Repetition Structures

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Repetition Structures
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3 Types of Control Structures

• Sequence
• Selection
• If
• IfElse
• Repetition
• Looping

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Types of Loops

• While Loop ? Pre-Test Loop
• Test loop condition prior to loop code
• Minimum number of times executed is 0
• Do While Loop ? Post-Test Loop
• Test loop condition after loop code
• Minimum number of times executed is 1
• For / Counted Loop

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While Loop

• General Syntax
• while (condition) //while condition is true
• // do everything inside curly braces
• // end of while loop

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While Loop

• int number 3
• while (number lt 5)
• JOptionPane.showMessageDialog (Null, Hello)
• number
• // end while

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3 Keys to Successful Looping

• Initialize the Loop Control Variable (LCV)
• (Give the LCV a beginning value)
• Test the LCV for Exit Condition
• Update the LCV
• (Most overlooked step)

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While Loop

• initialize LCV //Key 1
• While (condition) // Key 2
• loop business
• update LCV // Key 3
• // close loop

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Example of While Loop

• Input / Data Validation
• Ask the user for input, make sure the user inputs
  a proper data value (example 0 100)

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While Loop
input JOptionPane.showInputDialog (Enter
Score ) number Integer.parseInt
(input) while (number lt 0 number gt 100)
JOptionPane.showMessageDialog (null, Bad
Input) input JOptionPane.showInputDialog
(Enter a new Score ) number
Integer.parseInt (input) // end while
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Do While Loop

• Post Test ? Move the test condition to after the
  looping code
• General Syntax
• do
• init LCV // initialize update
• Loop business
• while (condition) //test exit condition

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Do While Loop
do input JOptionPane.showInputDialog
(Enter Score ) number Integer.parseInt
(input) if (number lt 0 number gt 100)
JOptionPane.showMessageDialog (null, Bad
Input) input JOptionPane.showInputDialog
(Enter a new Score ) number
Integer.parseInt (input) // end if while
(number lt 0 number gt 100)
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Sentinel Value Loop

• User enters a Sentinel Value (a point as which to
  stop execution)
• Example
• Read in numbers from the user until a value of
  -1 (sentinel value) is input. Then output the
  count, sum and average of those numbers.

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Pseudocode

• Get first data value
• While data value ! sentinel value
• Process current value
• Add to sum
• Increment count
• Get next data value / Update LCV
• Determine Average
• If count gt 0 then average sum/count
• Else average 0
• Output
• Sum
• Count
• Ave

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Data Dictionary
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import javax.swing.JOptionPane class
Nums public static void main (String args)
String input int count 0
int sum 0 int number float
average final int END_MARKER 999
input JOptionPane.showInputDialog (Enter
Number ) number Integer.parseInt
(input) while (number !
END_MARKER) count // count count 1
(increment counter) // can also use
count(count count 1) sum number //sum
sum number input JOptionPane.showInputDialog
(Enter Number ) number Integer.parseInt
(input) // end while if (count
0) JOptionPane.showMessageDialog (null, No
Numbers Entered) else average
(float)sum / (float)count JOptionPane.showMessag
eDialog (null, Count is count) JOptionPane
.showMessageDialog (null, Sum is
sum) JOptionPane.showMessageDialog (null,
Average is average) // end else
// end main // end Nums
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Control of Loops with Flags

• Using boolean variables for control purposes
• Set a flag when a condition is met
• Change value of boolean to True

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Boolean Flag
finished false // boolean flag while
(!finished) 1st part of loop body if
(criteria check) finished true else 2nd
part of loop // end while In this case the
2nd part of the loop body is only executed if
the exit criteria is not met.
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Counted / For Loops

• A Pre-test loop in which a counter is
  initialized, updated and checked to see if it has
  reached a terminal value
• Runs a given number of times

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General Syntax

• for (initialization loop criteria step)
• // looping code
• // end for loop
• Initialization clause initialize LCV
• Loop clause Check LCV
• Step clause Update LCV

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Example

• for (int I0 I lt 20 I)
• JOptionPane.showMessageDialog (null, Counting
  Loop)
• // end loop

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Syntax to Remember

• A semicolon at the end of a loop header causes a
  NULL Loop Statement
• No Error Message
• Loop Code never executes
• For (int count 1 count lt 10 count)
• System.out.print (Null Loop)
• // end loop
• While (counter lt -1)
• // loop code