Repetition Structures

Transcription

1 Repetition Structures Chapter 5 5-1

2 Chapter 5 Objectives The Loop Instruction The While Instruction Nested Loops 5-2

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

4 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 5-4

5 5.1 The Loop Instruction Simplifies to: 5-5

6 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 5-6

7 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? o Example: getting a ball to roll across into a goal regardless of where the ball is initially placed o o Use the soccerball s distance to function to calculate distance to goal Loop uses truncation: decimal portions of numbers are discarded o Examples: objects loop 4 times not 4.8 or 3 times not

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 5-8

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 5.1 Infinite Loops The loop that never stops o 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. 5-10

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 5-11

12 Infinite Loop Continual Motion Helicopter blades are turning while helicopter rescues diver 5-12

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? 5-13

14 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 o Otherwise it skips those instructions 5-14

15 5.2 While Instruction Flowchart Loop s condition is tested before each repetition of loop o If true it performs a repetition and starts over o If false the loop terminates 5-15

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 5-16

17 5.2 Tennis Racket Flowchart Start Racket Distance Behind ball > 0? Racket turns forward 0.06 revolution Test loop s condition: is racket behind ball? o o Yes? Turn racket forward 0.06 revolutions No? Simulate racket hitting ball by moving forward 5 meters Ball moves forward 5 meters End 5-17

18 Inserting While Statement Drag While tile up to Method Editor Choose True or False as placeholder from popup menu 5-18

19 Vanishing Cookie Example Magician says Abracadabra In While loop o Cookie reduces in opacity by 10% until opacity is 0% or cookie disappears Uses o Opacity property o Relational operation o Uses math to change while condition 5-19

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 o o Two function calls Relational operation 5-20

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 o To prevent jerky movement Ladder is lowered to diver 5-21

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? o Shark should not immediately catch goldfish Otherwise there would be no chase o Goldfish should appear to be fleeing

24 Shark Chases Fish Solution To create a chase scene o Shark will swim short distance toward fish o At same time fish will swim short distance away from shark o Fish will flee to a random location o As long as goldfish is 0.5 meter away from shark, repeat above actions

25 Stepwise Refinement Process of breaking problem into smaller tasks o 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? 5-28

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? 5-29

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? o 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