Repetition Structures

1
Repetition Structures

• Chapter 5 Part 2

2
The While Instruction

• Combines Loop and the condition that is also used
  in If/else statements
• The loop repeats as long as its Boolean condition
  is true
• Called conditional loop since loop is controlled
  by a condition
• Also called pretest loop since a test has to pass
  before it does the instructions in the loop
• Otherwise it skips those instructions

3
While Instruction Flowchart

• Loops condition is tested before each repetition
  of loop
• If true it performs a set of instructions and
  loop starts over
• If false then the loop terminates

4
Inserting While Statement

• Drag While tile up to Method Editor
• Choose True or False as placeholder from popup
  menu

5
Proximity Example

• Tennis racket is rotated 0.06 revolutions until
  it is next to the ball
• Ball moves forward looking like it was hit
• Uses
• Function call
• Relational operation

6
Tennis Racket Flowchart
Start

• Test loops condition is racket behind ball?
• Yes? Turn racket forward 0.06 revolutions
• No? Simulate racket hitting ball by moving
  forward 5 meters

Racket Distance Behind ball gt 0?
Yes
Racket turnsforward 0.06 revolution
No
Ball movesforward 5 meters
End
7
Object Slowly Disappears

• Magician says Abracadabra
• In While loop
• Cookie reduces in opacity by 10 until opacity is
  0 or cookie disappears
• Uses
• Opacity property
• Relational operation
• Uses math to change
• while condition
• Notice Condition is changed in While

8
Change Size in While

• In Alice in Wonderland, Alice drinks something
  which causes her to shrink which allows her to go
  thru a small door
• While loop is used to constantly shrink her by
  half until she is smaller than door
• Uses
• Two function calls
• Relational operation
• Notice Condition is changed in While

9
Use While to Move an Object

• Helicopter moves to scuba diver
• In While loop Helicopter descends to diver ½
  meter at time until 1 meter above diver
• Notice style is abruptly
• Prevents jerky movement
• After While is finished ladder is lowered to
  diver
• Event turns blades with infinite loops

10
Shark Chases Fish Example

• Common feature in popular "action films" is an
  exciting chase scene
• Hungry shark chases after and catches a fleeing
  fish
• Shark should not immediately catch goldfish
• Otherwise there would be no chase
• Goldfish should appear to be fleeing

11
Shark Chases Fish Solution

• To create a chase scene
• Shark will swim short distance toward fish
• At same time fish will swim short distance away
  from shark
• Fish will flee to a random location
• As long as goldfish is 0.5 meter away from shark
  repeat above actions

12
Algorithm Shark Chases Fish

• chase
• While the goldfish is more than 0.5 meters away
  from the shark
• Do in order
• shark point at the goldfish
• Do together
• shark swim (toward the goldfish)
• goldfish flee (away from the shark)
• shark eat (the goldfish)

• shark swim, goldfish flee, and shark eat actions
  are complex
• Use stepwise refinement to break them down into
  simple steps

13
Review

• Why is the While instruction considered a
  conditional loop?
• What causes the While loop to stop repeating?
• Why is the While instruction called a pretest
  loop?

14
Nested Loops

• A nested loop is a loop that is inside of another
  loop
• The inner loop executes all its iterations for
  every single iteration of the outer loop
• How many times does the bee pace or move forward?

15
Ferris Wheel Example

• Whole Ferris wheel will rotate clockwise
• Inner loop runs completely each time outer loop
  runs once

16
Ferris Wheel Example

• Outer loop executes 10 times and inner loop
  executes 2 times
• How many times does inner wheels rotate?
• Inner loop executed 20 times
• 2 wheel inner rotations 10 outer rotations

17
Homework

• Read chapter 5
• Do lab assignments
• Due in one week