Programming with microsoft visual basic 2008 fourth edition
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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

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Programming with Microsoft Visual Basic 2008 Fourth Edition

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Programming with microsoft visual basic 2008 fourth edition

Programming with Microsoft Visual Basic 2008 Fourth Edition

Chapter Three

Using Variables and Constants


Previewing the playtime cellular application

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

Programming with Microsoft Visual Basic 2008, Fourth Edition


Previewing the playtime cellular application continued

Previewing the Playtime Cellular Application (continued)

Figure 3-1: Name Entry dialog box

Programming with Microsoft Visual Basic 2008, Fourth Edition


Lesson a objectives

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

Programming with Microsoft Visual Basic 2008, Fourth Edition


Lesson a objectives continued

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

Programming with Microsoft Visual Basic 2008, Fourth Edition


Using variables to store information

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

Programming with Microsoft Visual Basic 2008, Fourth Edition


Selecting a data type for a variable

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

Programming with Microsoft Visual Basic 2008, Fourth Edition


Selecting a data type for a variable continued

Selecting a Data Type for a Variable (continued)

Figure 3-3: Basic data types in Visual Basic

Programming with Microsoft Visual Basic 2008, Fourth Edition


Selecting a data type for a variable continued1

Selecting a Data Type for a Variable (continued)

Figure 3-3: Basic data types in Visual Basic (continued)

Programming with Microsoft Visual Basic 2008, Fourth Edition


Selecting a data type for a variable continued2

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

Programming with Microsoft Visual Basic 2008, Fourth Edition


Selecting a name for a variable

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

Programming with Microsoft Visual Basic 2008, Fourth Edition


Selecting a name for a variable continued

Selecting a Name for a Variable (continued)

Figure 3-4: Variable naming rules and examples

Programming with Microsoft Visual Basic 2008, Fourth Edition


Declaring a variable

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

Programming with Microsoft Visual Basic 2008, Fourth Edition


Assigning data to an existing variable

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

Programming with Microsoft Visual Basic 2008, Fourth Edition


The tryparse method

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

Programming with Microsoft Visual Basic 2008, Fourth Edition


The tryparse method continued

The TryParse Method (continued)

Figure 3-8: Results of the TryParse method for the Double, Decimal, and Integer data types

Programming with Microsoft Visual Basic 2008, Fourth Edition


The convert class

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

Programming with Microsoft Visual Basic 2008, Fourth Edition


The convert class continued

The Convert Class (continued)

Figure 3-9: Syntax and examples of the Convert class methods

Programming with Microsoft Visual Basic 2008, Fourth Edition


The scope and lifetime of a variable

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

Programming with Microsoft Visual Basic 2008, Fourth Edition


The scope and lifetime of a variable continued

The Scope and Lifetime of a Variable (continued)

Figure 3-11: Click event procedure using procedure-level variables

Programming with Microsoft Visual Basic 2008, Fourth Edition


The scope and lifetime of a variable continued1

The Scope and Lifetime of a Variable (continued)

Figure 3-13: Code using a module-level variable

Programming with Microsoft Visual Basic 2008, Fourth Edition


Static variables

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

Programming with Microsoft Visual Basic 2008, Fourth Edition


Static variables continued

Static Variables (continued)

Figure 3-14: Code using a static variable

Programming with Microsoft Visual Basic 2008, Fourth Edition


Named constants

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

Programming with Microsoft Visual Basic 2008, Fourth Edition


Named constants continued

Named Constants (continued)

Figure 3-15: Syntax and examples of the Const statement

Programming with Microsoft Visual Basic 2008, Fourth Edition


Named constants continued1

Named Constants (continued)

Figure 3-17: Calculate Area button’s Click event procedure

Programming with Microsoft Visual Basic 2008, Fourth Edition


Option explicit option infer and option strict

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

Programming with Microsoft Visual Basic 2008, Fourth Edition


Option explicit option infer and option strict continued

Option Explicit, Option Infer, and Option Strict (continued)

Figure 3-18: Rules and examples of type conversions

Programming with Microsoft Visual Basic 2008, Fourth Edition


Option explicit option infer and option strict continued1

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

Programming with Microsoft Visual Basic 2008, Fourth Edition


Option explicit option infer and option strict continued2

Option Explicit, Option Infer, and Option Strict (continued)

Figure 3-19: Option statements entered in the General Declarations section

Programming with Microsoft Visual Basic 2008, Fourth Edition


Lesson a summary

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

Programming with Microsoft Visual Basic 2008, Fourth Edition


Lesson b objectives

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

Programming with Microsoft Visual Basic 2008, Fourth Edition


Modifying the playtime cellular application

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

Programming with Microsoft Visual Basic 2008, Fourth Edition


Modifying the playtime cellular application continued

Modifying the Playtime Cellular Application (continued)

