Repetition

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Chapter 6

• Repetition

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Outline Objectives

• Loop Structure
• Elements of a Loop Structure
• Processing Lists of Data with Do Loops
• Option buttons vs. Check boxes
• Multiple forms

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Types of LOOP Structures

• Do While . Loop
• Do Until Loop
• For Next loop

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Basic Definition

• Looping the process of repeating a series of
  statements as many times as needed.
• Looping also called iteration.

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Basic Components of Loops

• Loop control variable A variable used to
  determine whether a loop will be executed
• Loop body The statement (s) that are executed
  each time a loop repeats

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The Do While . Loop

• Do While condition is true
• statement(s)
• Loop
• Compare this to IF . Then

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Flowchart for a Do While Loop
Is the condition true
No
Yes
Execute statements within the loop
Execute statements that follow the loop
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The Do While . Loop

• Is executed as long as the condition is True.
• If condition is False then the next statement
  after the Loop is executed.
• Step Demo DoCount

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Controlling Loops

• Methods of controlling loops
• Counter-controlled loops
• repeat a specific number of times
• Event-controlled loops
• repeat until something happens in the loop body
  to change the value of loop control variable.

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Example of event-controlled loops

• passWord ""
• Do While passWord ltgt "SHAZAM"
• passWord UCase(InputBox("What is the
  password?"))
• Loop

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Counter-controlled Loops

• Is useful when the programmer knows how many
  times the loop should be executed.
• Initialize the counter by setting it to a
  beginning value before entering the loop.
• The counter is increased (or decreased) by the
  same value during each repetition.

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Example

• num 1
• Do While num lt 10
• picOutput.Print num
• num num 1
• Loop
• Watch out for infinite loops!

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Do Until . Loop

• Is executed until the condition becomes True
• Any Do While. Loop can be rewritten as a Do
  Until .. Loop

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Example (requires the user to give a password
before opening a file)

• Do
• passWord UCase(InputBox("What is the
  password?"))
• Loop Until passWord "SHAZAM
• Demo - CTRLBreak

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Example (years to deplete a saving account)

• Jane has saved 150,000 for retirement. She
  is now 65 and wants to draw 15,000 per year to
  supplement her pension. She is earning interest
  on her account at the rate of 7.5 per year. How
  old will Jane be before all her savings are used
  up?
• Demo Savings

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Comparing While and Until Loops

• The Do While Loop executes while the condition
  is true
• The Do Until.. Loop executes until the condition
  is true
• Both can be used to create any type of loop

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Schneider p. 255 - 261
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Processing List of Data with Loops

• EOF Function
• The EOF function tells us if we have read to the
  end of a file.
• Checks for the end-of-file marker.

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EOF Function

• EOF(n) where n is the reference number for the
  file.
• If there are more records to be read, EOF is
  False.
• When the end-of-file marker is reached EOF
  becomes True.

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Example of EOF

• Open Phone.txt for Input As 1
• Do While Not EOF(1)
• Input 1, nom, phoneNum
• picOutput.Print nom, phoneNum
• Loop
• Close 1

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Counters and Accumulators

• A Counter is a numeric variable that keeps track
  of the number of items that have been processed
  in the loop.
• An Accumulator is a numeric variable that totals
  numbers.
• Demo Average

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Example Counter Accumulator

• Private Sub cmdAnalyze_Click()
• Dim numCoins As Integer, sum As Single, value
  As Single
• Open "COINS.TXT" For Input As 1
• numCoins 0
• sum 0
• Do While Not EOF(1)
• Input 1, value
• numCoins numCoins 1
• sum sum value
• Loop
• picValue.Print "The value of the" numCoins
  "coins is" sum "cents."
• Close 1
• End Sub

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Example of a Data File

• 1, 1, 5, 10, 10, 25

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Output for the Previous Example

• The value of the 6 coins is 52 cents.

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For ... Next Loop

• Is used to create a counting loop.
• Loop control variable has an initial value.
• Loop control variable has a terminating value.
• Loop control variable has a Step value.

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Example ( display a table of the first 5 numbers
and their square)
Terminating value

• For i 1 To 5
• picTable.Print i i 2
• Next i
• Demo counter

Initial Value
Loop Control variable
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Rules for Using For ... Next loop

• The initial, terminal, and step values cannot be
  modified in the loop body.
• You should never modify the value of the loop
  control variable in the loop body.
• Each For statement must end with a Next statement.

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Example (the step value is negative)

• For Counter 8 To 1 Step -2
• picOutput.Print Counter
• Next Counter

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Nested Loops

• For Outer 1 To 4
• For Inner 1 To 2
• ..
• ..
• Next Inner
• Next Outer

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• For i 1 To 10
• For j 1 To 10
• picOutput.Print ""
• Next j
• picOutput.Print
• Next I
• What wil this code do?

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Schneider p. 268 275Schneider p. 282 - 289
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Option Buttons

• Used in situations where you want to limit the
  user to only one of two or more related and
  mutually exclusive choices
• Only one in a group can be selected (on) at any
  one time
• When selected, an option buttons Value property
  contains the Boolean value True otherwise it
  contains the Boolean value False

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Option Buttons

• Minimum number in an interface is two
• Recommended maximum number is seven
• A default button should be selected when the
  interface first appears (use form_load event)
• Demo State_opt

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Frame Control

• Acts as a container for other controls
• Used to visually separate controls from one
  another
• The frame and the controls contained within the
  frame are treated as one unit
• You must use a frame control if you want to have
  more than one group of option buttons (Demo)

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Check Box

• Used in situations where you want to allow the
  user to select any number of choices from one or
  more independent and non-exclusive choices
• Any number of check boxes can be selected at any
  one time
• When selected, a check boxs Value property
  contains the number 1 (vbChecked). When
  unselected, it contains the number 0 (vbUnchecked)

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Check Box

• Assign a unique access to each check box
• You also can use the spacebar to select/deselect
  a check box that has the focus
• Unlike option buttons, you must test a checkbox
  when it is clicked to see if it is checked or
  unchecked

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• Demo Rentals_opt
• Demo Math and Math2

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Multiple Forms

• Adding a second form
• Form load and unload commands
• Form hide and show commands
• Syntax - referring to controls on other forms
• Demo - Order