Repetition Structures

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

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

5.1 The Loop Instruction Simplifies to:

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

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

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?

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

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

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 turnsforward 0.06 revolution Ball movesforward 5 meters End

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

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 • 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 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

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

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

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

Repetition Structures