unit 8 list boxes and the do while looping structure chapter 5 lists, loops, validation, and more

© 2006 ITT Educational Services Inc. Introduction to Computer Programming: Unit 8: Chapter 5: Slide 1

Unit 8

List Boxes and the Do While Looping Structure

Chapter 5

Lists, Loops, Validation, and More


Chapter 5 Topics

• This chapter covers the Visual Basic .NET looping statements Do … While Do … Until For … Next

• It also discusses the use of List Boxes Combo Boxes


Input Boxes

Input Boxes Provide a Simple Way to Gather Input Without Placing a Text

Box on a Form


Format of the InputBox Function

• Prompt - message to the user• Title - text for the box's title bar• Default - default text for user's input• Xpos - X coordinate for the box's position• Ypos - Y coordinate for the box's position• Title and beyond are optional arguments

InputBox(Prompt [,Title] [,Default] [,Xpos] [,Ypos])


Sample InputBox Usage

• userInput = InputBox("Enter the distance.", "Provide a Value", "150")


Xpos, Ypos, and Twips

• Xpos specifies the distance from the left of the screen to the left side of the box

• Ypos, from the top of the screen to the top of the box

• Both are specified in twips• One twip is 1/440 inch


List Boxes

List Boxes Display a List of Items and Allow the User to Select an Item

From the List


ListBox Items Property

• This property holds the list of items from which the user may choose

• The value may be established at design time and/or run time


ListBox Items.Count Property

• This property holds the number of items that are stored in the Items property

• Example of use:

If lstEmployees.Items.Count = 0 ThenMessageBox.Show("There are no items in the list!")

End If


Item Indexing

• The Item property values can be accessed under program control

• Each item value is given a sequential index The first item has an index of 0 The second item has an index of 1, etc.

• Example:

name = lstCustomers.Items(2)' Access the 3rd item value


ListBox SelectIndex Property

• The index of the user selected item is available as the value of the SelectIndex property

• If the user did not select any item, the value is set to -1 (an invalid index value)

• Example:

If lstLocations.SelectedIndex <> -1 Thenlocation = lstLocations.Items(lstLocations.SelectedIndex)

End If


ListBox SelectedItem Property

• When an item has been selected, this property, SelectedItem, contains the selected item itself


ListBox Sorted Property

• The value of this property, if true, causes the items in the Items property to be displayed in alphabetical order


ListBox Items.Add Method

• To add items to the end of a ListBox list at run time, use the Add method

• ListBox.Items.Add(Item)• Example

lstStudents.Items.Add("Sharon")


ListBox Items.Insert Method

• To add items at a specific position in a ListBox list at run time, use the Insert method

• ListBox.Items.Insert(Index, Item)• Example making the 3rd item "Jean"

lstStudents.Items.Insert(2, "Jean")


ListBox Items.Remove, Items.Clear,and Items.RemoveAt Methods

• ListBox.Items.Remove(Item) Removes the item named by value

• ListBox.Items.RemoveAt(Index) Removes the item at the specified index

• ListBox.Items.Clear() Removes all items in the Items property


The Do While Loop

A Loop Is Part of a ProgramThat Repeats


Repetition Structure (or Loop)

• Visual Basic .NET has three structures for repeating a statement or group of statements Do While Do Until For Next


Do While Flowchart

• The Do While loop• If/While the

expression is true,the statement(s)are executed Expression statement(s)

False

True


Do While Syntax

• "Do", "While", and "Loop" are new keywords• The statement, or statements are known as

the body of the loop

Do While expressionstatement(s)

Loop


Do While Example

Private Sub btnRunDemo_Click(ByVal sender As System.Object, _ByVal e As System.EventArgs) Handles btnRunDemo.Click' Demonstrate the Do While loop

Dim count As Integer = 0Do While count < 10

lstOutput.Items.Add("Hello")count += 1

LoopEnd Sub


Infinite Loops

• Generally, if the expression is true and, hence the starts executing:

• Something with the body of the loop must eventually make the test expression false

• Otherwise, the Do While loop will continuously loop forever - called an infinite loop


Counters

• Variables called counters are frequently used to control Do While loops (see count in the previous example

• Counters are invariably initialized before the loop begins (above: Dim count As Integer = 0)

• They are also usually modified within the body of the loop (above: count += 1)


Pretest vs. Posttest Loops

• The preceding Do While loops were written in their pretest syntax

• The expression is always tested before the body of the loop is executed

• Do While loops also have a posttest form• In these, the body of the loop is always

executed first, then the expression is evaluated to check to see if additional iterations are needed


Posttest Do While Syntax and Flowchart

• The statement(s) willalways be done once,irrespective of theexpression used

Dostatement(s)

Loop While expression

Expression

statement(s)

False

True


Example: Keeping a Running Total

count = 1 ' Initialize the countertotal = 0 ' Initialize totalDo

input = InputBox("Enter the sales for day " & _count.ToString, "Sales Amount Needed")

If input <> "" Thensales = CDec(input) total += sales ' Add sales to totalcount += 1 ' Increment the counter

End IfLoop While count <= 5

unit 8 list boxes and the do while looping structure chapter 5 lists, loops, validation, and more

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