1. CS320n –Visual Programming Indefinite Loops (Slides 7-2) Thanks to Wanda Dann, Steve Cooper, and Susan Rodger for slide ideas.

2. Visual ProgrammingIndefinite Loops2 What We Will Do Today Learn about indefinite loops (while) Learn about event loops Look at a case study involving loops

3. Visual ProgrammingIndefinite Loops3 Repetition Sometimes don’t know or can’t computer exactly how many times a set of instructions are repeated Stopping is based on some condition (a boolean) Examples: –Game of Chess, how many moves until win Stop: when markers are in a check mate position –The bunny world. Bunny two jump up and down until Bunny one done eating. Actually could roughly calculate this based on distances

4. Visual ProgrammingIndefinite Loops4 Indefinite Repetition In programs where a count of repetitions is not known (indefinite), we can use one of two repetition control mechanisms: – While statement (today) – Recursion (next time)

5. Visual ProgrammingIndefinite Loops5 While statement While some condition is true –execute statements 1 time, go and recheck condition To write a While statement, you need to know the condition that determines whether the loop will be repeated.

6. Visual ProgrammingIndefinite Loops6 Example In the garden scene from last time bunny one went around and ate up the food bunny two hopped in place forever what if we want bunny two to hop until bunny one is done eating all the food? –in other words we want bunny two to hop while what is true?

7. Visual ProgrammingIndefinite Loops7 Original Version

8. Visual ProgrammingIndefinite Loops8 Change to Garden World What condition can we check to determine if bunny two should hop again or stop?

9. Visual ProgrammingIndefinite Loops9 Example Two A chase scene hungry shark chasing fleeing goldfish –repeat: fish swim away from shark and shark swim toward fish –shark swim distance a little more than fish swim distance –eventually shark will catch up with fish

10. Visual ProgrammingIndefinite Loops10 Storyboard World.chase while goldfish more than 0.5 meters from shark Do in order shark point at goldfish Do together shark swim (toward goldfish) goldfish flee (away from shark) resolve chase (shark eat goldfish or goldfish eat shark)

11. Visual ProgrammingIndefinite Loops11 World.chase methods to be written: top down design

12. Visual ProgrammingIndefinite Loops12 Will the loop end? What does it depend on?

13. Visual ProgrammingIndefinite Loops13 Ending the Loop Assume the goldfish and the shark move a random distance –make the shark move forward between 0.1 and 0.6 meters –make the goldfish move between 0.0 and 0.3 meters –Shark will eventually catch up, and the loop will end. –not sure how many steps it will take, but the shark will eventually catch the goldfish

14. Visual ProgrammingIndefinite Loops14 goldfish.flee

15. Visual ProgrammingIndefinite Loops15 goldfish.random Motion Will the goldfish ever move backwards? How could the random motion be made more realistic?

16. Visual ProgrammingIndefinite Loops16 Shark.swim Shark pointed in correct direction move random distance

17. Visual ProgrammingIndefinite Loops17 The loop will end –Geometrically, the fish can never move more than 0.52 meters away the distance formula – The shark has a distance advantage and will eventually catch up. The loop will end. 0.3 0.52

18. Visual ProgrammingIndefinite Loops18 General “Rule of Thumb” As a general rule, a While loop should be written so the loop will eventually end. – Requires that statements within the loop change the conditions of the world such that the condition for the While statement will eventually become false. If the While loop never ends, it is an infinite while loop.

19. Visual ProgrammingIndefinite Loops19 Begin – During – End Events From chapter 7 tips and techniques

20. Visual ProgrammingIndefinite Loops20 Storyboard Helicopter circling the rabbit on the island rabbit turns to look at the helicopter as long as the helicopter is in front of him –no “Exorcist” rabbit 2 repeated actions –helicopter circling –rabbit moving head when rabbit can no longer see helicopter turn to look at camera

21. Visual ProgrammingIndefinite Loops21 While something is true

22. Visual ProgrammingIndefinite Loops22 Begin – During - End Selecting the event “While Something is True” creates 3 options begin: something done when event becomes true during: something done while event is true end: what to do when event is no longer true

23. Visual ProgrammingIndefinite Loops23 Completed World