Chapter 5: Control Structures II

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Chapter 5 Control Structures II

• Java Programming
• From Problem Analysis to Program Design,
• Second Edition

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Chapter Objectives

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

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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.
• Three looping structures
• Whlie
• For
• Dowhile

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The while Looping (Repetition) Structure

• Syntax
• while (expression)
• statement
• Expression is always true in an infinite loop.
• Statements must change value of expression to
  false.

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The while Looping (Repetition) Structure

• Example 5-1
• i 0 //Line 1
• while (i lt 20) //Line 2
• System.out.print(i " ") //Line 3
• i i 5 //Line 4
• System.out.println() //Line 5
• Output
• 0 5 10 15 20

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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)
• .
• .
• .

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• public class loop1
• public static void main(Stringargs)
• int i // i is the loop control variable
  (LCV)
• double j
• i0 // the loop must be initialized
• while (ilt25) // remember no after the
  expression
• jMath.pow(i,3) // to raise i to the power of 3
• System.out.println(i","j)
• ii1 // if this statement is omitted, we
  have an infinite loop

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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
• .
• .
• .

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• import java.util.
• import java.io.
• public class loop2
• static Scanner console new Scanner(System.in)
• public static void main(Stringargs)
• throws FileNotFoundException
• Scanner inFile new Scanner(new
  FileReader("loop.txt"))
• int n
• System.out.println("Please enter number of
  students in the class")
• nconsole.nextInt()
• int counter
• counter1
• while (counterltn)

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• String fname
• String lname
• int score1,score2,score3
• int tot
• double avg
• fnameinFile.next()
• lnameinFile.next()
• score1inFile.nextInt()
• score2inFile.nextInt()
• score3inFile.nextInt()
• tot score1score2score3
• avgtot/3
• System.out.println(fname " " lname " "
  score1" " score2" " score3" " tot" "
  avg)
• counter
• inFile.close()

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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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• import java.util.
• import java.io.
• public class loop2sentinel
• static Scanner console new Scanner(System.in)
• static final int SENTINEL-999
• public static void main(Stringargs)
• throws FileNotFoundException
• Scanner inFile new Scanner(new
  FileReader("sentinel.txt"))
• int number, tot, count1
• tot0
• count10
• numberinFile.nextInt()
• while (number ! SENTINEL)
• tot tot number
• numberinFile.nextInt()
• System.out.println(count1 " " number)

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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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• import java.util.
• import java.io.
• public class loopflag
• static Scanner console new Scanner(System.in)
• public static void main(Stringargs)
• throws FileNotFoundException
• Scanner inFile new Scanner(new
  FileReader("loopflag.txt"))
• int number, tot
• tot0
• boolean found
• foundfalse
• numberinFile.nextInt()
• while (!found)
• if (numberlt0)
• System.out.println("Wehave encountered a
  negative number")

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EOF(End of File)-Controlled while Loop

• Used when input is from files.
• Sentinel value is not always appropriate.
• In an EOF-controlled while loop that uses the
  Scanner object console to input data, console
  acts at the loop control variable.
• The method hasNext, of the class Scanner, returns
  true if there is an input in the input stream
  otherwise, it returns false.
• The expression console.hasNext() acts as the loop
  condition.
• Expressions such as console.nextInt() update the
  value of the loop condition.

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EOF-Controlled while Loop

• A general form of the EOF-controlled while loop
  that uses the Scanner object console to input
  data is
• while (console.hasNext())
• //Get the next input and store in an
• //appropriate variable
• //Process data

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EOF-Controlled while Loop

• Suppose that inFile is a Scanner object
  initialized to the input file. In this case, the
  EOF-controlled while loop takes the following
  form
• while (inFile.hasNext())
• //Get the next input and store in an
• //appropriate variable
• //Process data

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• import java.util.
• import java.io.
• public class loopeof
• static Scanner console new Scanner(System.in)
• public static void main(Stringargs)
• throws FileNotFoundException
• Scanner inMike new Scanner(new
  FileReader("loopeof.txt"))
• int number
• int tot 0
• while (inMike.hasNext())
• numberinMike.nextInt()
• tot tot number
• System.out.println(number)
• System.out.println("Total" " " tot)
• inMike.close()

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Programming Example Checking Account Balance

• Input file Customers account number, account
  balance at beginning of month, transaction type
  (withdrawal, deposit, interest), transaction
  amount.
• Output Account number, beginning balance, ending
  balance, total interest paid, total amount
  deposited, number of deposits, total amount
  withdrawn, number of withdrawals.

