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Chapter 3: Using Variables and Constants. Programming 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

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Chapter 3 using variables and constants

Chapter 3: Using Variables and Constants

Programming with Microsoft Visual Basic 2005, Third Edition


Creating variables and named constants lesson a objectives
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

Programming with Microsoft Visual Basic 2005, Third Edition


Creating variables and named constants lesson a objectives continued
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

Programming with Microsoft Visual Basic 2005, Third Edition


Previewing the completed application
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

Programming with Microsoft Visual Basic 2005, Third Edition


Previewing the completed application continued
Previewing the Completed Application (continued)

Figure 3-1: Name Entry dialog box

Programming with Microsoft Visual Basic 2005, Third 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

    • Allows for more precise treatment of numeric data

    • Enables code to run more efficiently

Programming with Microsoft Visual Basic 2005, Third Edition


Selecting a data type for a variable
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

Programming with Microsoft Visual Basic 2005, Third Edition


Selecting a data type for a variable continued
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

Programming with Microsoft Visual Basic 2005, Third Edition


Selecting a name for a variable
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

Programming with Microsoft Visual Basic 2005, Third Edition


Selecting a name for a variable continued
Selecting a Name for a Variable (continued)

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

Programming with Microsoft Visual Basic 2005, Third Edition


Declaring a variable
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”

Programming with Microsoft Visual Basic 2005, Third Edition


Assigning data to an existing variable
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

Programming with Microsoft Visual Basic 2005, Third Edition


Assigning data to an existing variable continued
Assigning Data to an Existing Variable (continued)

Figure 3-7: Literal type characters

Programming with Microsoft Visual Basic 2005, Third Edition


The tryparse method
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)

Programming with Microsoft Visual Basic 2005, Third Edition


The convert class
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)

Programming with Microsoft Visual Basic 2005, Third Edition


Using a variable in an arithmetic expression
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

Programming with Microsoft Visual Basic 2005, Third Edition


The scope and lifetime of a variable
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

Programming with Microsoft Visual Basic 2005, Third Edition


The scope and lifetime of a variable continued
The Scope and Lifetime of a Variable (continued)

Figure 3-14: Total Sales application’s code using a module-level variable

Programming with Microsoft Visual Basic 2005, Third Edition


Static variables
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

Programming with Microsoft Visual Basic 2005, Third Edition


Named constants
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

Programming with Microsoft Visual Basic 2005, Third Edition


Option explicit and option strict
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

Programming with Microsoft Visual Basic 2005, Third Edition


Option explicit and option strict continued
Option Explicit and Option Strict (continued)

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

Programming with Microsoft Visual Basic 2005, Third Edition


Option explicit and option strict continued1
Option Explicit and Option Strict (continued)

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

Programming with Microsoft Visual Basic 2005, Third Edition


Summary lesson a
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

Programming with Microsoft Visual Basic 2005, Third Edition


Modifying the skate away sales application lesson b objectives
Modifying the Skate-Away Sales ApplicationLesson 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

Programming with Microsoft Visual Basic 2005, Third Edition


Modifying the skate away sales application lesson b objectives continued
Modifying the Skate-Away Sales ApplicationLesson B Objectives (continued)

  • Designate the default button for a form

  • Format numbers using the ToString method

Programming with Microsoft Visual Basic 2005, Third Edition


Revising the application s documents
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

Programming with Microsoft Visual Basic 2005, Third 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 Strict On statement

    • Suppresses implicit type conversions

Programming with Microsoft Visual Basic 2005, Third Edition


Modifying the calculate order button s code continued
Modifying the Calculate Order Button’s Code (continued)

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

Programming with Microsoft Visual Basic 2005, Third Edition


Modifying the calculate order button s code continued1
Modifying the Calculate Order Button’s Code (continued)

Figure 3-29: Calculated amounts shown in the interface

Programming with Microsoft Visual Basic 2005, Third Edition


Concatenating strings
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

Programming with Microsoft Visual Basic 2005, Third Edition


Concatenating strings continued
Concatenating Strings (continued)

Figure 3-30: Examples of string concatenation

Programming with Microsoft Visual Basic 2005, Third Edition


The inputbox function
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

Programming with Microsoft Visual Basic 2005, Third Edition


The inputbox function continued
The InputBox Function (continued)

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

Programming with Microsoft Visual Basic 2005, Third Edition


The inputbox function continued1
The InputBox Function (continued)

Figure 3-36: MainForm’s Load event procedure

Programming with Microsoft Visual Basic 2005, Third Edition


The controlchars newline constant
The Controlchars.Newline Constant

  • Issues a carriage return followed by a line feed

  • Using the ControlChars.NewLine constant

    • Type ControlChars.NewLine at appropriate location

Programming with Microsoft Visual Basic 2005, Third Edition


The controlchars newline constant continued
The Controlchars.Newline Constant (continued)

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

Programming with Microsoft Visual Basic 2005, Third Edition


Designating a default button
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

Programming with Microsoft Visual Basic 2005, Third Edition


Using the tostring method to format numbers
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

Programming with Microsoft Visual Basic 2005, Third Edition


Using the tostring method to format numbers continued
Using the ToString Method to Format Numbers (continued)

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

Programming with Microsoft Visual Basic 2005, Third Edition


Summary lesson b
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

Programming with Microsoft Visual Basic 2005, Third Edition


Modifying the skate away sales application s code lesson c objectives
Modifying the Skate-Away Sales Application’s CodeLesson C Objectives

  • 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 2005, Third Edition


Modifying the code in the mainform s load and xcalcbutton click procedures
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

Programming with Microsoft Visual Basic 2005, Third Edition


Modifying the code in the mainform s load and xcalcbutton click procedures continued
Modifying the Code in the MainForm’s Load and xCalcButton Click Procedures (continued)

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

Programming with Microsoft Visual Basic 2005, Third Edition


Using a static variable
Using a Static Variable Click Procedures (continued)

  • 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

Programming with Microsoft Visual Basic 2005, Third Edition


Coding the textchanged event procedure
Coding the TextChanged Click Procedures (continued)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

Programming with Microsoft Visual Basic 2005, Third Edition


Associating a procedure with different objects and events
Associating a Procedure Click Procedures (continued)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

Programming with Microsoft Visual Basic 2005, Third Edition


Associating a procedure with different objects and events continued
Associating a Procedure Click Procedures (continued)with Different Objects and Events (continued)

Figure 3-56: Completed ClearControls procedure

Programming with Microsoft Visual Basic 2005, Third Edition


Summary lesson c
Summary – Lesson C Click Procedures (continued)

  • 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

Programming with Microsoft Visual Basic 2005, Third Edition


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