Transcript Document 7174817

Programming with
Microsoft Visual Basic 2008
Fourth Edition
Chapter Three
Using Variables and Constants
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
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Previewing the Playtime Cellular
Application (continued)
Figure 3-1: Name Entry dialog box
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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 (continued)
• 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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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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Selecting a Data Type for a Variable
(continued)
Figure 3-3: Basic data types in Visual Basic
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Selecting a Data Type for a Variable
(continued)
Figure 3-3: Basic data types in Visual Basic (continued)
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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
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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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Selecting a Name for a Variable
(continued)
Figure 3-4: Variable naming rules and examples
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Declaring a Variable
• Declaration statement: Used to declare (create)
a variable and reserves space in memory for it
• Syntax:
{Dim|Private|Static} 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
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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: sales=2356D
• Integer cast to Decimal
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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
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The TryParse Method (continued)
Figure 3-8: Results of the TryParse method for the Double, Decimal, and
Integer data types
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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
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The Convert Class (continued)
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: 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
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The Scope and Lifetime of a Variable
(continued)
Figure 3-11: Click event procedure using procedure-level variables
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The Scope and Lifetime of a Variable
(continued)
Figure 3-13: Code using a module-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 module-level variables,
but have narrower scope
– Can only be used within procedure where
declared
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Static Variables (continued)
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 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
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Named Constants (continued)
Figure 3-15: Syntax and examples of the Const statement
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Named Constants (continued)
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: Cata expanded
• 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 (continued)
Figure 3-18: Rules and examples of type conversions
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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
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Option Explicit, Option Infer, and
Option Strict (continued)
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, 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
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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
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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 button’s Click event and the
form’s Load event
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Modifying the Playtime Cellular
Application (continued)
Figure 3-20: Revised TOE chart for the Playtime Cellular application
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Modifying the Playtime Cellular
Application (continued)
Figure 3-20: Revised TOE chart for the Playtime Cellular application
(continued)
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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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Modifying the Calculate Order
Button’s Code (continued)
Figure 3-22: Jagged blue lines indicate the statements contain an error
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Modifying the Calculate Order
Button’s Code (continued)
Figure 3-23: Lines to delete from the procedure
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Modifying the Calculate Order
Button’s Code (continued)
Figure 3-24: Revised pseudocode for the btnCalc control’s Click event
procedure
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Modifying the Calculate Order
Button’s Code (continued)
Figure 3-25: Named constants and variables for the btnCalc control’s Click
event procedure
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Modifying the Calculate Order
Button’s Code (continued)
Figure 3-26: Const and Dim statements entered in the 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 (continued)
Figure 3-29: Examples of string concatenation
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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 box’s title bar
– defaultResponse: Text to be displayed in the
input field
• Arguments are String literals, constants, or
variables
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The InputBox Function (continued)
Figure 3-32: Example of a dialog box created by the InputBox function
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The InputBox Function (continued)
Figure 3-34: Module-level variable declared in the form’s Declarations section
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The InputBox Function (continued)
Figure 3-35: frmMain Load event procedure
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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
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The ControlChars.Newline Constant
(continued)
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 desired
button to specify the default button
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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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Using the ToString Method to Format
Numbers (continued)
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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Modifying the Load
and Click Event Procedures
(continued)
Figure 3-45: Revised TOE chart
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Modifying the Load
and Click Event Procedures
(continued)
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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Associating a Procedure
with Different Objects and Events
• Handles keyword: 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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Associating a Procedure
with Different Objects and Events
(continued)
Figure 3-48: txtBlue control’s TextChanged event procedure
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Associating a Procedure
with Different Objects and Events
(continued)
Figure 3-49: Completed ClearLabels procedure
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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
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