Week 6
The Repetition Structure
1
The Repetition Structure (Looping)
Lesson A Objectives
After completing this lesson, you will be able to:
 Code the repetition structure using the For…Next
and Do…Loop statements
 Write pseudocode for the repetition structure
 Create a flowchart for the repetition structure
 Display a message in the Output window while an
application is running
 Change the location and size of a control while an
application is running
 Initialize and update counters and accumulators
2
The Repetition Structure
 Most programs also contain the selection structure, which
you learned about in Tutorials 4 and 5
 Programmers use the repetition structure, referred to more
simply as a loop, when they need the computer to
repeatedly process one or more program instructions until
some condition is met, at which time the loop ends
 In a pretest loop, the evaluation occurs before the
instructions within the loop are processed
 In a posttest loop, the evaluation occurs after the
instructions within the loop are processed
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The For … Next Loop
 You can use the For…Next statement to code a loop whose
instructions you want processed a precise number of times
 Syntax:
For counter = startValue To endValue [Step stepValue]
[instructions you want repeated]
Next [counter]
 counter is the name of a numeric variable and it keeps track
of how many times the loop instructions are repeated
 startValue, endValue, and stepValue must be numeric and
they can be either positive or negative, integer or noninteger (default stepValue is 1)
4
A For … Next Example
Dim intCount As Integer
For intCount = 0 to 3 Step 1
Debug.WriteLine(intCount)
Next intCount
Dim intCount As Integer
For intCount = 3 to 0 Step -1
Debug.WriteLine(intCount)
Next intCount
Dim intCount As Integer
For intCount = 0 to 10 Step 2
Debug.WriteLine(intCount)
Next intCount
Dim sngLoc As Single
For sngLoc = 0.5 To 15 Step 0.5
Debug.WriteLine(sngLoc)
Next sngLoc
5
Flowchart and Pseudocode
Repeat for intCount = 1 to 3 by 1
Hexagon
intCount
Display intCount
Next Iteration
>3
+=1
1
Display intCount
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The Do…Loop Statement
• Unlike the For…Next statement, the Do…Loop statement can be used to
Code both a pretest loop and a posttest loop
• The Do…Loop statement begins with the Do clause and ends with the
Loop clause
Pretest condition
Pretest condition
Do While Condition
[loop instructions]
Loop
Do Until Condition
[loop instructions]
Loop
Posttest condition
Posttest condition
Do
[loop instructions]
Loop While Condition
Do
[loop instructions]
Loop Until Condition
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Loop Examples
Dim intCount As Integer = 1
Do While intCount < 3
Debug.WriteLine(intCount)
intCount += 1
Loop
Dim intCount As Integer = 1
Do Until intCount > 3
Debug.WriteLine(intCount)
intCount += 1
Loop
Dim intCount As Integer = 1
Do
Debug.WriteLine(intCount)
intCount += 1
Loop While intCount < 3
Dim intCount As Integer = 1
Do
Debug.WriteLine(intCount)
intCount += 1
Loop Until intCount > 3
8
Do While Pretest Loop
intCount = 1
intCount <= 3
F
intCount = 1
Repeat while intCount < 3
Display intCount
Add 1 to intCount
End Repeat
T
Display intCount
intCount += 1
9
Do Until Posttest Loop
intCount = 1
intCount = 1
Repeat
Display intCount
Add 1 to intCount
End Repeat until intCount > 3
Display intCount
intCount += 1
intCount > 3
T
F
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Using Counters and Accumulators
 Counters and accumulators are used within a repetition
structure to calculate subtotals, totals, and averages
 Initialized (usually to 0 or 1) outside the loop and updated
within the loop
 A counter is a numeric variable used for counting
something and is typically updated by 1
 An accumulator is a numeric variable used for
accumulating (adding together) and is updated by an
amount that varies
 Initializing means to assign a beginning value to the
counter or accumulator
 Updating, also called incrementing, means adding a
number to the value stored in the counter or accumulator
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Using Collections
Lesson B Objectives
After completing this lesson, you will be able to:
 Access the controls in the Controls collection
 Code the repetition structure using the For
Each…Next statement
 Create an object variable
 Create a collection
 Create parallel collections
 Enable and disable a control
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The Controls Collection
