chapter 3: using variables and constants programming with microsoft visual basic 2005, third edition

Chapter 3: Using Variables and Constants

Programming with Microsoft Visual Basic 2005, Third Edition

2Programming with Microsoft Visual Basic 2005, Third Edition

Creating Variables and Named Constants

Lesson A Objectives

• 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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Creating Variables and Named Constants

Lesson A Objectives (continued)

• Explain the scope and lifetime of variables and named constants

• Explain the purpose of the Option Explicit and Option Strict statements

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Previewing the Completed Application

• Previewing the Skate-Away Sales application

– Access the Run command on the Start menu

– Browse to the VB2005\Chap03 folder

– Open the SkateAway (SkateAway.exe) file

– View the completed order form

• Completed application resembles Chapter 2 version

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Previewing the Completed Application (continued)

Figure 3-1: Name Entry dialog box

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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

– Allows for more precise treatment of numeric data

– Enables code to run more efficiently

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Selecting a Data Type for a Variable

• Data type– Specifies type of data variable can store– Provides a class template for creating variables

• Integer variables: Integer, Long, Short

• Floating-point number– Expressed as a power of 10– Written in E (exponential) notation; e.g., 3.2E6

• Floating-point variables: Single, Double

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Selecting a Data Type for a Variable (continued)

• Fixed decimal point variable: Decimal

• Character variable: Char

• Text variable: String

• Boolean variables: True, False

• The Object variable– Default data type assigned by Visual Basic– Can store many different types of data– Less efficient than other data types

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Selecting a Name for a Variable

• Variables are referred to by name

• Identifier: another term for a variable name

• Basic guidelines for naming variables

– Name should be descriptive; e.g., length and width

– Enter the name in camel case; e.g., salesAmount

• Certain rules must be followed

– Example: a name begins with a letter or underscore

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Selecting a Name for a Variable (continued)

Figure 3-4: Rules for variable names along with examples of valid and invalid names

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Declaring a Variable

• Declaration statement

– Used to declare, or create, a variable

• Syntax: {Dim | Private | Static} variablename [As datatype][= initialvalue]

• Examples

– Dim hoursWorked As Double ‘note: no initial value

– Dim isDataOk As Boolean = True ‘ variable initialized

– Dim message As String = “Good Morning”

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Assigning Data to an Existing Variable

• Assignment statement

– Assigns a value to a variable at runtime

• Syntax: variablename = value

– Example: quantityOrdered = 500

• Literal constant: data item that does not change

– Example: the string “Mary”

• Literal type character: changes type of a literal

– Example: sales = 2356R ‘ integer cast to Double

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Assigning Data to an Existing Variable (continued)

Figure 3-7: Literal type characters

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The TryParse Method

• Syntax: dataType.TryParse(string, variable)

– dataType: numeric data type, such as Integer

– TryParse method is a member of dataType class

– string argument: string to convert to a number

– variable argument: names numeric storage unit

• Example – Dim sales As Decimal

Decimal.TryParse(Me.xSalesTextBox.Text, sales)

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The Convert Class

• Syntax: Convert.method(value)

– Convert: the name of the class

– method: converts value to specified data type

– value: numeric data to be converted

• Example– Dim sales As Integer = 4500 Dim

newSales As Double newSales = Convert.ToDouble(sales)

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Using a Variable in an Arithmetic Expression

• Data stored in variables can be used in calculations

• Example 1– Dim age As Integer ‘ Dim allocates memory for age

age = age + 1 ‘ A new value is assigned

• Example 2– Dim totalAmountDue As Double = 250.55

Me.xTotalLabel.Text = _ Convert.ToString(totalAmountDue)

– Line continuation character: underscore in line 2

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The Scope and Lifetime of a Variable

• Scope: indicates where a variable can be used

• Lifetime: indicates how long a variable can be used

• Scope and lifetime determined by declaration site

• Three types of scope

– Block: variable used within a specific code block

– Procedure: variable only used within a procedure

– Module: variable used by all procedures in a form

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The Scope and Lifetime of a Variable (continued)

Figure 3-14: Total Sales application’s 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– Declare a variable using the Static keyword

• Example: Static totalSales As Decimal– Value in totalSales persists between calls– During a current call, value may be altered

• Static variables act like module-level variables– Difference: static variable has narrower scope

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Named Constants

• Named constant

– Memory location inside the computer

– Contents cannot be changed at runtime

• Const statement: creates a named constant

• Syntax: Const constantname As datatype = expression

• Example: Const PI As Double = 3.141593

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Option Explicit and Option Strict

• Option Explicit On statement

– Prevents you from using undeclared variables

• Implicit type conversion

– Converts right-side value to datatype of left side

– Promotion: data expanded; e.g., Integer to Decimal

– Demotion: data truncated; e.g., Decimal to Integer

• Option Strict On statement

– Suppresses implicit conversions

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Option Explicit and Option Strict (continued)

