: in middle SIGN• Penguin center: in middle of FEET• I want to control stopping by distance of

penguin center from BASE of stop sign!• Use math – again

SQRT( c*c – b*b)SQRT (penguin.distanceTo(stopSign) * penguin.distanceTo(stopSign) – stopSign.height * stopSign.height)