 The controls contained on a Windows form belong to the
Controls collection in Visual Basic .NET
 A collection is simply a group of one or more individual
objects treated as one unit
 Identifies by an index, automatically assigned by Visual
Basic .NET when object is created
 Refer to a control Controls.Item(index)
 Controls are numbered LIFO – that is, the last control
object has an index = 0
 The value of Controls.Count gives the number of
controls on a form
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Accessing the Controls Collection
Dim intX As Integer = 0
For intX = 0 To Controls.Count – 1
Debug.WriteLine(Controls(intX).Name)
Next intX
Dim intX As Integer = 0
Do While intX < Controls.Count
Debug.WriteLine(Controls(intX).Name)
If TypeOf Controls.Item(intX) Is TextBox Then
Controls.Item(intX).Text = “”
End If
intX += 1
Loop
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Object Variables
 An object variable is a memory location that can
store the address of an object
 The address indicates where the object is located
in the computer’s internal memory
 An object variable is initialized to the keyword
Nothing, which simply means that the object
variable does not currently contain an address
 You assign an object variable
objStateTextBox = Me.StateTextBox
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The For Each…Next Statement
 The For Each…Next statement is used to code a loop whose
instructions you want processed for each object in a
collection
For Each element In group
[processing statements for element]
Exit For
Next element
Dim objTextBox As TextBox
For Each objTextBox In Me.Controls
If objTextBox.Text = “Hello” Then
Exit For
End If
Next
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Flowchart and Pseudocode
for the For Each … Next
Repeat for
each Control
in Controls
collection
stop
Is control a label? T
Repeat for each
Control in Collection
if control is a label
remove border
end if
end repeat
Remove the border
F
17
Creating a User-Defined Collection
 A user-defined collection allows you to group related
controls together
 To define a collection
Dim collectionName As New Collection()
 To insert an object into the collection
collectionName.Add(object[, key])
 To access an object in the collection
objMyObject = collectionName(index)
objMyObject = collectionName(key)
 To remove an object from the collection
collectionName.Remove(index|key)
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Creating a User-Defined Collection
'declare form-level collections
Private mCheckBoxCollection As New Collection()
Private Sub GradeForm_Load(ByVal sender As Object, _
ByVal e As System.EventArgs) Handles MyBase.Load
Dim intCtr As Integer
For intCtr = 0 To Controls.Count - 1
If TypeOf (Controls(intCtr)) Is CheckBox Then
mCheckBoxCollection.Add(Controls(intCtr))
End If
Next
…
19
Parallel Collection


Collections whose objects are related in some way are called
parallel collections
You can indicate to the computer that two collections are parallel
collections by setting the key argument for each object in one of
the collections to the name of the corresponding object in the
other collection
For intCtr = 0 To Controls.Count - 1
If TypeOf (Controls(intCtr)) Is TextBox Then
mTextBoxCollection.Add(Controls(intCtr), _
Replace(Controls(intCtr).Name, "TextBox", _
"CheckBox"))
End If
Next
20
The Enabled Property
Private Sub ProcessCheckBox(ByVal sender As Object, _
ByVal e As System.EventArgs) Handles Proj1CheckBox.Click, …
Dim objCB As CheckBox, objTB As TextBox
objCB = sender ' assign sender to the object variable
objTB = mTextBoxCollection(oCB.Name)
If objCB.Checked Then
objTB.Enabled = True
objTB.Focus()
Else
objTB.Text = ""
objTB.Enabled = False
End If
End Sub
21
Completing the Grade
Calculator Application
Lesson C Objectives
After completing this lesson, you will be able to:
 Select the existing text when the user tabs to a
text box
 Prevent a form from closing
22
Coding the DisplayButton’s
Click Event Procedure
 You still need to code the DisplayButton’s
Click event procedure, the GradeForm’s
Closing event procedure, and the Enter event
procedure for the text boxes
 You begin with the DisplayButton’s Click
event procedure as the pseudocode in Figure
6-50 illustrates
23
24
Coding the GradeForm’s
Closing Event Procedure
 A form’s Closing event occurs when a
form is about to be closed
 You can close a form using either the
Close button on its title bar, or the
Me.Close( ) statement in code
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