Programming with
Microsoft Visual Basic 2010
5th Edition
Chapter Three
Using Variables and Constants
Previewing the Modified Playtime
Cellular Application
 Previewing the Playtime Cellular application
 Access Run command on Start menu
 Browse to VB2010\Chap03 folder
 Click the Playtime Cellular (Playtime Cellular.exe) file
 View order form
 Enter customer information from pages 119-120
 Completed application resembles Chapter 2 version
2
Figure 3-2 Completed order form
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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
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Lesson A Objectives (cont’d.)
 Explain the scope and lifetime of variables and named
constants
 Explain the purpose of the Option Explicit, Option Infer, and
Option Strict
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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
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Using Variables to Store Information
(cont’d.)
 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
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Figure 3-3
Basic data types in
Visual Basic
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Using Variables to Store Information
(cont’d.)
 For this course:
 Use Integer data type for all integers
 Use either Decimal or Double data type for 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
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Using Variables to Store Information
(cont’d.)
 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 variable’s data type
 Name should be descriptive: e.g., dblLength
 Use camel case: e.g., dblSalesAmount
 Must follow variable naming rules
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Figure 3-4 Variable naming rules and examples
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Using Variables to Store Information
(cont’d.)
 Declaring a variable
 Declaration statement: Used to declare (create) a variable and
reserve space in memory for it
 Syntax shown in Figure 3-5 on next slide
 If no initial value is given to variable when declaring it,
computer stores default value
 Numeric variables are set to 0
 Boolean variables are set to False
 Object and String variables are set to Nothing
 Date variables are set to 1/1/0001 12:00:00AM
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Figure 3-5 Syntax and examples of a variable declaration statement
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Assigning Data to an Existing Variable
 Assignment statement: Assigns value to variable at run time
 Syntax: variablename = expression
 Expression may include literal constants, object properties,
variables, keywords, arithmetic operators
 Literal constant
 Data item whose value does not change
 Example: The string “Mary”
 Literal type character
 Forces literal constant to change data type
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Figure 3-6 Assignment statements in which the value’s data
type matches the variable’s data type
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Assigning Data to an Existing Variable
(cont’d.)
 TryParse method: Converts string to number
 TryParse is preferred over Val
 Allows programmer to specify data type
 Val only returns a type Double value
 Syntax shown in Figure 3-7 on next slide
 dataType: Numeric data type, such as Integer
 string : String to be converted
 variable : Variable that receives the numeric value
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Figure 3-7 Basic syntax and examples of the TryParse method
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Assigning Data to an Existing Variable
(cont’d.)
 Convert class: Can be used to convert a number from one
type to another
 Syntax shown in Figure 3-9 on next slide
– 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
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Figure 3-9 Syntax and examples of the Convert class methods
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The Scope and Lifetime of a Variable
 Scope: Indicates where variable can be used
 Lifetime: How long variable remains in memory
 Scope and lifetime are determined by where variable is
declared
 Three types of scope
 Class: 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
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The Scope and Lifetime of a Variable
(cont’d.)
 Variables with procedure scope
 Can be used only by that procedure
 Declared at beginning of procedure
 Removed from memory when procedure ends
 Declared using Dim keyword
 Most variables used in this course will be procedure-level
variables
 Sales tax example UI and code given on following slides
illustrate use of procedure variables
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The Scope and Lifetime of a Variable
(cont’d.)
Figure 3-10 User interface for the Sales Tax Calculator application
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Figure 3-11
Click event
procedures using
procedure-level
variables
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The Scope and Lifetime of a Variable
(cont’d.)
 Variables with class scope
 Can be used by all procedures in a form
 Declared in form’s Declarations section
 Remain in memory until application ends
 Declared using Private keyword
 Total Sales example UI and code given on following slides
illustrate use of class-level variables
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The Scope and Lifetime of a Variable
(cont’d.)
