Chapter 4: Control Structures II

1
Chapter 4 Control Structures II
2
Chapter Objectives

• Learn about repetition (looping) control
  structures.
• Explore how to construct and use
  counter-controlled, sentinel-controlled, and
  flag-controlled structures.
• Examine break and continue statements.
• Discover how to form and use nested control
  structures.

3
Why is Repetition Needed?

• There are many situations in which the same
  statements need to be executed several times.
• Example
• Formulas used to find average grades for students
  in a class.

4
Repetition

• Java has three repetition, or looping, structures
  that let you repeat statements over and over
  again until certain conditions are met
• while
• for
• dowhile

5
The while Looping (Repetition) Structure

• Syntax
• while (expression)
• statement
• Statements must change value of expression to
  false.
• A loop that continues to execute endlessly is
  called an infinite loop (expression is always
  true).

6
The while Looping (Repetition) Structure

• Example 5-1
• i 0
• while (i lt 20)
• System.out.print(i " ")
• i i 5
• System.out.println()
• Output
• 0 5 10 15 20

7
The while Looping (Repetition) Structure

• Typically, while loops are written in the
  following form
• //initialize the loop control variable(s)
• while (expression) //expression tests the LCV
• .
• .
• .
• //update the loop control variable(s)
• .
• .
• .

8
Counter-Controlled while Loop

• Used when exact number of data or entry pieces is
  known.
• General form
• int N //value input by user or specified
• //in program
• int counter 0
• while (counter lt N)
• .
• .
• .
• counter
• .
• .

9
Counter-Controlled while Loop-Example 5-3

• //Counter-controlled while loopimport
  java.util.public class CounterControlledWhileLo
  op static Scanner console new
  Scanner(System.in) public static void
  main(String args) int limit
  //store the number of items
  //in the list int number
  //variable to store the number int sum
  //variable to store the sum int
  counter //loop control variable
  System.out.print("Enter the number of "
  "integers in the list ")
  limit console.nextInt()
• System.out.println() sum 0
• counter 0 System.out.println("Ent
  er " limit " integers.")

10
Counter-Controlled while Loop-Example 5-3
(continued)

• while (counter lt limit)
  number console.nextInt()
• sum sum number counter
  System.out.printf("The sum of the d " "numbers
  dn", limit, sum)
• if (counter ! 0)
• System.out.printf("The average
  dn",(sum / counter)) else
  System.out.println(
  "No input.")

Sample Run Enter the number of integers I the
list 4 Enter 4 Integers 2 1 5 8 The sum of the 4
numbers 16 The average 4
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Sentinel-Controlled while Loop

• Used when exact number of entry pieces is
  unknown, but last entry (special/sentinel value)
  is known.
• General form
• Input the first data item into variable
• while (variable ! sentinel)
• .
• .
• .
• input a data item into variable
• .
• .
• .

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Sentinel-Controlled while LoopExample 5-4

• //Sentinel-controlled while loopimport
  java.util.public class SentinelControlledWhile
  Loop static Scanner console new
  Scanner(System.in) static final int
  SENTINEL -999 public static void main
  (String args) int number
  //variable to store the number int sum
  0 //variable to store the sum int
  count 0 //variable to store the total
  //numbers read
  System.out.println("Enter positive integers "
  "ending with " SENTINEL)
  

13
Sentinel-Controlled while LoopExample 5-4
(continued)

• number console.nextInt()
  while (number ! SENTINEL)
• sum sum number
  count
• number console.nextInt()
• System.out.printf("The sum of the d "
  "numbers dn", count, sum)if
  (count ! 0) System.out.printf("
  The average dn",(sum / count)) else
  
  System.out.println("No input")

14
Sentinel-Controlled while LoopExample 5-5

• //This program converts uppercase letters to
  their// corresponding telephone
  digits.//
  import java.util.public
  class TelephoneDigit static Scanner input
  new Scanner (System.in) public static void
  main (String args) char letter
  String
  inputMessage
  String inputString
  String outputMessage
  inputMessage "Program to
  convert uppercase "
  "letters to their corresponding "
  "telephone digits.\n"
  "To stop the program enter .\n"
  "Enter a letter"
  System.out.println(inputMessage)
  

