Iteration Structures

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Lesson 7

• Iteration Structures

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Iteration Structures

• Iteration is the third control structure we will
  explore.
• Iteration simply means to do something
  repeatedly.
• All iteration control structures test a condition
  each time before the loop is executed.

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Types of Loops

• There are three types of loops that Java employs.
• Pretest Loop- test a condition each time before
  the loop is executed.
• Posttest Loop - test a condition after each loop
  execution.
• Fixed repetition - cause a loop to be executed a
  predetermined number of times.

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Three Iteration Control Structures

• The three iteration control structures are the
  while, do/while, and for.
• The difference between them is the means by which
  they control the exiting of the loop.
• The while is a pretest loop.
• The do/while is a posttest loop.
• The for is a fixed repetition loop.

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The WHILE loop

• If the test expression is true the loop
  statements are executed.
• If the test expression is false the loop
  statements are bypassed.
• To get out of the loop something must change the
  expression to false, otherwise the the result
  will be an infinite loop.

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Syntax for the While Loop
while (test expression) statement1 statement2
statementn
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While Loop Example (counter)

• //Compute 1 2 . . . 100
• class counter
• public static void main (String Args)
• int sum 0, cntr 1
• while (cntr lt100)
• sum cntr
• cntr
• System.out.println (sum)

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While Loop Example (Square Roots)

• //Display the square roots of 25, 20, 15, and 10
• class SquareRoots
• public static void main (String args)
• int number 25
• while (number gt10)
• System.out.println ("The square root of "
  number " is Math.sqrt(number))
• number -5

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The DO/WHILE loop

• The do/while loop is tested at the end of the
  loop compared to the while which is tested at the
  beginning.
• The loop statements will always be executed once.
• To break the loop the test expression must become
  false.

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Syntax for DO/WHILE
do statement1 statement2 statementn whil
e (test expression)
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• class dowhile
• public static void main(String args)
• int myNum
• //generate random numbers until we get 5
• do
• myNum (int)(Math.random() 10)
• System.out.print(myNum)
• while (myNum !5)

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Using Counters and Accumulators

• Numeric variables used within a repetition
  structure to calculate subtotals, totals, and
  averages
• Counter
• Used for counting something
• Accumulator
• Used for accumulating (adding together) something

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Sentinel Values

• Values used to end loops
• Should be easily distinguishable from the valid
  data used by the program
• Also called trip values or trailer values

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Sentinel values

• Our program can read numbers repeatedly until it
  encounters a special value called a sentinel that
  marks the end of the list.
• For example, if all the numbers in the list are
  positive, then the sentinel could be -1.

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• import TerminalIO.KeyboardReader
• class sentinel
• public static void main(String args)
• KeyboardReader reader new KeyboardReader()
• double number, sum 0
• int count 0
• while (true)
• number reader.readDouble("Enter a positive
  number or -1 to quit ")
• if (number -1)
• break
• sum number
• count
• System.out.println("The average is "
  sum/count)

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The FOR loop

• The for loop runs a fixed number of times
• The first thing that is done is the
  initialization of the counter.
• If the results of the test expression is true,
  then the statements are executed.
• Each time the loop is executed, the loop counter
  must be incremented or decremented.

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Syntax for FOR loop
for (initialize counter test counter update
counter) statement1 statement2 statementn

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• class Racer
• public static void main (String args)
• System.out.println("GO!")
• for (int lap1 lap lt10 lap)
• System.out.println("Completed " lap "
  laps.")
• System.out.println("Finish!")

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Nested Loops

• Many times it is desirable to have looping
  operations within loops.
• This is called nested looping.
• Each inner loop will as many times as desired X
  the number of times the outer loop runs.

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Each iteration of the outer loop causes multiple
iterations of the inner loop.

• class NestedForLoop
• public static void main (String args)
• for (int i 0 i lt3 i)
• for (int j 0 j lt 3 j)
• System.out.println("i " i ", j "
  j)

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Break and Continue Option

• If you want a loop to terminate if some desired
  value is found, it is possible to use the break
  statement to force the loop to quit.
• If you want one iteration of the loop to be
  terminated, it is possible to use a continue
  statement to force the loop to quit one iteration
  and continue on with another.

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Syntax for BREAK
while (test expression) statement1 statement2
if (test expression) break statementn
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The loop will iterate once because the condition
is true, but the break statement takes the
program right out of the loop.

• class breakExample
• public static void main (String args)
• while (true)
• System.out.print ("true")
• break
• System.out.println(" out of loop")

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Syntax for CONTINUE
for (initialize counter test counter update
counter) statement1 statement2 if (test
expression) continue statementn
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The continue statement causes the loop to stop
where it is and loop again from the beginning,
causing an infinite loop.

• class ContinueExample
• public static void main (String args)
• boolean b true
• while (b)
• if (b) continue
• System.out.println("never gets here")
• b false

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Bottoms Up On The

• Fifth Cup