Go Over Test

1
Go Over Test 1

• Grade Breakdown (Fall 2005)
• A's 6
• B's 2
• C's 3
• How to Study for CSc-037
• Visit the instructor's office for help
• Work on HW Early (as opposed to starting it the
  night before it is due)
• Read Text Before After Class
• Make sure you understand what's happening in Lab
• Play with Sample Programs in Course Folder
• Work on Exercises in Textbook
• Try Sample Tests without looking at answers
• (not just the night before the test)

2
Week 04 - b

• The while Loop The do-while LoopReading Input
  in a Loop
• The for Loop

3
Sample Programs Referenced(code located in
course folder)

• BubbleGum
• java loops.WhileLab
• (AnimatedIllustrations)
• Count Digits
• InfiniteLoop

• Display Word
• Target
• BouncingBall
• 23Matches Game

4
Introduction to Repetition

• Control structures regulate flow of execution in
  a program or method
• Individual instructions combined into a single
  logical unit with
• One entry point
• One exit point
• 3 Types of Control Structures
• Sequence
• Selection
• Repetition

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Repetition

• Two types of Repetition
• Condition Controlled
• repeat something while a condition remains true
• e.g. while not at your destination, keep driving
• Counter Controlled
• repeat something a certain number of times
• e.g. drive 100 miles (or drive a mile 100 times!)

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Bubble Gum ExampleBlueJ

• Make an applet that shows a bubble popping
• The applet should make a bubble and ask it to
  pop
• The bubble should grow larger and larger until it
  fills the applet window and pops
• Demonstrate the applet
• The classes
• BubbleApplet - applet with paint
• Bubble
• has a maximum radius and a Graphics object
• has a pop method
• draws larger and larger circles

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Popping the Bubble

• Algorithm for pop method
• initialize the bubbles radius to 1
• while the bubble fits in the window
• draw the bubble in the center of the window
• increase the bubbles radius
• wait a short time
• clear the window (to simulate popped bubble)
• write the word POP in the center of the window
• How do we implement the loop?

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while Loop

• Repeatedly executes body of the loop which can be
  a single or compound statement
• A while loop will execute as long as its
  condition is true
• An infinite loop will occur if the condition
  never becomes false

while (condition) statement
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Review the Bubble Code

• Applet
• Bubble
• Constructor
• What does it receive as parameters?
• What does it do?
• Trace the execution of the pop method
• What happens in the loop?
• How does it end?
• What would happen if we didnt add 1 to the
  radius inside the loop body?
• What would happen if we started the radius at
  maxRadius?
• Aside Take a look at the wait method in the
  Timer class
• Note use of static. What kind of method is it?
  How is it called?

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while Loop(Animated Illustrations)

• java loops.WhileLab

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Danger Semicolon Ahead

• What would happen in the following code?
• x 5
• while (xgt0)
• out.println(x)
• x--

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Stopping "Infinite" Loops

• Click "Halt" in the Debugger Window
• Click "Terminate" in the Debugger Window
• OR ltCtrl/Alt/Deletegt
• and run "Task Manager"

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do-while Loop

• Alternative to the while loop for conditional
  repetition
• Test done at the bottom of the loop
• Always executes the body of the loop at least
  once
• while loop can be used to solve the same
  problems, but for some problems the do-while is
  easier to read

do statement while (condition)
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A do-while ExampleBlueJ

• Count the number of digits in a whole number
• Idea keep dividing the number by ten (removing a
  digit) until the number reaches 0 and count the
  number of times you divide.
• We want 0 to yield "1 digit"
• Example number 251
• divide by 10 ? 25 (count is 1)
• divide by 10 ? 2 (count is 2)
• divide by 10 ? 0 (count is 3)

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Reading in a Loop

• What if we want to do something with the input as
  we read it?
• Example
• read in a string
• calculate the number of characters in the string
• display the string and the number of characters
• repeat until the user enters the string STOP
• STOP is called the sentinel (signal)
• We could be reading in numbers and stop when we
  get a negative value (if negative values were not
  legal input). -1 would be the sentinel

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Sentinel-Controlled Loops 1 of 2

• do forever
• read input
• if input is the sentinel value, exit the loop
• process the input
• Pro more natural sequence in loop body
• Con exit in middle of loop
• How do we loop forever? exit a loop in the
  middle?