15
Sentinel-Controlled while LoopExample 5-5
(continued)

• letter input.next().charAt(0)
  
• while (letter ! '' )
  outputMessage "The letter you entered
  is " letter "\n"
  "The corresponding
  telephone " "digit
  is "
• if (letter gt 'A' letter lt 'Z')
  switch (letter)
  
  case 'A' case 'B'
  case 'C'
  outputMessage outputMessage "2"
  break
  case 'D'
  case 'E' case 'F'
  outputMessage outputMessage "3"
  break
  

16
Sentinel-Controlled while LoopExample 5-5
(continued)

• case 'G'
  case 'H' case 'I'
  outputMessage outputMessage "4"
  break
  case 'J'
  case 'K' case 'L'
  outputMessage outputMessage
  "5" break
• case 'M' case 'N'
  case 'O'
  outputMessage outputMessage "6"
  break
  case 'P'
  case 'Q' case 'R'
  case 'S' outputMessage
  outputMessage "7"
  break

17
Sentinel-Controlled while LoopExample 5-5
(continued)

• case 'T' case
  'U' case 'V'
  outputMessage outputMessage "8"
  break
  case 'W'
  case 'X' case 'Y'
  case 'Z' outputMessage
  outputMessage "9"
  else
  
  outputMessage outputMessage "Invalid
  input"
• System.out.println(outputMessage)
  inputMessage "Enter another uppercase
  letter " "to find its
  corresponding "
  "telephone digit.\n"
  "To stop the program enter .\n"
  "Enter a letter"
  System.out.println (inputMessage)
  letter input.next().charAt(0)
  //end while
  

18
Flag-Controlled while Loop

• Boolean value used to control loop.
• General form
• boolean found false
• while (!found)
• .
• .
• .
• if (expression)
• found true
• .
• .
• .

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Flag-Controlled while Loop-Example 5-6

• /Flag-controlled while loop.
• //Guessing the number game.
• import java.util.public class
  FlagControlledLoop static Scanner console
  new Scanner(System.in) public static void
  main (String args) //declare the
  variables int num //variable to
  store the random number int guess
  //variable to store the number
  //guessed by the user boolean done
  //boolean variable to control the loop
  num (int) (Math.random() 100)
• done false

20
Flag-Controlled while Loop-Example 5-6
(continued)

• while (!done)
• System.out.print ("Enter an integer greater"
  " than or equal to 0
  and " "less than
  100 ")
• guess console.nextInt()
  System.out.println()
• if (guess num)
  System.out.println("You guessed the "
  "correct number.")
• done true
• else if (guess lt num)
• System.out.println("Your guess is "
  "lower than "
  "the number.\n"
  "Guess again!")
• else
  System.out.println("Your
  guess is
• "higher
  than " "the
  number.\n"
  "Guess again!") //end while
  

21
While Loop Programming Example Fibonacci Number

• Fibonacci formula for any Fibonacci sequence
• an an-1 an-2
• Input First two Fibonacci numbers in sequence,
  position in sequence of desired Fibonacci number
  (n).
• int previous1 Fibonacci number 1
• int previous2 Fibonacci number 2
• int nthFibonacci Position of nth Fibonacci
  number
• Output nth Fibonacci number.

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While Loop Programming Example Fibonacci Number

• if (nthFibonacci 1)
• current previous1
• else if (nthFibonacci 2)
• current previous2
• else
• counter 3
  
• while (counter lt nthFibonacci)
• current previous2 previous1
• previous1 previous2
• previous2 current
• counter
  
• Final result found in last value of current.

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The for Looping (Repetition) Structure

• Specialized form of while loop.
• Its primary purpose is to simplify the writing of
  counter-controlled loops. For this reason, the
  for loop is typically called a counted or indexed
  for loop. .
• Syntax
• for (initial statement loop condition update
  statement)
• statement

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The for Looping (Repetition) Structure

• Execution
• Initial statement executes.
• Loop condition is evaluated.
• If loop condition evaluates to true,
• execute for loop statement and
• execute update statement.
• Repeat step 2 until loop condition is false.

