lecture vb - program controls

Check boxes provide a way to make choices from a list of potential

candidates. Some, all, or none of the choices in a group may be

selected.

Check Box Properties:

Text Identifying text next to box.

Font Sets font type, style, size.

CheckState Indicates if unchecked, checked or

indeterminate

Check Box Events:

Checked Triggered when a box is clicked. Value

property is automatically changed by Visual

Basic.

Radio buttons provide the capability to make a mutually exclusive

choice among a group of potential candidate choices. Hence, option

buttons work as a group, only one of which can have a True (or

selected) value.

Radio Button Properties:

Text Identifying text next to button.


Checked Indicates if selected (True) or not (False). Only

one option button in a group can be True. One

Check Boxes

Radio Buttons

button in each group of option buttons should

always be initialized to True at design time.

Option Button Events:

Click Triggered when a button is clicked. Value

property is automatically changed by Visual

Basic.

GroupBox provide a way of grouping related controls on a form.

And, in the case of option buttons, frames affect how such buttons

operate.

To group controls in a groupbox, you first draw the frame. Then, the

associated controls must be drawn in the groupbox. This allows you

to move the groupbox and controls together.

Drawing the controls outside the groupbox and dragging them in,

copying them into a groupbox, or drawing the groupbox around

existing controls will not result in a proper grouping. It is perfectly

acceptable to draw groupbox within other groupbox.

As mentioned, groupbox affect how option buttons work. Option

buttons within a frame work as a group, independently of option

buttons in other frames. Option buttons on the form, and not in

groupbox, work as another independent group. That is, the form is

itself a groupbox by default. We'll see this in the next example.

GroupBox

It is important to note that an independent group of option buttons is

defined by physical location within groupbox, not according to naming

convention.

Groupbox Properties:

Text Title information at top of frame.


Program-Control Statements

Like any programming language, the control statements are the

essence of VB programming language because it governs the flow of

program execution. The way they are implemented in many ways defines

the personality of the VB language.

The program-control statements may be separated into three

categories. The first consists of the conditional instructions if and case.

The second are the loop-control statements such as while, for and do-

while. The final category is the unconditional branch instruction label.

The If Then Statement

If statement executes a statement or a block of codes if an expression is true.

General syntax:If (boolean expression) Then

statementEnd If

where:

Boolean expression is a combination of relational operators, conditional operators, and values resulting in a value of true or false

Statement represents the lines of code that are executed if expression is true

Example:

If (grade >= 75) Then Label2.Text = "Passed" End If

Implementation:

1. Design the following form.

2. Set properties as:

Label 1:

Text: Input Grade

Label 2:Text: [BLANK]

Button1:Text: Evaluate

3. Declare a variable that will hold the value of TextBox1 as:

Dim grade As Short

The program will check if the grade in the TextBox1 is a passing grade once the Button1 is clicked. Double click on Button1 and inside the procedure type:

grade = Val(TextBox1.Text)If (grade >= 75) Then

Label2.Text = "Passed" End If

The If Then Else Statement

If statement executes a statement or a block of codes if an

expression is true, otherwise the else statement will be executed.


statementElse

statementEnd If

where:

Boolean expression is a combination of relational operators,

conditional operators, and values resulting in a value of true

or false

Statement1 represents the lines of code that are executed if

expression is true and statement2 represents the lines of

code that are executed if expression is false.

Example:

If (grade >= 75) Then Label2.Text = "Passed"Else

Label2.Text = "Failed"End If

Implementation:

Modify the Evaluate Button above and change the code into: grade = Val(TextBox1.Text)

If (grade >= 75) Then Label2.Text = "Passed" Else Label2.Text = "Failed" End If

The If Then ElseIf Statement

Another variation of if statement is if then elseif statements to state

multiple outcomes for several different expressions.


statement1Else if (boolean expression) Then

statement2Else

statement3

where:

Boolean expression is a combination of relational operators,

conditional operators, and values resulting in a value of true

or false


expression is true. Statement2 represents the lines of code

that are executed if the else if expression is true and

statement3 represents the lines of code that are executed if

both Boolean expressions are false.

Example:

If (grade >= 75 And grade <= 100) Then Label2.Text = "Passed" ElseIf (grade <= 74) Then Label2.Text = "Failed" Else

Label2.Text = "Invalid Input" End If

Implementation:

Modify the program again, this time we will limit the input up to

100, since there is no grade higher than 100.

grade = Val(TextBox1.Text) If (grade >= 75 And grade <= 100) Then Label2.Text = "Passed" ElseIf (grade <= 74) Then Label2.Text = "Failed" Else Label2.Text = "Invalid Input" End If

The Select Case Statement

Another control statement that runs one of several groups of

statements, depending on the value of an expression.

General syntax:Select Case TestExpression

Case ExpressionList1

Statement1

Case ExpressionList2

Statement1

Case Else

Statement3

End Select

where:

TestExpression is the value to be compared to the case

statements


expression is ExpressionList1 is true. Statement2 represents

the lines of code that are executed if the else if

ExpressionList2 is true and statement3 represents the lines of

code that are executed if both Boolean expressions are false.

