CS320n

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CS320n Visual Programming

• Indefinite Loops
• (Slides 7-2)

Thanks to Wanda Dann, Steve Cooper, and Susan
Rodger for slide ideas.
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What We Will Do Today

• Learn about indefinite loops (while)
• Learn about event loops
• Look at a case study involving loops

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Repetition

• Sometimes dont know or cant 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

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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)

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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.

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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?

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Original Version
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Change to Garden World

• What condition can we check to determine if bunny
  two should hop again or stop?

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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

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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)

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World.chase
methods to be written top down design
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Will the loop end?

• What does it depend on?

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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

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goldfish.flee
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goldfish.random Motion
Will the goldfish ever move backwards? How could
the random motion be made more realistic?
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Shark.swim

• Shark pointed in correct direction
• move random distance

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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.

0.52
0.3
0.3
0.3
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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.

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Begin During End Events

• From chapter 7 tips and techniques

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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

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While something is true
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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

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Completed World