Figure 3-12 User interface for the Total Sales application
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Figure 3-13 Code using a class-level variable
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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 class-level variables but have narrower
scope
 Can only be used within procedure where declared
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Figure 3-14 Code using a static variable
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Named Constants
 Named constant
 Memory location inside computer whose contents cannot be
changed at run time
 Const statement: Creates named constant
 Stores value of expression in named constant
 expression: Can be literal constant, another named constant, or
an arithmetic operator
 Cannot contain a variable or method
 Syntax and examples shown in Figure 3-15 on next slide
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Figure 3-15 Syntax and examples of the Const statement
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Figure 3-16 User interface for the Area Calculator application
Figure 3-17 Calculate Area button’s Click event procedure
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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
 Data converted to greater precision number
 e.g., Integer to Decimal
 Demotion
 Data truncated
 e.g., Decimal to Integer
 Data loss can occur when demotion occurs
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Option Explicit, Option Infer, and Option
Strict (cont’d.)
 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
 Figure 3-18 on following slide contains examples
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Figure 3-18 Rules and examples of type conversions
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Option Explicit, Option Infer, and Option
Strict (cont’d.)
Figure 3-19 Option statements entered in the General Declarations section
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Lesson A Summary
 Declare a variable using {Dim | Private | Static}
 Assignment statement: Assigns value to a variable
 Three levels of scope: Block, procedure, class
 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
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Lesson B Objectives
After studying Lesson B, you should be able to:
 Include procedure-level and class-level variables 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
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Modifying the Playtime Cellular
Application
 Modifications needed:
 Calculate and display the sales tax
 Display salesperson name
 Revise the TOE chart to reflect the new tasks
 Must modify btnCalc button’s Click event and the form’s
Load event
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Figure 3-20 Revised TOE chart for the Playtime Cellular application
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Modifying the Calculate Order Button’s
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
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Figure 3-22 Jagged blue lines indicate errors in the statements
Figure 3-23 Lines to delete from the procedure
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Modifying the Calculate Order Button’s
Code (cont’d.)
Figure 3-24 Revised pseudocode for the btnCalc control’s Click event procedure
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Modifying the Calculate Order Button’s
Code (cont’d.)
Figure 3-25 List of named constants and variables
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Modifying the Calculate Order Button’s
Code (cont’d.)
Figure 3-26 Const and Dim statements entered in the procedure
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Figure 3-27
Code entered in the btnCalc
control’s Click event procedure
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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
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Concatenating Strings (cont’d.)
Figure 3-29 Examples of string concatenation
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The InputBox Function
 InputBox function
 Displays dialog box and retrieves user input
 Arguments
 prompt: Message to display inside dialog box
 title: Text to display in the dialog box’s title bar
 defaultResponse: Text to be displayed in the input field
 Returned value most often assigned to String variable
 Syntax shown in Figure 3-33 on next slide
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Figure 3-33
Basic syntax and examples of
the InputBox function
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The ControlChars.Newline Constant
 ControlChars.NewLine constant
 Advances the insertion point to the next line in a control
 Also 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
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The ControlChars.Newline Constant
(cont’d.)
Figure 3-37 Modified assignment statement
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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 button’s task is destructive and irreversible, such as
deleting data
 Set form’s AcceptButton property to button name
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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
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Figure 3-40
Syntax and examples of the
ToString method
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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 form’s AcceptButton property
 ToString method
 Formats number for string output
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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
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Modifying the Load
and Click Event Procedures
 Capability needed when each order is calculated
 Order form should ask for the salesperson’s name
 Revise TOE chart before implementing changes
 Shift task of retrieving name to btnCalc’s Click event
 Use static variable for the salesperson’s name
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Figure 3-45 Revised TOE chart
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Modifying the Load
and Click Event Procedures (cont’d.)
Figure 3-46 Revised Pseudocode for the Calculate Order button
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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 control’s Text property
 Example:
 A change is made to the number of items ordered
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Coding the TextChanged
Event Procedure (cont’d.)
 Associating a procedure with different objects and events
 Handles clause
 Appears in an event procedure’s 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
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Coding the TextChanged
Event Procedure (cont’d.)
Figure 3-48 Completed ClearLabels procedure
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Figure 3-49
Playtime Cellular application’s code at
the end of Lesson C (continues)
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Figure 3-49
Playtime Cellular application’s code
at the end of Lesson C (cont’d.)
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Lesson C Summary
 TextChanged event procedure responds to change in value of
control’s 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 keyword
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