Repetition StructuresChapter 5 Part 2

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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 o Otherwise it skips those instructions

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While Instruction Flowchart

Loop’s condition is tested before each repetition of loopo If true it performs a

set of instructions and loop starts over

o If false then the loop terminates

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Inserting While Statement Drag While tile up to Method Editor

Choose True or False as placeholder from popup menu

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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

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Tennis Racket Flowchart

Test loop’s condition: is racket behind ball?o Yes? Turn racket

forward 0.06 revolutions

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

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Start

Racket DistanceBehind

ball > 0?

Racket turnsforward 0.06

revolution

Ball movesforward 5 meters

End

Yes

No

Object Slowly Disappears Magician says

Abracadabra In While loop

o Cookie reduces in opacity by 10% until opacity is 0% or cookie disappears

Uses o Opacity propertyo Relational operationo Uses math to

change while condition

Notice Condition is changed in While

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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

Useso Two function callso Relational

operation Notice Condition is

changed in While 8

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 divero Notice style is abruptly

Prevents jerky movement

After While is finished ladder is lowered to diver

Event turns blades with infinite loops

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Shark Chases Fish Example Common feature in popular "action films" is an

exciting chase scene Hungry shark chases after and catches a fleeing fish

o Shark should not immediately catch goldfish Otherwise there would be no chase

o Goldfish should appear to be fleeing

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Shark Chases Fish Solution

To create a chase scene o Shark will swim short distance toward fisho At same time fish will swim short distance

away from sharko Fish will flee to a random locationo As long as goldfish is 0.5 meter away from

shark repeat above actions

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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

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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?

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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?

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Ferris Wheel Example Whole Ferris wheel will rotate

clockwise Inner loop runs completely

each time outer loop runs once

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Ferris Wheel Example

Outer loop executes 10 times and inner loop executes 2 times

How many times does inner wheels rotate?o Inner loop executed 20 times

2 wheel inner rotations * 10 outer rotations

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Homework Read chapter 5 Do lab assignments Due in one week

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