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Programming with Microsoft Visual Basic 2008 Fourth Edition

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Title: Programming with Microsoft Visual Basic 2008 Fourth Edition


1
Programming with Microsoft Visual Basic 2008
Fourth Edition
  • Chapter Three
  • Using Variables and Constants

2
Previewing the Playtime Cellular Application
  • Previewing the Playtime Cellular application
  • Access Run command on Start menu
  • Browse to VB2008\Chap03 folder
  • Click the Playtime Cellular (Playtime Cellular
    .exe) file
  • View completed order form
  • Completed application resembles Chapter 2 version

3
Previewing the Playtime Cellular Application
(continued)
Figure 3-1 Name Entry dialog box
4
Lesson A Objectives
  • After studying Lesson A, you should be able to
  • Declare variables and named constants
  • Assign data to an existing variable
  • Convert string data to a numeric data type using
    the TryParse method
  • Convert numeric data to a different data type
    using the Convert class methods

5
Lesson A Objectives (continued)
  • Explain the scope and lifetime of variables and
    named constants
  • Explain the purpose of the Option Explicit,
    Option Infer, and Option Strict

6
Using Variables to Store Information
  • Controls and variables temporarily store data
  • Variable Temporary storage location in main
    memory
  • Specified by data type, name, scope, and lifetime
  • Reasons to use variables
  • Hold information that is not stored in control on
    form
  • Allow for more precise treatment of numeric data
  • Enable code to run more efficiently

7
Selecting a Data Type for a Variable
  • Data type Specifies type of data a variable can
    store
  • Provides a class template for creating variables
  • Unicode Universal coding scheme for characters
  • Assigns unique numeric value to each character in
    the written languages of the world

8
Selecting a Data Type for a Variable (continued)
Figure 3-3 Basic data types in Visual Basic
9
Selecting a Data Type for a Variable (continued)
Figure 3-3 Basic data types in Visual Basic
(continued)
10
Selecting a Data Type for a Variable (continued)
  • For this course
  • Use Integer data type for all integers
  • Use either Decimal or Double data type of numbers
    containing decimal places or numbers used in
    calculations
  • Use String data type for text or numbers not used
    in calculations
  • Use Boolean data type for Boolean values

11
Selecting a Name for a Variable
  • Variables are referred to by name
  • Identifier Another term for variable name
  • Guidelines for naming variables
  • Use Hungarian notation, with a three character
    prefix representing the variables data type
  • Name should be descriptive e.g., dblLength
  • Use camel case e.g., dblSalesAmount
  • Must follow variable naming rules

12
Selecting a Name for a Variable (continued)
Figure 3-4 Variable naming rules and examples
13
Declaring a Variable
  • Declaration statement Used to declare (create) a
    variable and reserves space in memory for it
  • Syntax
  • DimPrivateStatic variablename As
    datatype initialvalue
  • If no initial value is given to variable when
    declaring it, computer stores default value
  • Numeric data types are set to 0
  • Boolean data types are set to False
  • Object and String data types are set to Nothing

14
Assigning Data to an Existing Variable
  • Assignment statement Assigns value to variable
    at runtime
  • Syntax variablename expression
  • Expression may contain literal constants,
    properties of controls, variables, or arithmetic
    operators
  • Literal constant Data item whose value does not
    change
  • Example The string Mary
  • Literal type character Changes type of a literal
  • Example sales2356D
  • Integer cast to Decimal

15
The TryParse Method
  • TryParse method Converts string to number
  • Syntax
  • dataType.TryParse(string, variable)
  • dataType Numeric data type, such as Integer
  • string String to be converted
  • variable Variable that receives the numeric
    value
  • TryParse is preferred over Val
  • Val only returns a type Double value

16
The TryParse Method (continued)
Figure 3-8 Results of the TryParse method for
the Double, Decimal, and Integer data types
17
The Convert Class
  • Convert class Can be used to convert a number
    from one type to another
  • Syntax Convert.method(value)
  • Convert Name of class
  • method Converts value to specified data type
  • value Numeric data to be converted
  • TryParse is recommended for converting strings to
    numeric data types
  • Will not produce an error if conversion fails