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Programming Example Checking Account Balance

• Solution
• Read data.
• EOF-controlled loop.
• switch structure of transaction types.
• Determine action (add to balance or subtract from
  balance depending on transaction type).

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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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Programming Example Fibonacci Number (Solution)

• 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.
• Simplifies the writing of count-controlled loops.
• 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 (once).
• Loop condition is evaluated.
• If loop condition evaluates to true, execute for
  loop statement and execute update statement.
• Repeat until loop condition is false.

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The for Looping (Repetition) Structure

• Example 5-8
• The following for loop prints the first 10
  nonnegative integers
• for (i 0 i lt 10 i)
• System.out.print(i " ")
• System.out.println()

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The for Looping (Repetition) Structure

• Example 5-9
• 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 initial condition is 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.
• e.g. for ( )
• System.out.println (Hello World) //
  prints this line infinitely
• If loop condition is omitted, it is assumed to be
  true.
• for statement ending in semicolon is empty ie
  has empty action.
• We can also count backwards using a for loop

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• public class forloop
• public static void main(Stringargs)
• int x
• for(x0 xlt10 x)
• System.out.println(x" ")
• // the initial statement x0 initializes the
  variable x, Next the loop condition
• // ilt10 is evaluated. Because ilt10 is true, the
  print statement executes and
• // outputs 0. The update statement x then
  executes, which sets the value of x to 1.
• // Again the loop condition is evaluated, which
  is still true, and so on

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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.
• Could be simple or compound (enclosed in curly
  braces)

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dowhile Loop (Post-Test Loop)
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Dowhile VS for and while loops

• Do...while always activates, for and while may
  never activate
• In Do.. While loop condition is evaluated after
  execution of the loop (post- test loop), while in
  for and while, the loop condition is evaluated
  before the execution (pre-test loop)
• Do while does not have an empty condition. For
  or while can have an empty condition

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• public class dowhile
• public static void main(String args)
• int i
• i1
• do
• System.out.println(i "," 3i)
• ii2
• while(ilt40) // note the semicolon after the
  while statement

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• // do...while and while loops compared
• i11
• while(ilt10)
• System.out.println()
• System.out.println(i"," ii)
• ii2
• //This produces nothing because the loop
  condition is false (gt10) from the beginning.
• i11
• do
• System.out.println()
• System.out.println(i"," ii)
• ii2
• while(ilt10)
• // Here the loop outputs the number 11, and also
  changes the value of i to 112 before
• // evaluating the loop condition

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break Statements

• Used to exit early from a loop.
• Used to 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 starting from the first
  statement after the structure

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• import java.util.
• import java.io.
• public class loopbreak
• static Scanner console new Scanner(System.in)
• public static void main(Stringargs)
• throws FileNotFoundException
• Scanner inFile new Scanner(new
  FileReader("loopeof.txt"))
• int number
• int tot 0

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• while (inFile.hasNext())
• numberinFile.nextInt()
• if(numberlt0)
• System.out.println("A Negative number has been
  encountered")
• break
• tot tot number
• System.out.println(number)
• System.out.println("Total" " " tot)
• inFile.close()

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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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• import java.util.
• import java.io.
• public class loopcontinue
• static Scanner console new Scanner(System.in)
• public static void main(Stringargs)
• throws FileNotFoundException
• Scanner inFile new Scanner(new
  FileReader("loopeof.txt"))
• int number
• int tot 0
• while (inFile.hasNext())
• numberinFile.nextInt()
• if(numberlt0)
• System.out.println("A Negative number has been
  encountered")
• continue

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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)

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

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• import java.util.
• import java.io.
• public class nestedloop
• static Scanner console new Scanner(System.in)
• public static void main(Stringargs)
• throws FileNotFoundException
• Scanner inFile new Scanner(new FileReader
  ("nestedloop.dat"))
• char grade
• int score
• int count 0
• String stdid
• stdid""

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• System.out.println("STUDENT-ID" " " "SCORE""
  " "GRADE")
• while(!stdid.equals("XXXX"))
• count
• stdidinFile.next()
• scoreinFile.nextInt()
• if(scoregt90)
• grade'A'
• else if(scoregt80)
• grade'B'
• else if(scoregt70)
• grade'C'
• else if(scoregt60)
• grade'D'-
• else grade'F'
• System.out.println(stdid" "score"
  " grade)
• // endwhile
• System.out.println("\nStudents in
  class"""count)
• inFile.close()

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Chapter Summary

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