1. Tutorial 6 The Repetition Structure

2. 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
Tutorial 6: The Repetition Structure

3. 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
Tutorial 6: The Repetition Structure

4. 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 non-integer (default stepValue is 1)
Tutorial 6: The Repetition Structure

5. A For … Next Example Dim intCount As Integer
For intCount = 0 to 3 Step 1
Debug.WriteLine(intCount)
Next intCount
For intCount = 3 to 0 Step -1
For intCount = 0 to 10 Step 2
Dim sngLoc As Single
For sngLoc = 0.5 To 15 Step 0.5
Debug.WriteLine(sngLoc)
Next sngLoc
Tutorial 6: The Repetition Structure

6. Flowchart and Pseudocode
Hexagon
Repeat for intCount = 1 to 3 by 1
Display intCount
Next Iteration
intCount
> 3
+=1 1
Display intCount
Tutorial 6: The Repetition Structure

7. 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
Do While Condition
[loop instructions]
Loop
Do Until Condition
Posttest condition Do
Loop While Condition
Loop Until Condition
Tutorial 6: The Repetition Structure

8. Loop Examples Dim intCount As Integer = 1 Do While intCount < 3
Debug.WriteLine(intCount)
intCount += 1
Loop
Do Until intCount > 3 Do
Loop While intCount < 3
Loop Until intCount > 3
Tutorial 6: The Repetition Structure

9. Do While Pretest Loop intCount = 1 intCount <= 3 F T
Repeat while intCount < 3
Display intCount
Add 1 to intCount
End Repeat
intCount <= 3 F T
Display intCount
intCount += 1
Tutorial 6: The Repetition Structure

10. Do Until Posttest Loop intCount = 1 Display intCount intCount += 1
Repeat
Display intCount
Add 1 to intCount
End Repeat until intCount > 3
Display intCount
intCount += 1
intCount > 3 T F
Tutorial 6: The Repetition Structure

11. 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
Tutorial 6: The Repetition Structure

12. 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
Tutorial 6: The Repetition Structure

13. 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
Tutorial 6: The Repetition Structure

14. Accessing the Controls Collection
Dim intX As Integer = 0
For intX = 0 To Controls.Count – 1
Debug.WriteLine(Controls(intX).Name)
Next intX
Do While intX < Controls.Count
If TypeOf Controls.Item(intX) Is TextBox Then
Controls.Item(intX).Text = “”
End If
intX += 1
Loop
Tutorial 6: The Repetition Structure

15. 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
Tutorial 6: The Repetition Structure

16. 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
End If
Next
Tutorial 6: The Repetition Structure

17. Flowchart and Pseudocode for the For Each … Next
Repeat for each Control in Controls collection
Repeat for each Control in Collection
if control is a label
remove border
end if
end repeat
stop
Is control a label? T
Remove the border F
Tutorial 6: The Repetition Structure

18. 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)
Tutorial 6: The Repetition Structure

19. 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 …
Tutorial 6: The Repetition Structure

20. 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
Tutorial 6: The Repetition Structure

21. 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
Tutorial 6: The Repetition Structure

22. 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
Tutorial 6: The Repetition Structure

23. 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 illustrates
Tutorial 6: The Repetition Structure

24. Tutorial 6: The Repetition Structure

25. 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
Tutorial 6: The Repetition Structure