Tutorial 6 The Repetition Structure

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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 Tutorial 6: The Repetition Structure

Tutorial 6: The Repetition Structure

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