programming with microsoft visual basic 2008 fourth edition chapter three using variables and...

Programming with Microsoft Visual Basic 2008

Fourth Edition

Chapter ThreeUsing 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

Programming with Microsoft Visual Basic 2008, Fourth Edition 2

Previewing the Playtime Cellular Application (continued)


Figure 3-1: Name Entry dialog box

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


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


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


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


Selecting a Data Type for a Variable (continued)


Figure 3-3: Basic data types in Visual Basic



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


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


Selecting a Name for a Variable (continued)


Figure 3-4: Variable naming rules and examples

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


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


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



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

The TryParse Method (continued)

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



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

The Convert Class (continued)

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



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

The Scope and Lifetime of a Variable (continued)


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

The Scope and Lifetime of a Variable (continued)

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



Figure 3-14: Code using a static variable

Static Variables (continued)

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



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

Named Constants (continued)

Named Constants (continued)


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

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



Figure 3-18: Rules and examples of type conversions

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




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

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


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


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


Modifying the Playtime Cellular Application (continued)


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

Modifying the Playtime Cellular Application (continued)


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

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


Modifying the Calculate Order Button’s Code (continued)


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



Figure 3-23: Lines to delete from the procedure



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



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



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

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



Figure 3-29: Examples of string concatenation

Concatenating Strings (continued)

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


The InputBox Function (continued)


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


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



Figure 3-35: frmMain Load event procedure


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


The ControlChars.Newline Constant (continued)


Figure 3-37: Modified assignment statement

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


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



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

Using the ToString Method to Format Numbers (continued)

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


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


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



Figure 3-45: Revised TOE chart


(continued)


(continued)


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

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


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



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


(continued)


Figure 3-49: Completed ClearLabels procedure


(continued)

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 chapter three using variables and...

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