programming with microsoft visual basic 2010 5 th edition chapter three using variables and...

Programming with Microsoft Visual Basic 2010 5th Edition

CHAPTER THREE

USING VARIABLES AND CONSTANTS

Programming with Microsoft Visual Basic 2010, 5th Edition

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

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Programming with Microsoft Visual Basic 2010, 5th Edition3

Figure 3-2 Completed order form


Lesson A ObjectivesAfter 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



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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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



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


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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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


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


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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Figure 3-10 User interface for the Sales Tax Calculator application


Figure 3-11 Click event

procedures using procedure-level

variables



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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Figure 3-12 User interface for the Total Sales application


Figure 3-13 Code using a class-level variable


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


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


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

Figure 3-16 User interface for the Area Calculator application


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


Option Explicit, Option Infer, and Option Strict (cont’d.)

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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, 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 ObjectivesAfter 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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Modifying Playtime Application

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Modifying Playtime Application

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


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


Modifying the Calculate Order Button’s Code (cont’d.)

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



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Figure 3-25 List of named constants and variables



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


Figure 3-27 Code entered in the btnCalc

control’s Click event 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

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Concatenating Strings (cont’d.)

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Figure 3-29 Examples of string concatenation


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 3-33Basic syntax and examples of

the InputBox function


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.)

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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 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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Printing Euro sign Thread.CurrentThread.CurrentCulture = New CultureInfo("fr-FR")

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Figure 3-40

Syntax and examples of the ToString method


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 ObjectivesAfter 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


Modifying the Load and Click Event Procedures (cont’d.)

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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

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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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Event Procedure (cont’d.)

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Figure 3-48 Completed ClearLabels procedure


Figure 3-49 Playtime Cellular application’s code at

the end of Lesson C (continues)


Figure 3-49 Playtime Cellular application’s code

at the end of Lesson C (cont’d.)


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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