Figure 3-19: Rules and examples of implicit type conversions

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Option Explicit and Option Strict (continued)

Figure 3-20: Option statements entered in the General Declarations section

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Summary – Lesson A

• 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

• Avoid programming errors by using Option Explicit On and Option Strict On

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Modifying the Skate-Away Sales Application

Lesson B Objectives

• 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

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Modifying the Skate-Away Sales Application

Lesson B Objectives (continued)

• Designate the default button for a form

• Format numbers using the ToString method

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Revising the Application’s Documents

• Modifications needed

– Display message, sales tax amount, salesperson

– Calculate the sales tax

• Revise TOE chart to reflect new tasks

• Three controls are impacted

– xCalcButton, MainForm, xMessageLabel

• Modify button’s Click event and form’s Load event

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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 Strict On statement

– Suppresses implicit type conversions

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Modifying the Calculate Order Button’s Code (continued)

Figure 3-25: Revised pseudocode for the xCalcButton’s Click event procedure

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Modifying the Calculate Order Button’s Code (continued)

Figure 3-29: Calculated amounts shown in the interface

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Concatenating Strings

• Concatenate: connect strings together

• Concatenation operator: the ampersand (&)

• Include a space before and after the & operator

• Numbers after & operator are converted to strings

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Concatenating Strings (continued)

Figure 3-30: Examples of string concatenation

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The InputBox Function

• InputBox function

– Displays a dialog box and retrieves user input

• Syntax: InputBox(prompt[, title][, defaultResponse])

– prompt: the message to display inside dialog box

– title: text to display in the dialog box’s title bar

– defaultResponse: text you want displayed

• Arguments are String literals, constants, or variables

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The InputBox Function (continued)

Figure 3-33: Example of a dialog box created by the InputBox function

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The InputBox Function (continued)

Figure 3-36: MainForm’s Load event procedure

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The Controlchars.Newline Constant

• Issues a carriage return followed by a line feed

• Using the ControlChars.NewLine constant

– Type ControlChars.NewLine at appropriate location

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The Controlchars.Newline Constant (continued)

Figure 3-39: ControlChars.NewLine constant added to the assignment statement

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Designating a Default Button

• Default button

– Can be selected by pressing the Enter key

– Button is not required to have the focus

• The default button is typically the first button

• Button’s deleting data should not be made default

• Specifying the default button (if any)

– Set form’s AcceptButton property to desired button

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Using the ToString Method to Format Numbers

• Formatting– Specifying decimal places and special characters

• ToString method is replacing the Format function

• Syntax: variablename.ToString(formatString)– variablename: name of a numeric variable– formatString: string specifying format you want to use

• Form Axx consists of a format and precision specifier

• Example: C2 formatString converts 75.312 to $75.31

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Using the ToString Method to Format Numbers (continued)

Figure 3-46: Order form showing the formatted total price

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Summary – Lesson B

• Concatenation operator (&): used to link strings

• InputBox function: displays interactive dialog box

• Use ControlChars.NewLine to go to a new line

• Set default button in form’s AcceptButton property

• ToString method: formats number for string output

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Modifying the Skate-Away Sales Application’s Code

Lesson C Objectives• 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 Code in the MainForm’s Load and xCalcButton Click

Procedures

• Capability needed when each order is calculated

– Order form to ask for the salesperson’s name

• Revise TOE chart before implementing changes

• Objects impacted: xCalcButton and MainForm

• Shift task of retrieving name to xCalcButton

• Use a static variable to store salesperson’s name

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Modifying the Code in the MainForm’s Load and xCalcButton Click

Procedures (continued)

Figure 3-51: Revised pseudocode for the Calculate Order button

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Using a Static Variable

• Static variable

– Retains its value between procedure calls

– Like a module-level variable with reduced scope

• Syntax

– Static variablename [As datatype] [= initialvalue]

• Example of declaring a static variable

– Static salesPerson As String = String.Empty

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Coding the TextChanged Event Procedure

• Control’s TextChanged event

– Occurs when the Text property value changes

• Triggering events

– 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

• The keyword Handles

– Appears in a procedure header

– Indicates object and event associated with procedure

• Procedures can relate to multiple objects and events

• Associating procedures with extra objects and events

– Go to the Handles section of the procedure header

– List each object and event, separated by commas

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Associating a Procedure with Different Objects and Events

(continued)

Figure 3-56: Completed ClearControls procedure

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Summary – Lesson C

• Static variables retain their value between calls

• TextChanged event procedure responds to change in value of control’s Text Property

• Handles clause determines when TextChanged event procedure is invoked

• To create a procedure for more than one object or event, list each object and event after Handles