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The for Looping (Repetition) Structure

• Example 5-9
• The following for loop prints the first 10
  nonnegative integers
• for (i 0 i lt 10 i)
• System.out.print(i " ")

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The for Looping (Repetition) Structure

• Example 5-10
• The following for loop outputs the word Hello and
  a star (on separate lines) five times
• for (i 1 i lt 5 i)
• System.out.println("Hello")
• System.out.println("")
• 2. The following for loop outputs the word Hello
  five times and the star only once
• for (i 1 i lt 5 i)
• System.out.println("Hello")
• System.out.println("")

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The for Looping (Repetition) Structure

• Does not execute if loop condition is initially
  false.
• Update expression changes value of loop control
  variable, eventually making it false.
• If loop condition is always true, result is an
  infinite loop.
• Infinite loop can be specified by omitting all
  three control statements.
• If loop condition is omitted, it is assumed to be
  true.
• Action of for loop ending in semicolon is empty.

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For Loop Programming Example Classify Numbers

• Input N integers (positive, negative, and
  zeros).
• int N 20 //N easily modified
• Output Number of 0s, number of even integers,
  number of odd integers.

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For Loop Programming Example Classify Numbers
(solution)

• for (counter 1 counter lt N counter)
• number console.nextInt()
• System.out.print(number " ")
• switch (number 2)
• case 0 evens
• if (number 0)
• zeros
• break
• case 1
• case -1 odds
• //end switch
• //end for loop

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The dowhile Loop (Repetition) Structure

• Syntax
• do
• statement
• while (expression)
• Statements are executed first and then expression
  is evaluated.
• Statements are executed at least once and then
  continued if expression is true.

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dowhile Loop (Post-Test Loop)
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dowhile Loop (Post-Test Loop)

• Example
• i 0
• do
• System.out.print(i )
• i i 5
• while ( i lt 30 )
• output 0 5 10 15 20 25 30

33
break Statements

• Used to
• exit early from a loop. (while, for, and
  do...while)
• skip remainder of switch structure.
• Can be placed within if statement of a loop.
• If condition is met, loop is exited immediately.
• After the break statement executes, the program
  continues to execute with the first statement
  after the structure

34
break Statements

• Example
• int count
• for ( count 1 count lt 10 count )
• if ( count 5)
• break
• System.out.print(count )

Output 1 2 3 4
35
continue Statements

• Used in while, for, and do...while structures.
• When executed in a loop, the remaining statements
  in the loop are skipped proceeds with the next
  iteration of the loop.
• When executed in a while/dowhile structure,
  expression is evaluated immediately after
  continue statement.
• In a for structure, the update statement is
  executed after the continue statement the loop
  condition then executes.

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continue Statements

• Example
• int count
• for ( count 1 count lt 10 count )
• if ( count 5)
• continue
• System.out.print(count )

Output 1 2 3 4 6 7 8 9 10
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Nested Control Structures

• Provides new power, subtlety, and complexity.
• if, ifelse, and switch structures can be placed
  within while loops.
• for loops can be found within other for loops.

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Nested Control Structures (Example 5-18)

• for (int i 1 i lt 5 i)
• for (int j 1 j lt i j)
• System.out.print(" ")
• System.out.println()
• Output

39
Nested Control Structures (Example 5-19)

• //printing a multiplication table
• for (i 1 i lt 5 i) for (j
  1 j lt 10 j) System.out.printf("3
  d", ij) System.out.println()
  

Output 1 2 3 4 5 6 7 8 9 102 4 6 8 10
12 14 16 18 203 6 9 12 15 18 21 24 27 304 8
12 16 20 24 28 32 36 405 10 15 20 25 30 35 40 45
50
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Chapter Summary

• Looping mechanisms
• Counter-controlled while loop
• Sentinel-controlled while loop
• Flag-controlled while loop
• for loop
• dowhile loop
• break statements
• continue statements
• Nested control structures