1. Chapter 3 Variables, Constants and Calculations
Programming In
Visual Basic .NET

2. Data - Variables and Constants
Memory locations that hold data that can be changed during project execution
Example: customer’s name
Named Constant
Memory locations that hold data that cannot be changed during project execution
Example: sales tax rate
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3. Data - Variables and Constants (continued)
In Visual Basic when you declare a Variable or Named Constant
An area of memory is reserved
A name is assigned called an Identifier
Follow rules and naming conventions
Use Declaration Statements to establish Variables and Constants
Assign name and data type
Not executable unless initialized on same line
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4. Data Types (p 87 Table 3.1) Boolean Byte (0 to 255) Char Date String
Decimal
Object
Short (-32,768 to 32,767)
Integer (-2,147,483,648 to 2,147,483,647)
Long (larger whole numbers)
Single (floating point accuracy to 6 digits)
Double (floating point accuracy to 14 digits)
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5. Data Types – Memory Usage
Boolean – 2 bytes
Byte – 1 byte
Char – 2 bytes
Date – 8 bytes
String – varies
Decimal – 16 bytes
Object – 4 bytes
Short – 2 bytes
Integer – 4 bytes
Long – 8 bytes
Single – 4 bytes
Double – 8 bytes
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6. Naming Variables and Constants
Must follow Visual Basic Naming Rules
Should follow Naming Conventions
Meaningful names
Include class (data type) of variable
Use mixed case for variables and uppercase for constants
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7. Constants Named User assigned name, data type and value
Use CONST keyword to declare
Intrinsic
System defined within Visual Studio
In Chapter 2 we used the Intrinsic Color Constants
Const COMPANY_ADDRESS_String As String = "101 S. Main Street"
Const SALES_TAX_RATE_Decimal As Decimal = .08D
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8. Assigning Values to Constants
Declare the data type of numeric constants by appending a type-declaration character
Decimal - D
Double - R
Integer - I
Long - L
Short - S
Single - F
Integer – I Single – F
Decimal – D Double – R
Long – L
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9. Declaring Variables Declared inside a procedure using a Dim statement
Declared outside a procedure using Public, Private or Dim statements
Always declare the variable’s data type
May declare several variables with one statement
Use IntelliSense to assist in writing statements
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10. Declaration Examples Dim customerNameString As String
Private totalSoldInteger As Integer
Dim temperatureSingle As Single
Dim priceDecimal As Decimal
Private priceDecimal As Decimal
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11. Scope and Lifetime of Variables
Visibility of a variable is its scope
Scope may be
Namespace
Module-level
Local
Block-level
Lifetime of a variable is the period of time the variable exists
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12. Scope and Lifetime of Variables (continued)
Namespace
Available to all procedures of project
Good programming practice excludes use of Namespace variables
Module
Available to all procedures within that module (often a form)
Use Public or Private keywords
Place in declaration section of module (form)
Local
Available only to the procedure it is declared in
Declare with Dim keyword and place at top of procedure
Block (not used until later in this course)
Available only in block of code where declared
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13. Module Level Variable Declaration Example
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14. Calculations
Calculations can be performed with variables, constants, properties of certain objects, and numeric literals
Do not use strings in calculations
Values from Text property of Text Boxes
Are strings, even if they contain numeric data
Must be converted to a numeric data type before performing a calculation
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15. Converting Strings to a Numeric Data Type
Use Parse methods to convert Text to numeric data
Each numeric data type class has a Parse method
Parse method returns a value that can be used in calculations
Parse method fails if user enters nonnumeric data or leaves data blank
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16. Converting to String
Values assigned to string variables or Text properties must be string
Convert any numeric data type to string using .ToString method
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17. Method Convert To Conversion Methods Integer.Parse Integer
Decimal.Parse Decimal
.ToString String
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18. Conversion Examples
quantityInteger = Integer.Parse(quantityTextBox.Text)
priceDecimal = Decimal.Parse(priceTextBox.Text)
wholeNumberInteger = Integer.Parse(digitString)
resultLabel.Text = resultDecimal.ToString( )
countTextBox.Text = countInteger.ToString( )
idString = idInteger.ToString( )
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19. Arithmetic Operations
Operator Operation
+ Addition
– Subtraction
* Multiplication
/ Division
\ Integer Division
Mod Modulus – Remainder of division
^ Exponentiation
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20. Order of Operations
Order of precedence in arithmetic expressions from highest to lowest
1. Any operation inside parentheses
2. Exponentiation
3. Multiplication and division
4. Integer division
5. Modulus
6. Addition and subtraction
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21. Mathematical Examples
Note the use of parentheses to control order of precedence
3+4*2 = 11 Multiply then add
(3+4)*2 = 14 Parentheses control: add then multiply
8/4*2 = 4 Same level, left to right: divide then multiply
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22. Using Calculations in Code
Perform calculations in assignment statements
What appears on right side of assignment operator is assigned to item on left side
Assignment operators
=, +=, -=, *=, /=, \=, &=
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23. Option Explicit and Option Strict
Option Explicit forces variables to be declared before using
Option Strict
Makes VB a strongly typed language
