Clearly Visual Basic: Programming
with Visual Basic 2008
Chapter 7
Where Can I Store This?
Design the following Interface, and program using
methods learned in Chapter 6
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Objectives
• Declare variables and named constants
• Convert text to a numeric data type using the
TryParse method
• Understand the scope and lifetime of variables
• Desk-check a program
• Format a program’s numeric output
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Using Storage Bins
• Internal memory
– Composed of memory locations
• Memory location
– Can hold only one item of data at a time
• To reserve a memory location:
– Use a Visual Basic instruction that assigns both a
name and data type to the memory Location
• Memory locations are called variables
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So, What’s Your Type?
• Variables
– Come in different types and sizes
• Item that a variable will accept for storage
– Determined by the variable’s data type
• Decimal variables
– Take twice as much room in memory as Double
variables
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Clearly Visual Basic: Programming with Visual Basic 2008
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Let’s Play the Name Game
• Variable name
– Should be descriptive
– Should be meaningful right after you finish a
program and years later
– No punctuation marks or spaces allowed
– Cannot be a reserved word, such as Val
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Clearly Visual Basic: Programming with Visual Basic 2008
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You’ll Need a Reservation
• Reserving a variable
– Often referred to as declaring a variable
• Dim statement
– Declares a variable in an event procedure
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Clearly Visual Basic: Programming with Visual Basic 2008
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Mini Quiz
Which of the 3 data types are appropriate for storing
the number of disks purchased by a customer:
a. Integer b. Decimal C. Double
Which of the following is not a valid name for a
variable?
A. decRate B. dblRate C. decRate_Of_Pay
D. dblPay.rate
Write a Dim statement to declare a Double variable
named dblHoursWorked
Dim dblHoursWorked as Double
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How Many Variables Should I Use?
• Circle Area problem’s solution (Figure 7-5)
– Utilizes two different variables
• One to store the output item and the other to store the
input item
• Before coding the Circle Area application
– View the Ch07-Circle Area video
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Clearly Visual Basic: Programming with Visual Basic 2008
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The TryParse Method
• Every numeric data type in Visual Basic has:
– A TryParse method that can be used to convert text
to that numeric data type
• Syntax of the TryParse method
– dataType.TryParse(text, variable)
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Dim Area as a Double
Dim PI as a Constant- Double
Dim Radius as a double
Using TryParse-change the Radius to a numeric variable
Double.TryParse(txtRadius.Text, dblRadius)
Check, Please … I’m Ready to Go
• Desk-check table for a program
– Will contain one column for each variable
• Desk-check the Circle Area program
– Use radius values of 6.5 and 10
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Clearly Visual Basic: Programming with Visual Basic 2008
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Using Constants to Keep Things …
Well, the Same
• Named constant
– Memory location whose value cannot change while
the application is running
– Used to give names to constant values
– Makes code more self-documenting and easier to
modify
– Created using the Const statement
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Dressing Up the Output
• Formatting
– Specifying number of decimal places and the special
characters to display in a number
• variable.ToString(formatString)
– Formats a number
• string
– Text enclosed in double quotation marks
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Try entering 6.5 for the Radius
This is what your results should look like:
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Private Sub btnCalc_Click(ByVal sender As Object, ByVal e As System.EventArgs)
Handles btnCalc.Click
' calculates and displays the area of a circle
' declare named constant
Const dblPI As Double = 3.141593
' declare variables
Dim dblArea As Double
Dim dblRadius As Double
' assign radius to a variable
Double.TryParse(txtRadius.Text, dblRadius)
' calculate area
dblArea = dblRadius * dblRadius * dblPI
' display area
lblArea.Text = dblArea.ToString("N2")
End Sub
1. Which of the following declares a variable named intNumSold?
A. Dim intNumSold As Integer B. Dim As Integer intNumSold
C. Const intNumSold As Integer D. None of the Above
2. If you enter A34 in a txtPrice textbox, what does the TryParse
control assign to it? Decimal.TryParse(txtPrice.Text, decPrice)
A. A
B. A34
C. 0
D. 34
3. Which adds intScore and intScore2 and then multiplies the sum
by 2, assigning the results to intTotal?
A. intScore1 +intScore2 *2 = intTotal
B. (intScore1 +intScore2)*2= intTotal
C. intTotal = IntScore1 + intScore2 * 2
D. None of these
4. Which of the following statements declares the decRate as a
constant set to .15? A. Con decRate as Decimal = .15
B. Const decRate as Decimal = .15
C. Constant decRate as Decimal = .15 D. None of these
5. Which formats dblDue with dollar sign and 2 decimal places?
A. lblDue.text = dblDue.ToString(“C2”)
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B. lblDue.Text = ToSting(dblDue, “C2”)
Jackson College charges $1800 for room and board plus
$100 per semester hour. The Cashier will enter the number
of hours the student enrolls in. The output is Total Cost
Assign the variable txtHours for the textBox using the
TryParse command. Format the output as currency
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Design and program the following Concert Revenue Calculator
Finding the area, and number of pizza slices from a given Radius
Area = Pi * R^2
Open SUN Project and Dimension decHours, decPayRate,
decGross, decFwt, decFica, decState, decNet As Decimal
Summary
• Internal memory of a computer
– Composed of memory locations
• Memory location
– Can store only one value at any one time
• Value in a variable
– Can change as the application is running
• Integer data type
– Stores integers
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Summary (continued)
• Dim statement
– Reserves a procedure-level variable
• TryParse method
– Converts text to numbers
• Const statement
– Declares named constants
• ToString method
– Converts a number to formatted text
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