Repetition structures
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Repetition Structures. Chapter 5. Chapter 5 Objectives. The Loop Instruction The While Instruction Nested Loops. 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.

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Repetition Structures

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Repetition structures

Repetition Structures

Chapter 5

Chapter 5 objectives

Chapter 5 Objectives

  • The Loop Instruction

  • The While Instruction

  • Nested Loops

The loop instruction

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

Flowcharting a loop instruction


Flowcharting a Loop Instruction

Set index to 0

Index < 12?


Clock, minute rollleft 1 revolution

Clock, hour rollleft 1/12threvolution

Add 1 to index


The loop instruction1


The Loop Instruction

Simplifies to:

Using loop instruction


Using Loop Instruction

  • 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

Computing number of times to repeat


Computing Number of Times to Repeat

  • 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 soccerBall’s 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

Loops controlled by variable

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

Loop with function call

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

Infinite loops


Infinite Loops

  • The loop that never stops

    • Use for objects that shouldn’t stop

  • If placed in consecutive order…the next instructions will NEVER occur, since the loop NEVER ends!

  • Place an infinite loop into a Do Together structure with other items.

Infinite loop background example

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

Infinite loop continual motion

Infinite Loop Continual Motion

  • Helicopter blades are turning while helicopter rescues diver



  • 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?

The while instruction


The While Instruction

  • 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

While instruction flowchart


While Instruction Flowchart

  • Loop’s condition is tested before each repetition of loop

    • If true it performs a repetition and starts over

    • If false the loop terminates

Tennis racket example

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

Tennis racket flowchart


Tennis Racket Flowchart


  • Test loop’s condition: is racket behind ball?

    • Yes? Turn racket forward 0.06 revolutions

    • No? Simulate racket hitting ball by moving forward 5 meters

Racket Distance


ball > 0?

Racket turnsforward 0.06


Ball movesforward 5 meters


Inserting while statement

Inserting While Statement

  • Drag While tile up to Method Editor

  • Choose True or False as placeholder from popup menu

Vanishing cookie example

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

Shrinking alice example

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

Rescue scuba diver example

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

Shark chases fish example

Shark Chases Fish Example

  • Common feature in popular "action films" is exciting chase scene

  • Hungry shark chases after and catches a fleeing fish

Shark chases fish problem

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

Shark chases fish solution

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

Stepwise refinement

Stepwise Refinement

  • Process of breaking problem into smaller tasks

    • Then break each task into simpler steps

  • Once storyboard completed write method for each task

Storyboard shark chases fish

Storyboard: Shark Chases Fish


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

Chase storyboards

Chase Storyboards


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)


Do in order

turn torso left and move forward

turn torso right and move forward

turn torso left and move forward


Do together

wiggle tail

move to random location


Parameter: what

Do in order

shark points at what

shark opens jaw and what disappears

shark closes jaw



  • 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?

Nested loops

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?

Ferris wheel example

Ferris Wheel Example

  • Whole Ferris wheel will rotate clockwise

  • Inner loop runs completely each time outer loop runs once

Ferris wheel example1

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



  • Read chapter 5

  • Answer questions in handout

  • Do lab assignments after handing in answered questions

  • Due one week after assigned with 1 week grace

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