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Forever loops

• What if the condition is always true?
• Infinite loop
• How can we make it be true always?
• while (true)
• How can we exit the loop?
• break statement (as used in switch)

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Sentinel-Controlled Loops 2 of 2

• Code pattern using forever loop
• while (true)
• read input
• if ( end-of-input ) break
• process input
• // end while loop

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Error Handling

• A forever loop is also useful for fool-proof
  input.
• Pattern

Prompt for value while (true) Read
value if (value is valid) break
Display error message
This is good because control will only leave the
loop if/when the user enters a valid value.
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Sentinel Example(Part of HW 4)

• Want to read in a set of exam scores and
• count the number of scores read
• compute the sum (for average) of the scores
• determine the highest and lowest scores
• Algorithm
• do forever
• read a score
• if SENTINNEL was read, exit loop
• process the score

21
Problem Analysis

• Read in a set of exam scores and evaluate them
• count the number of Outstanding, etc.
• compute the average of the scores
• determine the highest and lowest scores

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Counting in a Loop

• How do we count the scores?
• What is the process step?
• What variable(s) will be required?
• What should the scope of the variable(s) be?

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Accumulating in a Loop

• Our program should compute the average of the
  scores
• Average is the sum divided by the count
• How do we compute the sum?
• Manually?
• What is the process step?
• What variable(s) will be required?
• How and where should we compute the average?

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Finding a Max and Min

• Our program should determine the max and min
  scores
• How can we do this? Ideas?
• initialize min to Integer.MAX_VALUE
• initialize max to Integer.MIN_VALUE
• Modify the algorithm
• New variables?
• Walk through the algorithm to test it

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HW 4

• A. Grade Analysis
• (Problem 4, page 268)
• B. Fading into the Horizon
• (Problem 12, page 269)

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2 "Volunteers" To the Board

• Write a while loop (2nd person write a do-while
  loop) that helps to launch the space
  shuttle--print out
• 10
• 9
• 8
• ...
• 1
• Blast Off!

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Java BREAK
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Additional Integer Operators

• Increment
• num
• equivalent to ? num num 1
• Decrement
• num--
• equivalent to ? num num - 1
• Compound Operations
• num 2
• equivalent to ? num num 2
• num - 2
• equivalent to ? num num 2

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Repetition

• Two types of repetition
• condition controlled
• repeat something while a condition remains true
• e.g. while not at your destination, keep driving
• counter controlled
• repeat something a certain number of times
• e.g. drive 100 miles (or drive a mile 100 times!)
• We now look at implementing counter-controlled
  loops in Java

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Counter-Controlled Loops

• Frequently we know how many times we want to
  repeat a task a priori.
• Examples
• A baseball game has 9 complete innings (ignore
  tie games).
• 3 outs per inning
• Display 5 circles in an applet
• Process the characters in a String
• (know the length)
• A course usually has a fixed number of students ?
  fixed number of scores
• Algorithm
• For each student
• Read a score
• Process the score

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Counter-Controlled Loops

• The for loop is normally used to count through a
  range of values

for (int count first count lt last
count3) Statement
Such a loop will count from first to last,
executing Statement once for each value in the
range first..last.
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for Loop Example

• An example first
• for (count 1 // 1. Initialization
• count lt 5 // 2. Test
• count) // 3. Update
  (Increment)
• // statements to do // body of loop
• 1. Initialize count to 1
• 2. Evaluate count lt 5
• true,
• body is executed
• increment count by 1
• repeat
• false, body is skipped

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The for Loop

• Syntax
• for (initialization-statement // 1.
  Initialization
• test-condition // 2. Test
• update-statement) // 3. Update
• statement-or-compound-statement // Body
• How it works
• 1. Perform initialization
• 2. Evaluate condition
• true,
• body is executed
• perform update statement
• repeat
• false, body is skipped

Could we do the same thing with a while loop ?
34
Count by tens

• Write a for loop to count from 0 to 100 by 10s.

