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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. Declare variables and named constants Assign data to an existing variable

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Chapter 3: Using Variables and Constants

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  1. Chapter 3: Using Variables and Constants Programming with Microsoft Visual Basic 2005, Third Edition

  2. 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

  3. 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

  4. 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

  5. Previewing the Completed Application (continued) Figure 3-1: Name Entry dialog box Programming with Microsoft Visual Basic 2005, Third Edition

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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

  13. Assigning Data to an Existing Variable (continued) Figure 3-7: Literal type characters Programming with Microsoft Visual Basic 2005, Third Edition

  14. 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

  15. 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

  16. 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

  17. 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

  18. 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

  19. 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

  20. 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

  21. 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

  22. Option Explicit and Option Strict (continued) Figure 3-19: Rules and examples of implicit type conversions Programming with Microsoft Visual Basic 2005, Third Edition

  23. 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

  24. 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

  25. 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

  26. 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

  27. 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

  28. 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

  29. 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

  30. 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

  31. 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

  32. Concatenating Strings (continued) Figure 3-30: Examples of string concatenation Programming with Microsoft Visual Basic 2005, Third Edition

  33. 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

  34. 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

  35. The InputBox Function (continued) Figure 3-36: MainForm’s Load event procedure Programming with Microsoft Visual Basic 2005, Third Edition

  36. 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

  37. The Controlchars.Newline Constant (continued) Figure 3-39: ControlChars.NewLine constant added to the assignment statement Programming with Microsoft Visual Basic 2005, Third Edition

  38. 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

  39. 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

  40. 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

  41. 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

  42. 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

  43. 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

  44. 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

  45. 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

  46. 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

  47. 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

  48. Associating a Procedure with Different Objects and Events (continued) Figure 3-56: Completed ClearControls procedure Programming with Microsoft Visual Basic 2005, Third Edition

  49. 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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