Example:

Select Case grade Case 0 To 74 Label2.Text = "Failed" Case 75 To 100 Label2.Text = "Passed" Case Else Label2.Text = "Invalid Input" End Select

Implementation:

Let’s convert the last example to the Select Case Statement by

changing the code now into:

grade = Val(TextBox1.Text) Select Case grade Case 0 To 74 Label2.Text = "Failed" Case 75 To 100 Label2.Text = "Passed" Case Else Label2.Text = "Invalid Input" End Select

A list box displays a list of items from which the user can select one

or more items. If the number of items exceeds the number that can

be displayed, a scroll bar is automatically added.

Some of the List Box Properties:

HorizontalScrollbar: Displays a horizontal scrollbar to the

ListBox.

MultiColumn: Works when the ListBox has

MultipleColumns.

The default value is set to False. Set it to

True if you want the list box to display

multiple columns.

ScrollAlwaysVisible: Default value is set to False. Setting it to

True will display both Vertical and

Horizontal scrollbar always.

SelectionMode: Default value is set to one. Select option

None if you do not any item to be selected.

Select it to MultiSimple if you want multiple

items to be selected. Setting it to

MultiExtended allows you to select multiple

items with the help of Shift, Control and

arrow keys on the keyboard.

Sorted: Default value is set to False. Set it to True if

you want the items displayed in the ListBox

to be sorted by alphabetical order.

Working with ListBoxes

To Add Contents to the ListBox

Contents in the ListBox can be added in two ways such as:

1. To the Properties Window, choose Items and you can directly supply the contents of your ListBox

2. To your program, using the ListBox.Items.Add (“Items to be Added”)

To Refer Items in the ListBox

Items in a ListBox are referred by index. When items are added to the ListBox they are assigned an index. The first item in the ListBox always has an index of 0 the next 1 and so on.

To Display the Index of an Item

TextBox1.Text = ListBox1.SelectedIndex'this line of code will display the index number of the selected item from the List in a TextBox

To Count the Number of Items in a ListBox

TextBox1.Text = ListBox1.Items.Count'this line will count the number of items in the ListBox and displays in a TextBox

To Display the Item Selected from ListBox in a TextBox

TextBox1.Text = ListBox1.SelectedItem'using this line will display the selected item in a TextBox

To Remove Items from a ListBox

You can remove all items or one particular item from the list box by

calling the Remove or RemoveAt method to delete items. Remove has

one argument that specifies the item to remove. RemoveAt removes the

item with the specified index number.

ListBox1.Items.RemoveAt(4)

'this removes an item at specific index 4 from the ListBox

ListBox1.Items.Remove(ListBox1.SelectedIndex)

‘this removes the selected item from the List Box

To Remove All Items

ListBox1.Items.Clear( )'using this line will clear or empty the ListBox

Example:1. Create a new Windows application and draw the following controls:

The combo box is similar to the list box. The differences are a combo box includes a text box on top of a list box and only allows selection of one item. In some cases, the user can type in an alternate response.

Butto

Butto

Butto

Butto

ListBo

Combo Boxes

DropDownStyle The default value is set to DropDown which means

that the ComboBox displays the Text set by it's

Text property in the Textbox and displays it's

items in the DropDownListBox below. Setting it to

simple makes the ComboBox to be displayed with

a TextBox and the list box which doesn't drop

down. Setting it to DropDownList makes the

ComboBox to make selection only from the drop

down list and restricts you from entering any text

in the textbox.

Sorted We can sort the ComboBox with this property

which is set to False by Default.

Items We can add items to the ComboBox with this

property.

ComboBox Event

The default event of ComboBox is SelectedIndexChanged which

looks like this in code:

Working with ComboBoxes

To display the selection made in the ComboBox in the Textbox the code looks like this:

To Add Contents to the ComboBox

Contents in the ComboBox can be added in two ways such as:

1. To the Properties Window, choose Items and you can directly

supply the contents of your ComboBox

2. To your program, using the ComboBox.Items.Add (“Items to be

Added”)

To Refer Items in the ComboBox

Items in a ComboBox are referred by index. When items are

added to the ComboBox they are assigned an index. The first item in

the ComboBox always has an index of 0 the next 1 and so on.

To Display the Index of an Item

TextBox1.Text = ComboBox1.SelectedIndex

'this line of code will display the index number of the

selected item from the ComboBox in a TextBox

To Count the Number of Items in a ComboBox

TextBox1.Text = ComboBox1.Items.Count

'this line will count the number of items in the

ComboBox and displays in a TextBox

To Display the Item Selected from ComboBox in a TextBox

TextBox1.Text = ComboBox1.SelectedItem

'using this line will display the selected item in a

TextBox

To Remove Items from a ComboBox

You can remove all items or one particular item from the list box

by calling the Remove or RemoveAt method to delete items. Remove

has one argument that specifies the item to remove. RemoveAt

removes the item with the specified index number.

ComboBox1.Items.RemoveAt(4)

'this removes an item at specific index 4 from the

ComboBox

ComboBox1.Items.Remove(ListBox1.SelectedIndex)

‘this removes the selected item from the Combo Box

To Remove All Items

ComboBox1.Items.Clear( )

'using this line will clear or empty the ComboBox

To Access a Specific Item

Accessing specific items in a Windows Forms combo box, list box,

or checked list box is an essential task. It enables you to

programmatically determine what is in a list, at any given position. You

can do this by using the GetItemText.

Example:

1. Create a new Windows form and design the following form:

ComboBox1

Label2

Label1

lecture vb - program controls

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