for (int count 0 count lt 100 count10)
System.out.println(count)
Modify the loop to count from 100 down to 0 by
10s.
for (int count 100 count gt 0 count-10)
System.out.println(count)
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For Loop Examples

• Experiment with Java code for the following
  examples
• "for" Example 1 (should be able to do these
  easily)
• Print the integers 1 through 5 - one at a time in
  order
• Print the even integers 0 through 6
• Print the integers 5 down to 1
• "for" Example 2 (see BLUEJ)
• Write a word down the page a character at a time
• Example 3 (next slide)
• Display a multi-circle target
• (next slide)

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BullsEye redux

• Display a "Target" using a for loop
• Review code
• Experiment with the for loop
• Try smaller number of circles
• Try incrementing by 2

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Compute N factorial

• N! N (N-1) (N-2) 2
• e.g. 4! 4 3 2 24
• Algorithm
• Initialize product to 1 (multiplicative identity)
• Initialize multiplier to number (N)
• Loop
• Compute product product multiplier
• Decrement multiplier by 1
• End loop
• Return product
• What terminates the loop? What kind of loop?

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factorial Function

• public static long factorial (int num)
• // long to allow big values
• long product 1 // multiplicative identity
• for (int m num m gt 2 m--)
• product m
• return product
• // end of factorial method

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"Perfect" Numbers

• An positive integer is said to be perfect if it
  is equal to the sum of its factors
• including 1
• obviously excluding itself
• Try a few integers such as 6, 17, 28, 59,

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Code to Check for a"Perfect" Number

• public static boolean isPerfect(int n)
• int sum 0
• for (int divisor1 divisorltn divisor)
• if (n divisor 0)
• sum divisor
• return (sum n)
• // end of isPerfect method

41
Bouncing Ball

• Suppose that when a ball is dropped, it bounces
  from the pavement to a height one-half of its
  previous height.
• We want to write a program that will simulate the
  behavior of the ball.
• The program should count the number of bounces
  that it will take before the height is less than
  some minimal height.

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Path of the Bouncing Ball
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Follow the Bouncing Ball
Algorithm Initialize bounce count to 0 Prompt
for and read height Loop Replace height by
height / 2 Add 1 to bounce End loop

• How/Where should repetition terminate?
• What kind of loop should be used?
• What if we were asking how high the ball would
  be after 10 bounces?

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Code for Bouncing Ball

• /
• Calculate how many times a ball will bounce.
• _at_author Dave Hannay
• _at_version CSc-105 Demo
• /
• public class BouncingBall
• final double MIN_HEIGHT 0.01 // somewhat
  arbitrary
• public int bounce(double height)
• int bounce 0
• while (height gt MIN_HEIGHT)
• height / 2.0
• bounce
• // end of while (height gt MIN_HEIGHT)
• return bounce
• // end of bounce method
• // end of BouncingBall class

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General Game Control(pseudocode)

• boolean computerTurn false
• Gameboard game new Gameboard()
• do
• if (computerTurn)
• game.makeComputerMove()
• else
• game.getHumanMove()
• computerTurn !computerTurn
• while (!game.gameOver())

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Game of 23 Matches

• Start with 23 matches lying on a table.
• On each turn, you may take 1, 2, or 3 matches.
• You alternate moves with the computer, and the
  one who takes the last match loses.
• Similar to "Old Maid"

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Code for "23 Matches" Game

• public class Matches23Client
• public static void main(String args)
• boolean computerTurn false
• Matches23 matches new Matches23()
• do
• matches.display()
• if (computerTurn)
• matches.makeMove()
• else
• matches.getHumanMove()
• computerTurn !computerTurn
• while (!matches.gameOver()) // game over if
  no more matches
• System.out.println()
• if (computerTurn)
• System.out.println("I win, you lose!")
• else
• System.out.println("Amazing as this may
  be, you beat me!")
• // end of main method
• // end of Matches23Client class

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Matches23 Class

• View class in BlueJ

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To the Computers...

• Try to beat the computer at "23 Matches"
• Drag from course folder
• (Matches Game on Palm Computer)

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Java BREAK