This document discusses various logical operators, generating random numbers, static variables, counter variables, the Select Case statement, displaying message boxes, the checkbox control, and the line continuation character in Visual Basic.NET. It provides examples and explanations of how to use these programming concepts and controls.

1. Module 7
Created by S. O’Brien-Duke

2. Logical
Operators
• Boolean Expressions
• Evaluates to True or False
• Can be formed using the logical
operator And and Or
• And
• Expression evaluates to True only
when both expressions (conditions)
are True
• If intGrade > 0 And intGrade < 100
Then
Me.lblDisplay.Text = “Valid
grade.”

3. Logical Operators
And
Expression 1
intGrade > 0
Expression 2
intGrade < 100
Result
Valid grade.
True True True
True False False
False True False
False False False

4. Logical
Operators
• Or
• If either expression (condition) is
true, then the entire expression is
True.
• If intGuess < 1 Or IntGuess> 50
Then
Me.lblMessage.Text = “Invalid
number”.

5. Logical Operators
Or
Expression 1
intGuess < 1
Expression 2
IntGuess> 50
Result
Invalid number
True True True
True False False
False True False
False False False

6. Logical
Operators
• Not
• Is True only when the expression it
is used with is False
• Example:
• strItem = “Pen”
• If Not strItem = “Pen” Then
Me.lblMessage.Text = “No
discount”.
• Would evaluate to False since
strItem does equal “Pen”

7. Logical Operators
Not
Expression
strItem = “Pen”
Result
No discount
True False
False True

8. Generating
Random
Numbers
• Random numbers are needed for
games, screen savers, and many other
types of applications.
• VB.NET has a built in Rnd() function
that generates random numbers.
• Using Rnd() alone generates number
greater than or equal to zero (0) and
less than one (1)
• What if a random number between 0
and 10 is needed?

9. Generating
Random
Numbers
• A range can be specified using this
formula
• (HighNumber – LowNumber + 1) *
Rnd() + LowNumber
• Example:
• We need a random number
between 10 and 30
• ((30-10+1 ) * Rnd() + 10)
• 21 * Rnd() + 10
• Using this formula we will have a
decimal number…Why?

10. Generating
Random
Numbers
• Rnd() generates number greater than or
equal to zero (0) and less than one (1)
• Rnd() always generates a decimal
• To generate an integer, the Int() function
can be used
• Int(21 * Rnd()) + 10
• The decimal potion of the calculation
is truncated
• Randomize()
• Initializes the Rnd() function
• Randomize() statement is added to the
event procedure before the Rnd()
function is used

11. Static Variables
• Variables have scope
• Local scope – declared in a
procedure
• Called a local variable
• Global scope – declared outside of
the procedure
• Called a global variable
• Variables also have a lifetime
• Local variable lifetime – duration of
the procedure
• Global variable lifetime – duration
of the program

12. Static Variables
• Lifetime of a local variable can be
extended by using a Static declaration
statement
• Static variableName as dataType = value
• Keyword Static replaces Dim
• Must be explicitly initialized in the
declaration
• Static variables
• Scope – local to the procedure
• Lifetime – duration of the program
• Necessary in event procedures with
variables that should be retained in
memory throughout program
execution

13. Counter
Variables
• A counter is a variable storing a
number that is incremented by a
constant value
• Useful for:
• Keeping track how many times a
button is clicked
• A guess is entered
• A password is typed
• A web site is visited

14. Counter
Variables
• Syntax:
• counter = counter + constant
• Counter is the numeric variable
that is updated
• Constant is the number that is
added to the current value of
the counter
• intNumTries = intNumTries + 1
• A counter should be declared as a
Static variable

15. The
Select…Case
Statement
• A decision structure that uses the
result of an expression to determine
which block of code to execute
• preferred to If…Then...ElseIf
• Takes the form:
Select expression
Case value
Statements
…
Case Else
Statements
End Select

16. The
Select…Case
Statement
Select Case intScore
Case 0, 10 'Score is 0 or 10
strMessage = "Nice try."
Case 20 To 25 'Score is 20, 21, 22, 23,
24, or 25
strMessage = "Great!"
Case Else 'Score other than 0, 10, 20,
21, 22, 23, 24, or 25
strMessage
End Select
‘Display message
Me.lblMessage.Text = strMessage

17. The
Select…Case
Statement
Start
Get User Input
Me.lblMessage.Text =
strMessage
Stop
Select
Case
intScore
Case 0, 10
strMessage = “Nice try”
Case 20 To 25
strMessage = "Great!"
Case Else
strMessage = "Invalid score."
Flowchart

18. The
Select…Case Is
Statement
• Compares the result of an expression
to a range of values
• Syntax:
SELECT CASE variableName
CASE IS < value
execute statement 1
CASE IS = value
execute statement 2
CASE IS > value
execute statement 3
END SELECT

19. The
Select…Case Is
Statement
Select Case intTestGrade
Case Is > 100
strMessage = "Invalid grade."
Case Is >= 93
strMessage = "A"
Case Is >= 85
strMessage = "B"
Case Is >= 78
strMessage = "C"
Case Is >= 69
strMessage = "D"
Case Is >= 0
strMessage = "F"
Case Else
strMessage = "Invalid grade."
End Select
Me.lblMessage.Text = strMessage

20. Review
Displaying a
Message Box
• A message box is a predefined dialog
box that displays a message for the
user
• Uses:
• Alert the user to invalid data
• A reminder of options required for
an application to continue
• MessageBox is a class that uses the
Show() method for displaying

21. More About Displaying a
Message Box
• Syntax:
• MessageBox.Show(message)
• Message is a variable
• A constant
• Or a string enclosed in quotation
marks
• MessageBox.Show(“Guess out of
range”)

22. More About Displaying a
Message Box
• Syntax:
• MessageBox.Show(message, title, button,
icon)
• Message is displayed in the dialog box
• Title is in the title bar
• Additional buttons can be added
• An icon can be displayed
• MessageBox.Show(“Guess out of range”,
“Guessing Game”,
MessageBoxButtons.RetryCancel,
MessageBoxIcon.Error)

23. The CheckBox
Control
• Allows the user to select options
• One or more check boxes can be
selected
• CheckBoxes can be placed inside a
GroupBox object
• CheckBox object has the properties:
• Name
• Identifies the control object
• Has prefix chk
• Text
• Text displayed next to the box
• Checked
• Can be set to True or False to display the
check box with or without a check

24. The CheckBox
Control
• CheckBoxes can have click events
• If…Then statement can be used in a
program to determine if a check box is
selected or cleared.
• Example:

25. Line-
Continuation
Character
• Visual Basic.NET statements that are
long can be typed onto two or more
lines when the line-continuation
character is used.
• The underscore (_) is the line-
continuation character and MUST have
a space before it and nothing after it
and cannot occur within quotation
marks.

26. Line-
Continuation
Character
• Example:
If Not (Me.chkBed.Checked And
Me.chkLunch.Checked _
And Me.chkHomework.Checked _
And Me.chkTeeth.Checked) Then
MessageBox.Show("Did you forget
something?")
• Properly dividing a statement into two
or more lines can make code easier to
read, which is good programming style