Repetition

1
Chapter 4

• Repetition

2
The Iteration Structure

• Iteration structure (or loop) repeats a process
• Uses a condition to control iteration
• Loop continues as long as condition is TRUE
• Pretest tests conditions at the start of the loop
• Posttest tests conditions at the end of the loop
• Endless loops never stop

3
Types of Loop Testing
4
Pretest Loops in Java

• One pretest loop in Java is the while statement
• A condition is used to control the loop
• The condition is put between parentheses

while (condition) statement
while (condition) statement1
statement2 . . .
5
Example of while loop
/ print the numbers from 1 to 10 using the
pretest while loop / public class PrintNum
public static void main(String args) int
number 1 while ( number lt 10)
System.out.print(number ) number
number 1 System.out.println("\nThank
you.")
Output 1 2 3 4 5 6 7 8 9 10 Thank you.
6
Posttest Loops in Java

• Posttest done with the do-while statement
• Also uses a condition placed in parentheses

do statement while (condition)
do statement1 statement2 . .
. while (condition)
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Example of do while
Output 1 2 3 4 5 6 7 8 9 10 Thank you.
/ print the numbers from 1 to 10 using the
pretest while loop / public class PrintNum
public static void main(String args) int
number 1 do System.out.print(numbe
r ) number number 1
while( number lt 10) System.out.println("\nT
hank you.")
8
Assignment

• Write a program to calculate the factorial of 10.

9
Increment/Decrement Operators

• Counting by 1 is a common operation
• Increment uses
• count or count
• equivalent to
• count count 1 or count 1
• Decrement uses --
• count-- or -- count
• equivalent to
• count count - 1 or count - 1

10
Increment/Decrement Operators (Cont ..)

• Increment and decrement can appear as a prefix or
  a postfix
• Prefix increment/decrement
• Changes before the expression
• count or --count
• Postfix increment/decrement
• Changes after the expression
• count or count--

11
Increment/Decrement Operators (Cont ..)
double total, price 3.5 int quantity 2
12
Counter-Controlled Loops

• Sometimes we want to execute a loop a specific
  number of times
• Elements necessary for CCLs
• Initialization give counter a starting value
• Test usually a pretest
• Body statement(s) to repeat
• Counter value changed in each iteration
• End sends program back to beginning of loop

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Example of a CCL
Output 1 2 3 The value of count is 4
public class MyLoop public static void
main(String args) int count count
1 / Initialization /
while (count lt 3) / Test /
System.out.println(count) / Body /
count / Counter
/ / End
/ System.out.println("The value of
count is count)
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The for Statement

• Specially designed for counting loops
• for (initialization test counter)
• statement
• for (initialization test counter)
• statement1
• statement2
• . . .

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The for Statement (Cont ..)

• Actions of the for spread out around the loop
• Initialization occurs only ONCE at the start
• Testing is the first repeated action of the loop
  (it gets done before the body each iteration)
• Counter occurs at the end of the loop body

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Example
public class MyLoop public static void
main(String args) int count for
(count 1 count lt 3 count )
System.out.println(count)
System.out.println("The value of count is
count)
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Example (Cont ..)
Output 1 2 3 The value of count is 4!
18
Assignment

• Write a program to ask the user to enter 10
  integers and then find the maximum value.

19
Nested Loops

• A loop within another is a nested loop
• Nest as deep as you want, but
• An inner loop must be entirely contained within
  an outer one
• If the loops are counter controlled, each loop
  must have a different loop counter variable
• Nested loops used often for rows and columns
• Outer loop controls the rows
• Inner loop controls the columns

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Example of Nested Loops
Output 1 2 3 4 5 2 4 6 8 10 3 6 9
12 15 4 8 12 16 20
public class MoreLoops public static void
main(String args) int column, row
for (row 1 row lt 4 row) for
(column 1 column lt 5 column)
System.out.print(row column)
System.out.print("\n")
21
Assignment

• Write a program to display the following output
• 1
• 1 2
• 1 2 3
• 1 2 3 4
• 1 2 3 4 5

22
Validation Loops

• Sometimes you may need to check for validity of
  input data
• The user can enter negative value where it is
  supposed to be positive (year)
• The user can enter a value that is outside the
  range of possible values (entering 110 for a test
  score)
• This can be done with validation loops
• Continue prompting the user for valid value as
  long as invalid value is entered

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Sentinel Values

• A special value of input that signals the program
  to behave in some special manner (like stopping a
  loop)
• Must be a value that would not occur in the
  normal course of operation
• i.e., a price and quantity of zero.

24
The break statement

• The break statement can be used with switch or
  any of the 3 looping structures.
• It causes an immediate exit from the switch,
  while, do while, or for structure in which it
  appears.
• If the break is inside nested structures, control
  exits only the innermost structure containing it.

25
The continue statement

• The continue statement is valid only within
  loops.
• It terminates the current loop iteration, but not
  the entire loop.
• In a for or while loop, continue causes the rest
  of the body statement to be skipped - in a for
  statement, the update is done.
• In a do while loop, the exit condition is tested,
  and if true, the next loop iteration starts.

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Loop Testing and Debugging

• Beware of infinite loops - program doesnt stop
• Check loop termination condition, and watch for
  off-by-1 problem
• Trace execution of loop by hand with code
  walk-through
• Use debug output statements