Asserting Java, Ch 7: Repetition

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Asserting Java Rick Mercer
Chapter 7 Repetition
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Chapter 7 Repetition

• Goals
• Recognize the Determinate loop pattern and use
  Java's while loop to implement it
• Recognize and use the indeterminate loop pattern
  and use Java's while loop to implement it
• Solve problems that require doing something over
  and over again
• either a pre-specified number of times,or
• until something happens to indicate you're done

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7.1 Repetitive Control

• The following algorithms involve repetition
• Add the remaining flour ¼ cup at a time whipping
  until smooth.
• While there are more burger/fries/soda orders,
  sum each item. Apply tax. Then display the
  total
• Compute a course grade for every student.
• While the ATM is running, process another
  customer, and for each customer allow many
  transactions.
• Microwave the food until the timer reaches 0, the
  cancel button is hit, or the door is opened.

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Why is repetition needed?

• Why Repetition?
• To take advantage of the computer's speed in
  order to perform the same tasks faster.
• To avoid writing the same statements over and
  over again (shorter programs).
• To examine all the objects in a collection of
  objects.
• To make programs general enough to handle various
  sized collections of data.
• Consider code intended to average exactly 100
  numbers (next slide)

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Average 100 values the hard way
How many statements are required for 100
inputs____? What changes are necessary to average
200 inputs ____?

• double sum 0.0
• double average, number
• System.out.print("Enter number ") // lt-
  Repeat
• number keyboard.nextDouble( ) // lt- these
• sum sum number // lt-
  statements
• System.out.print("Enter number ") // lt-
  Repeat
• number keyboard.nextDouble( ) // lt- these
• sum sum number // lt-
  statements
• // ... many statements not shown here ...
• average sum / 100.0
• System.out.println( "average " average )

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Algorithmic Pattern The Determinate loop

• There is a better way
• We often need to perform some action a specific
  number of times
• Produce 89 paychecks
• Count down to 0 (take 1 second of the clock).
• Send grade reports to 36,531 UofA students
• The determinate loop pattern repeats some action
  a specific number of times

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The while loop

• The indeterminate loop pattern can be
  imple-mented with the Java while loop there are
  other ways
• while ( loop-test )
• repeated-part
• When a while statement is encountered the block
  (statements between and ) executes while (as
  long as) the loop-test is true

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Flow chart view of while-loop execution
False
True
loop-test
statement-1
Iterative Part
statement-2
statement-n
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Determinate Loops

• This template repeats a process n times (you fill
  in the comments)
• int n / how often we must repeat the
  process /
• int counter 1 while ( counter lt n )
• // the process to be repeated
• counter counter 1
• determinate loops must know the number of
  repetitions before they begin know exactly how
  many employees, or students, or whatever that
  must be processed, for example

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Example for loop that produces an average

• Scanner keyboard new Scanner(System.in)
• double sum 0
• double number
• System.out.print("How many do you want to
  average? ")
• int n keyboard.nextInt()
• int counter 1
• // Do something n times
• while (counter lt n)
• System.out.print("Enter number ") // lt-
  Repeat 3
• number keyboard.nextDouble() // lt-
  statements
• sum sum number // lt- n times
• counter counter 1 // make sure the loop
  stops
• double average sum / n
• System.out.print("Average of " n " numbers is
  " average)

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Active Learning

• Write the output
• int n 5
• int j 1
• while(j lt n)
• System.out.print ( j " " )
• j j 1
• j 0
• while(j lt 2 n)
• System.out.print ( j " " )
• j j 2

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7.2 Indeterminate Loops

• Determinate loops have a limitation
• We must know n in advance
• Many situations when repeat a set of statements
  an unspecified number of times
• Processing report cards for every student in a
  school (or paychecks for all employees, or...)
• Allowing 1 to many ATM transactions
• Asking the user for specific input and allowing
  re-entry of input after invalid inputs
• NAME ANOTHER ? ________________ ?

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Some things that terminate indeterminate loops

• An indeterminate loop repeats a process until
  some stopping event terminates the repetition
• There are many such events, but we'll focus on
  these
• User enters a special value indicating end of
  data
• A logical expression becomes false
• The Grid's mover hits the wall or an edge
• The end of a file is encountered
• Indeterminate loops do not need to know n in
  advance
• Indeterminate loops can actually determine n

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Pattern Indeterminate loop Problem Some process
must repeat an unknown number of times so some
event is needed to terminate the
loop. Algorithm while( the termination event
has not occurred ) execute these
actions bring the loop closer to
termination Code while(myGrid.frontIsClear
()) Example myGrid.putDown( )
myGrid.move( )
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Sentinel Loops

• A sentinel is a specific input from the user to
  signal that there is no more data
• The sentinel should be the same type of data
• The sentinel must not be in the valid range of
  data
• Example Use -1 as the sentinel for test scores
  that can only be in the range of 0 through 100
  inclusive
• Enter test scores or -1 to quit
• 80 95 76 82 56 100 45 86 -1

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Sentinel loop continued

• Have to read until the sentinel is read
• process all inputs except the sentinel
• while ( inputValue ! sentinel )
• process inputValue
• But where do we input the values?
• check out the code on the next slide

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Sentinel loop complete this dialogue

• Scanner keyboard new Scanner(System.in)
• double sum 0.0
• double inputValue 0.0
• System.out.println("Enter numbers or -1 to quit"
  )
• while( inputValue ! -1)
• inputValue keyboard.nextDouble( )
• sum inputValue
• System.out.println( "Sum " sum )
• Complete this dialogue
• Enter numbers or -1.0 to quit
• 1.0 2.0 3.0 -1.0
• Sum _____ ?

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Complete the Dialogue -- what is sum?

• The Sum was not correctly computed
• Needed priming read and another read at bottom
• System.out.println("Enter numbers or -1 to quit"
  )
• currentInput keyboard.nextDouble( )
• while(currentInput ! -1)
• sum inputValue
• currentInput keyboard.nextDouble( )
• System.out.println( "Sum " sum )

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Tracing a dialogue AverageComputer.java

• Enter numbers or -1 to quit
• 70.0
• 90.0
• 80.0
• -1.0
• Average 80.0
• Iteration currentInput sum n
  currentInput!SENTINEL
• Before the loop 70.0 0.0 0 true
• End Loop 1 80.0 70.0 1 true
• End Loop 2 90.0 150.0 2 true
• End Loop 3 -1 240.0 3 false
• After the loop NA 240.0 3 NA

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Infinite loops

• Infinite loop a loop that never terminates.
• Infinite loops are usually not desirable.
• Here is an example of an infinite loop
• test keyboard.nextInt()
• while(test ! -1)
• // now process the test
• sum test
• n
• There is no step that brings the loop closer to
  termination.