# Repetition Structures

## Presentation on theme: "Repetition Structures"— Presentation transcript:

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

Flowcharting a Loop Instruction
5.1 Flowcharting a Loop Instruction Set index to 0 Index < 12? T Clock, minute roll left 1 revolution Clock, hour roll left 1/12th revolution Add 1 to index F

5.1 The Loop Instruction Simplifies to:

Using Loop Instruction
5.1 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
5.1 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
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 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 The loop that never stops
5.1 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 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
Helicopter blades are turning while helicopter rescues diver

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?

The While Instruction Combines Loop and If/else statements
5.2 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
5.2 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 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
5.2 Tennis Racket Flowchart Start 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 Behind ball > 0? Racket turns forward 0.06 revolution Ball moves forward 5 meters End

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

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

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
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 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
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
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 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

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?

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

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