Repetition Structures

1
Repetition Structures

• Chapter 5

2
Chapter 5 Objectives

• The Loop Instruction
• The While Instruction
• Nested Loops

3
The Loop Instruction

• Simple control structure allows
• Instruction (or block of instructions) to be
  repeated multiple times
• Repeated fixed amount of times
• A whole number

4
Flowcharting a Loop Instruction
5.1
Set index to 0
Index lt 12?
T
Clock, minute rollleft 1 revolution
Clock, hour rollleft 1/12threvolution
Add 1 to index
F
5
The Loop Instruction
5.1

• Simplifies to

6
Using Loop Instruction
5.1

• Drag Loop instruction from bottom of Method
  Editor
• It prompts you for amount of times to loop
• Creates empty slot where instructions can be
  placed

7
Computing Number of Times to Repeat
5.1

• What if you want something to loop and NOT have
  to specify a particular number of repetitions?
• Example getting a ball to roll across into a
  goal regardless of where the ball is initially
  placed
• Use the soccerBalls distance to function to
  calculate distance to goal
• Loop uses truncation decimal portions of numbers
  are discarded
• Examples objects loop 4 times not 4.8 or 3 times
  not 3.4

8
Loops Controlled by Variable

• Ask user for amount of times a lion should roar
• Place value in variable
• Loop the amount of times user entered

9
Loop With Function Call

• Loop count can be computed by calling a function
  that returns a number value
• Loop instruction automatically rounds returned
  value down to whole number

10
Infinite Loops
5.1

• The loop that never stops
• Use for objects that shouldnt stop
• If placed in consecutive orderthe next
  instructions will NEVER occur, since the loop
  NEVER ends!
• Place an infinite loop into a Do Together
  structure with other items.

11
Infinite Loop Background Example

• Infinite loops are good for moving objects in
  background
• This example simulates moving water by changing
  texture of water
• Called from main method

12
Infinite Loop Continual Motion

• Helicopter blades are turning while helicopter
  rescues diver

13
Checkpoint

• What type of structure is the Loop instruction?
• What are three different types of loops?
• What are different ways you can use a finite non
  fixed loop?
• What is an infinite loop?

14
The While Instruction
5.2

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

15
While Instruction Flowchart
5.2

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

16
Tennis Racket Example

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

17
Tennis Racket Flowchart
5.2
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?
Racket turnsforward 0.06 revolution
Ball movesforward 5 meters
End
18
Inserting While Statement

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

19
Vanishing Cookie Example

• 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

20
Shrinking Alice Example

• 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

21
Rescue Scuba Diver Example

• Helicopter moves to scuba diver
• With While loop Helicopter descends to diver ½
  meter at time until 1 meter above diver
• Notice style is abruptly
• To prevent jerky movement
• Ladder is lowered to diver

22
Shark Chases Fish Example

• Common feature in popular "action films" is
  exciting chase scene
• Hungry shark chases after and catches a fleeing
  fish

23
Shark Chases Fish Problem

• How do we get shark to chase goldfish in a
  chase-like action?
• Shark should not immediately catch goldfish
• Otherwise there would be no chase
• Goldfish should appear to be fleeing

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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

25
Stepwise Refinement

• Process of breaking problem into smaller tasks
• Then break each task into simpler steps
• Once storyboard completed write method for each
  task

26
Storyboard 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

27
Chase Storyboards
chase While the goldfish is more than 0.5 meters
from the shark Do in order Point the
shark at the goldfish Do together
shark swim goldfish flee shark eat
(goldfish)
swim Do in order turn torso left and move
forward turn torso right and move forward
turn torso left and move forward
flee Do together wiggle tail move to random
location
eat Parameter what Do in order shark points
at what shark opens jaw and what disappears
shark closes jaw
28
Checkpoint

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

29
Nested Loops

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

30
Ferris Wheel Example

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

31
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

32
Homework

• Read chapter 5
• Answer questions in handout
• Do lab assignments after handing in answered
  questions
• Due one week after assigned with 1 week grace