18
The Convert Class (continued)
Figure 3-9 Syntax and examples of the Convert
class methods
19
The Scope and Lifetime of a Variable
  • Scope Indicates where variable can be used
  • Lifetime Indicates how long variable remains in
    memory and can be used
  • Scope and lifetime are determined by where
    variable is declared
  • Three types of scope
  • Module Variable can be used by all procedures in
    a form
  • Procedure Variable can be used within procedure
  • Block Variable can be used within specific code
    block

20
The Scope and Lifetime of a Variable (continued)
Figure 3-11 Click event procedure using
procedure-level variables
21
The Scope and Lifetime of a Variable (continued)
Figure 3-13 Code using a module-level variable
22
Static Variables
  • Static variable Procedure level variable with
    extended lifetime
  • Remains in memory between procedure calls
  • Retains its value even when the procedure ends
  • Static keyword Used to declare static variable
  • Static variables act like module-level variables,
    but have narrower scope
  • Can only be used within procedure where declared

23
Static Variables (continued)
Figure 3-14 Code using a static variable
24
Named Constants
  • Named constant Memory location inside computer
    whose contents cannot be changed at runtime
  • Const statement Creates named constant
  • Syntax
  • Const constantname As datatype expression
  • expression Can be literal constant, another
    named constant, or an arithmetic operator
  • Cannot contain a variable

25
Named Constants (continued)
Figure 3-15 Syntax and examples of the Const
statement
26
Named Constants (continued)
Figure 3-17 Calculate Area buttons Click event
procedure
27
Option Explicit, Option Infer, and Option Strict
  • Option Explicit On statement
  • Prevents you from using undeclared variables
  • Implicit type conversion Converts right-side
    value to the data type of left side
  • Promotion Cata expanded
  • e.g., Integer to Decimal
  • Demotion data truncated
  • e.g., Decimal to Integer
  • Data loss can occur when demotion occurs

28
Option Explicit, Option Infer, and Option Strict
(continued)
Figure 3-18 Rules and examples of type
conversions
29
Option Explicit, Option Infer, and Option Strict
(continued)
  • Option Infer Off statement
  • Ensures that every variable is declared with a
    data type
  • Option Strict On statement
  • Disallows implicit conversions
  • Type conversion rules are applied when this
    option is on

30
Option Explicit, Option Infer, and Option Strict
(continued)
Figure 3-19 Option statements entered in the
General Declarations section
31
Lesson A Summary
  • Declare a variable using Dim Private Static
  • Assignment statement Assigns value to a variable
  • Three levels of scope Block, procedure, module
  • TryParse () converts strings to numeric data
  • Use Const to declare a named constant
  • Avoid programming errors by using Option Explicit
    On, Option Infer Off, and Option Strict On

32
Lesson B Objectives
  • After studying Lesson B, you should be able to
  • Include a procedure-level and module-level
    variable in an application
  • Concatenate strings
  • Get user input using the InputBox function
  • Include the ControlChars.NewLine constant in code
  • Designate the default button for a form
  • Format numbers using the ToString method

33
Modifying the Playtime Cellular Application
  • Modifications needed
  • Display message, sales tax amount, salesperson
  • Calculate the sales tax
  • Revise the TOE chart to reflect the new tasks
  • Must modify btnCalc buttons Click event and the
    forms Load event

34
Modifying the Playtime Cellular Application
(continued)
Figure 3-20 Revised TOE chart for the Playtime
Cellular application
35
Modifying the Playtime Cellular Application
(continued)
Figure 3-20 Revised TOE chart for the Playtime
Cellular application (continued)
36
Modifying the Calculate Order Buttons Code
  • General strategy
  • Remove existing code from Click event procedure
  • Recode the procedure using variables in equations
  • Use Option Explicit On statement Enforces full
    variable declaration
  • Use Option Infer Off statement Enforces that
    variables are declared with data types
  • Use Option Strict On statement Suppresses
    implicit type conversions

37
Modifying the Calculate Order Buttons Code
(continued)
Figure 3-22 Jagged blue lines indicate the
statements contain an error
38
Modifying the Calculate Order Buttons Code
(continued)
Figure 3-23 Lines to delete from the procedure
39
Modifying the Calculate Order Buttons Code
(continued)
Figure 3-24 Revised pseudocode for the btnCalc
controls Click event procedure
40
Modifying the Calculate Order Buttons Code
(continued)
Figure 3-25 Named constants and variables for
the btnCalc controls Click event procedure
41
Modifying the Calculate Order Buttons Code
(continued)
Figure 3-26 Const and Dim statements entered in
the procedure
42
Concatenating Strings
  • Concatenate Connect strings together
  • Concatenation operator Ampersand ()
  • Include space before and after operator
  • Numeric values used with the operator are
    converted to strings

