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

CS320n –Visual Programming

Indefinite Loops

(Slides 7-2)

Thanks to Wanda Dann, Steve Cooper, and Susan Rodger for slide ideas.

Visual Programming Indefinite Loops 2

What We Will Do Today• Learn about indefinite loops (while)

• Learn about event loops

• Look at a case study involving loops


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


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)


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.


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?


Original Version


Change to Garden World• What condition can we check to determine

if bunny two should hop again or stop?


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


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)


World.chase

methods to be written: top down design


Will the loop end?• What does it depend on?


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


goldfish.flee


goldfish.random Motion

Will the goldfish ever move backwards?How could the random motion be made more realistic?


Shark.swim• Shark pointed in correct direction• move random distance


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.

0.30.3

0.3

0.52


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.


Begin – During – End Events• From chapter 7 tips and techniques


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


While something is true


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


Completed World

Documents

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