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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
  • 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 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 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 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
  • 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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