43
Concatenating Strings (continued)
Figure 3-29 Examples of string concatenation
44
The InputBox Function
  • InputBox function Displays dialog box and
    retrieves user input
  • Syntax InputBox(prompt,title
  • ,defaultResponse)
  • prompt Message to display inside dialog box
  • title Text to display in the dialog boxs title
    bar
  • defaultResponse Text to be displayed in the
    input field
  • Arguments are String literals, constants, or
    variables

45
The InputBox Function (continued)
Figure 3-32 Example of a dialog box created by
the InputBox function
46
The InputBox Function (continued)
Figure 3-34 Module-level variable declared in
the forms Declarations section
47
The InputBox Function (continued)
Figure 3-35 frmMain Load event procedure
48
The ControlChars.Newline Constant
  • ControlChars.NewLine constant Issues carriage
    return followed by a line feed
  • Used to advance insertion point in file or on
    printer
  • To use, type ControlChars.NewLine at appropriate
    location
  • Can be used with string concatenation
  • Line continuation character (_) Used to break up
    long line of code into two or more lines

49
The ControlChars.Newline Constant (continued)
Figure 3-37 Modified assignment statement
50
Designating a Default Button
  • Default button Button that is activated by
    pressing Enter key
  • Button is not required to have the focus
  • Only one per form
  • Default button should be button used most often
    by the user
  • Except if buttons task is destructive and
    irreversible, such as deleting data
  • Set forms AcceptButton property to desired
    button to specify the default button

51
Using the ToString Method to Format Numbers
  • Formatting Specifying decimal places and special
    characters to display
  • ToString method is replacing Format function
  • Syntax variablename.ToString(formatString)
  • variablename Name of a numeric variable
  • formatString String specifying format you want
    to use
  • format String has form of Axx specifying a format
    and precision specifier

52
Using the ToString Method to Format Numbers
(continued)
Figure 3-40 Syntax and examples of the ToString
method
53
Lesson B Summary
  • Concatenation operator () Used to link strings
  • InputBox function Displays interactive dialog
    box
  • Use ControlChars.NewLine to move insertion point
    to a new line
  • Set default button in forms AcceptButton
    property
  • ToString method Formats number for string output

54
Lesson C Objectives
  • After studying Lesson C, you should be able to
  • Include a Static variable in code
  • Code the TextChanged event procedure
  • Create a procedure that handles more than one
    event

55
Modifying the Load and Click Event Procedures
  • Capability needed when each order is calculated
  • Order form should ask for the salespersons name
  • Revise TOE chart before implementing changes
  • Shift task of retrieving name to btnCalcs Click
    event
  • Use static variable for the salespersons name

56
Modifying the Load and Click Event Procedures
(continued)
Figure 3-45 Revised TOE chart
57
Modifying the Load and Click Event Procedures
(continued)
Figure 3-46 Revised Pseudocode for the Calculate
Order button
58
Coding the TextChanged Event Procedure
  • TextChanged event Occurs when the Text property
    value of a control changes
  • Can occur when
  • The user enters data into the control
  • Code assigns data to the controls Text property
  • Example
  • A change is made to the number of items ordered

59
Associating a Procedure with Different Objects
and Events
  • Handles keyword Appears in an event procedures
    header
  • Indicates object and event associated with
    procedure
  • Can associate an event procedure with more than
    one object and/or event
  • In Handles section of procedure header, list each
    object and event, separated by commas

60
Associating a Procedure with Different Objects
and Events (continued)
Figure 3-48 txtBlue controls TextChanged event
procedure
61
Associating a Procedure with Different Objects
and Events (continued)
Figure 3-49 Completed ClearLabels procedure
62
Lesson C Summary
  • TextChanged event procedure responds to change
    in value of controls Text Property
  • Handles clause determines which objects and
    events are associated with the event procedure
  • To create procedure for more than one object or
    event, list each object and event after Handles
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