Figure 3-20: Revised TOE chart for the Playtime Cellular application

Programming with Microsoft Visual Basic 2008, Fourth Edition


Modifying the playtime cellular application continued1

Modifying the Playtime Cellular Application (continued)

Figure 3-20: Revised TOE chart for the Playtime Cellular application (continued)

Programming with Microsoft Visual Basic 2008, Fourth Edition


Modifying the calculate order button s code

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

Programming with Microsoft Visual Basic 2008, Fourth Edition


Modifying the calculate order button s code continued

Modifying the Calculate Order Button’s Code (continued)

Figure 3-22: Jagged blue lines indicate the statements contain an error

Programming with Microsoft Visual Basic 2008, Fourth Edition


Modifying the calculate order button s code continued1

Modifying the Calculate Order Button’s Code (continued)

Figure 3-23: Lines to delete from the procedure

Programming with Microsoft Visual Basic 2008, Fourth Edition


Modifying the calculate order button s code continued2

Modifying the Calculate Order Button’s Code (continued)

Figure 3-24: Revised pseudocode for the btnCalc control’s Click event procedure

Programming with Microsoft Visual Basic 2008, Fourth Edition


Modifying the calculate order button s code continued3

Modifying the Calculate Order Button’s Code (continued)

Figure 3-25: Named constants and variables for the btnCalc control’s Click event procedure

Programming with Microsoft Visual Basic 2008, Fourth Edition


Modifying the calculate order button s code continued4

Modifying the Calculate Order Button’s Code (continued)

Figure 3-26: Const and Dim statements entered in the procedure

Programming with Microsoft Visual Basic 2008, Fourth Edition


Concatenating strings

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

Programming with Microsoft Visual Basic 2008, Fourth Edition


Concatenating strings continued

Concatenating Strings (continued)

Figure 3-29: Examples of string concatenation

Programming with Microsoft Visual Basic 2008, Fourth Edition


The inputbox function

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

Programming with Microsoft Visual Basic 2008, Fourth Edition


The inputbox function continued

The InputBox Function (continued)

Figure 3-32: Example of a dialog box created by the InputBox function

Programming with Microsoft Visual Basic 2008, Fourth Edition


The inputbox function continued1

The InputBox Function (continued)

Figure 3-34: Module-level variable declared in the form’s Declarations section

Programming with Microsoft Visual Basic 2008, Fourth Edition


The inputbox function continued2

The InputBox Function (continued)

Figure 3-35: frmMain Load event procedure

Programming with Microsoft Visual Basic 2008, Fourth Edition


The controlchars newline constant

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

Programming with Microsoft Visual Basic 2008, Fourth Edition


The controlchars newline constant continued

The ControlChars.Newline Constant (continued)

Figure 3-37: Modified assignment statement

Programming with Microsoft Visual Basic 2008, Fourth Edition


Designating a default button

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

Programming with Microsoft Visual Basic 2008, Fourth Edition


Using the tostring method to format numbers

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

Programming with Microsoft Visual Basic 2008, Fourth Edition


Using the tostring method to format numbers continued

Using the ToString Method to Format Numbers (continued)

Figure 3-40: Syntax and examples of the ToString method

Programming with Microsoft Visual Basic 2008, Fourth Edition


Lesson b summary

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

Programming with Microsoft Visual Basic 2008, Fourth Edition


Lesson c objectives

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

Programming with Microsoft Visual Basic 2008, Fourth Edition


Modifying the load and click event procedures

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

Programming with Microsoft Visual Basic 2008, Fourth Edition


Modifying the load and click event procedures continued

Modifying the Load and Click Event Procedures (continued)

Figure 3-45: Revised TOE chart

Programming with Microsoft Visual Basic 2008, Fourth Edition


Modifying the load and click event procedures continued1

Modifying the Load and Click Event Procedures (continued)

Figure 3-46: Revised Pseudocode for the Calculate Order button

Programming with Microsoft Visual Basic 2008, Fourth Edition


Coding the textchanged event procedure

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

Programming with Microsoft Visual Basic 2008, Fourth Edition


Associating a procedure with different objects and events

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

Programming with Microsoft Visual Basic 2008, Fourth Edition


Associating a procedure with different objects and events continued

Associating a Procedure with Different Objects and Events (continued)

Figure 3-48: txtBlue control’s TextChanged event procedure

Programming with Microsoft Visual Basic 2008, Fourth Edition


Associating a procedure with different objects and events continued1

Associating a Procedure with Different Objects and Events (continued)

Figure 3-49: Completed ClearLabels procedure

Programming with Microsoft Visual Basic 2008, Fourth Edition


Lesson c summary

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

Programming with Microsoft Visual Basic 2008, Fourth Edition


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