Does not allow implicit conversions from a wider data type to a narrower one or between String and numeric data types
Best practice to always turn on either in code or in Project Properties dialog box
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24. Converting Between Numeric Data Types
Implicit (automatic) conversion
Converts value from narrower data type to wider type where no danger of losing precision exists
Explicit conversion (casting)
Uses methods of Convert class to convert between data types
ToDecimal, ToSingle, ToDouble, ToInt16 (Short), ToInt32 (Integer), ToInt64 (Long)
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25. Rounding Numbers Round decimal fractions Use Decimal.Round method
Decimal.Round and Convert methods use technique called “rounding toward even”
See Appendix B for additional mathematical, financial and string functions
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26. Formatting Data for Display
To display numeric data in a label or text box, first convert value to string
Use ToString method
Format the data using formatting codes
Specifies use of dollar sign, percent sign and commas
Specifies number of digits that appear to right of decimal point
resultLabel.Text = resultDecimal.ToString( ) countTextBox.Text = countInteger.ToString( )
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27. Using Format Specifier Codes
"C" code
Currency – String formatted with dollar sign, commas separating each group of three digits and two digits to the right of decimal point
"N" code
Number – String formatted with commas separating each group of three digits and two digits to the right of decimal point
Can specify number of decimal positions
Example: "C0" zero digits
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28. Format Specifier Codes
Name
C or c
Currency
F or f
Fixed-point
N or n
Number
D or d
Digits
P or p
Percent
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29. Format Specifier Codes Examples
Variable
Value
Code
Output
totalDecimal
"C"
$1,125.67
"N0"
1,126
pinInteger
123
"D6"
000123
rateDecimal
0.075
"P"
7.50%
"P3"
7.500%
"P0" 8%
valueInteger
-10
($10.00)
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30. Date Specifier Code
Format DateTime values using format codes and ToString method
Date codes are case sensitive
d Short date D Long date
t Short time T Long time
f Full date/time F Full date/time
g General (short time) G General (long time)
M or m Month R or r GMT pattern
ToLongDateString, ToShortDateString, ToLongTimeString, ToShortTimeString methods also available
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31. Handling Exceptions
Use structured exception handling to catch errors before run-time error occurs
Catching exceptions referred to as error trapping
Code to handle exception called error handling
Error handling in Visual Studio .NET is standardized for all languages using the CLR
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32. Try/Catch Blocks
Enclose statements that might cause an error within Try/Catch block
If an error occurs control is transferred to the Catch Block
If a Finally statement is included, the code in that section executes last, whether or not an exception occurred
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33. Try Block - General Form
statements that may cause error
Catch [VariableName As ExceptionType]
statements for action when an exception occurs
[Finally
statements that always execute before exit of Try block]
End Try
See page 114 for list of common Exception Classes
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34. Try Block - Example Catches Any Exception
quantityInteger = Integer.Parse(quantityTextBox.Text)
quantityLabel.Text = quantityInteger.ToString( )
Catch
messageLabel.Text = "Error in input data."
End Try
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35. Try Block - Example Catches Specific Exception
quantityInteger = Integer.Parse(quantityTextBox.Text)
quantityLabel.Text = quantityInteger.ToString( )
Catch theException As FormatException
messageLabel.Text="Error in input data."
End Try
Failure of numeric conversion, usually blank or nonnumeric data
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36. Try Block - Example Handling Multiple Exceptions
' Statements that may cause errors.
Catch theException As FormatException
' Statements for nonnumeric data.
Catch theException As ArithmeticException
' Statements for calculation problem.
Catch theException As Exception
' Statements for any other exception.
End Try
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37. Displaying Messages in Message Boxes
Use Show method of MessageBox to display message box, a special type of window
Arguments of Show method
Message to display
Optional Title Bar Caption
Optional Button(s)
Optional Icon
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38. MessageBox Object
The MessageBox is an overloaded method
Signatures correspond to the argument list
There are multiple signatures to choose from
Do not reverse, transpose or leave out any of the arguments
IntelliSense displays argument list (also called signatures)
MessageBox.Show (TextMessage, TitlebarText, _ MessageBoxButtons, MesssageBoxIcon)
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39. MessageBox Object (continued)
TextMessage string
String literal or variable that displays message
Title Bar text
String that appears in title bar of message box
MessageBox Buttons
OK, OKCancel, RetryCancel, YesNo, YesNoCancel, AbortRetryIgnore
MessageBox Icons
Asterisk, Error, Exclamation, Hand, Information, None, Question, Stop, Warning
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40. Testing Multiple Fields
Each input field presents a chance for an error
To indicate specific field that caused error use nested Try/Catch blocks
After error set focus back to field in error
Use SelectAll method of text box to make text appear selected to aid user
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41. Counting and Accumulating Sums
Declare module-level variables, since local level variables reset to 0 each time the procedure is called
Summing Numbers
Counting
Calculating an Average
discountedPriceSumDecimal += discountedPriceDecimal
Private saleCountInteger As Integer
saleCountInteger += 1
averageDiscountedSaleDecimal = discountedPriceSumDecimal / saleCountInteger
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