1. Programming with Microsoft Visual Basic 2008 Fourth Edition Chapter Three Using Variables and Constants

2. Previewing the Playtime Cellular Application Previewing the Playtime Cellular application –Access Run command on Start menu –Browse to VB2008\Chap03 folder –Click the Playtime Cellular (Playtime Cellular.exe) file –View completed order form Completed application resembles Chapter 2 version Programming with Microsoft Visual Basic 2008, Fourth Edition2

3. Previewing the Playtime Cellular Application (continued) Programming with Microsoft Visual Basic 2008, Fourth Edition3 Figure 3-1: Name Entry dialog box

4. Lesson A Objectives After 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 Programming with Microsoft Visual Basic 2008, Fourth Edition4

5. Lesson A Objectives (continued) Explain the scope and lifetime of variables and named constants Explain the purpose of the Option Explicit, Option Infer, and Option Strict Programming with Microsoft Visual Basic 2008, Fourth Edition5

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: –Hold information that is not stored in control on form –Allow for more precise treatment of numeric data –Enable code to run more efficiently Programming with Microsoft Visual Basic 2008, Fourth Edition6

7. 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 Programming with Microsoft Visual Basic 2008, Fourth Edition7

8. Selecting a Data Type for a Variable (continued) Programming with Microsoft Visual Basic 2008, Fourth Edition8 Figure 3-3: Basic data types in Visual Basic

9. Selecting a Data Type for a Variable (continued) Programming with Microsoft Visual Basic 2008, Fourth Edition9 Figure 3-3: Basic data types in Visual Basic (continued)

10. Selecting a Data Type for a Variable (continued) For this course: –Use Integer data type for all integers –Use either Decimal or Double data type of 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 Programming with Microsoft Visual Basic 2008, Fourth Edition10

11. 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 Programming with Microsoft Visual Basic 2008, Fourth Edition11

12. Selecting a Name for a Variable (continued) Programming with Microsoft Visual Basic 2008, Fourth Edition12 Figure 3-4: Variable naming rules and examples

13. Declaring a Variable Declaration statement: Used to declare (create) a variable and reserves space in memory for it Syntax: {Dim|Private|Static} variablename [As datatype][= initialvalue] If no initial value is given to variable when declaring it, computer stores default value –Numeric data types are set to 0 –Boolean data types are set to False –Object and String data types are set to Nothing Programming with Microsoft Visual Basic 2008, Fourth Edition13

14. Assigning Data to an Existing Variable Assignment statement: Assigns value to variable at runtime Syntax: variablename = expression –Expression may contain literal constants, properties of controls, variables, or arithmetic operators Literal constant: Data item whose value does not change –Example: The string “Mary” Literal type character: Changes type of a literal –Example: sales=2356D Integer cast to Decimal Programming with Microsoft Visual Basic 2008, Fourth Edition14

15. The TryParse Method TryParse method: Converts string to number Syntax: dataType.TryParse(string, variable) –dataType : Numeric data type, such as Integer –string : String to be converted –variable : Variable that receives the numeric value TryParse is preferred over Val –Val only returns a type Double value Programming with Microsoft Visual Basic 2008, Fourth Edition15

16. Programming with Microsoft Visual Basic 2008, Fourth Edition16 Figure 3-8: Results of the TryParse method for the Double, Decimal, and Integer data types The TryParse Method (continued)

17. The Convert Class Convert class: Can be used to convert a number from one type to another Syntax: Convert.method(value) – 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 Programming with Microsoft Visual Basic 2008, Fourth Edition17

18. Programming with Microsoft Visual Basic 2008, Fourth Edition18 Figure 3-9: Syntax and examples of the Convert class methods The Convert Class (continued)

19. The Scope and Lifetime of a Variable Scope: Indicates where variable can be used Lifetime: Indicates how long variable remains in memory and can be used Scope and lifetime are determined by where variable is declared Three types of scope: –Module: 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 Programming with Microsoft Visual Basic 2008, Fourth Edition19

20. Programming with Microsoft Visual Basic 2008, Fourth Edition20 Figure 3-11: Click event procedure using procedure-level variables The Scope and Lifetime of a Variable (continued)

21. Programming with Microsoft Visual Basic 2008, Fourth Edition21 Figure 3-13: Code using a module-level variable The Scope and Lifetime of a Variable (continued)

22. 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 module-level variables, but have narrower scope –Can only be used within procedure where declared Programming with Microsoft Visual Basic 2008, Fourth Edition22

23. Programming with Microsoft Visual Basic 2008, Fourth Edition23 Figure 3-14: Code using a static variable Static Variables (continued)

24. Named Constants Named constant: Memory location inside computer whose contents cannot be changed at runtime Const statement: Creates named constant Syntax: Const constantname As datatype = expression expression : Can be literal constant, another named constant, or an arithmetic operator –Cannot contain a variable Programming with Microsoft Visual Basic 2008, Fourth Edition24

25. Programming with Microsoft Visual Basic 2008, Fourth Edition25 Figure 3-15: Syntax and examples of the Const statement Named Constants (continued)

26. Programming with Microsoft Visual Basic 2008, Fourth Edition26 Figure 3-17: Calculate Area button’s Click event procedure

27. 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: Cata expanded e.g., Integer to Decimal –Demotion: data truncated e.g., Decimal to Integer Data loss can occur when demotion occurs Programming with Microsoft Visual Basic 2008, Fourth Edition27

28. Programming with Microsoft Visual Basic 2008, Fourth Edition28 Figure 3-18: Rules and examples of type conversions Option Explicit, Option Infer, and Option Strict (continued)

29. 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 Programming with Microsoft Visual Basic 2008, Fourth Edition29

30. Option Explicit, Option Infer, and Option Strict (continued) Programming with Microsoft Visual Basic 2008, Fourth Edition30 Figure 3-19: Option statements entered in the General Declarations section

31. Lesson A Summary 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 Use Const to declare a named constant Avoid programming errors by using Option Explicit On, Option Infer Off, and Option Strict On Programming with Microsoft Visual Basic 2008, Fourth Edition31

32. Lesson B Objectives After studying Lesson B, you should be able to: 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 Designate the default button for a form Format numbers using the ToString method Programming with Microsoft Visual Basic 2008, Fourth Edition32

33. Modifying the Playtime Cellular Application Modifications needed: –Display message, sales tax amount, salesperson –Calculate the sales tax Revise the TOE chart to reflect the new tasks Must modify btnCalc button’s Click event and the form’s Load event Programming with Microsoft Visual Basic 2008, Fourth Edition33

34. Modifying the Playtime Cellular Application (continued) Programming with Microsoft Visual Basic 2008, Fourth Edition34 Figure 3-20: Revised TOE chart for the Playtime Cellular application

35. Modifying the Playtime Cellular Application (continued) Programming with Microsoft Visual Basic 2008, Fourth Edition35 Figure 3-20: Revised TOE chart for the Playtime Cellular application (continued)

36. 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 Programming with Microsoft Visual Basic 2008, Fourth Edition36

37. Modifying the Calculate Order Button’s Code (continued) Programming with Microsoft Visual Basic 2008, Fourth Edition37 Figure 3-22: Jagged blue lines indicate the statements contain an error

38. Modifying the Calculate Order Button’s Code (continued) Programming with Microsoft Visual Basic 2008, Fourth Edition38 Figure 3-23: Lines to delete from the procedure

39. Modifying the Calculate Order Button’s Code (continued) Programming with Microsoft Visual Basic 2008, Fourth Edition39 Figure 3-24: Revised pseudocode for the btnCalc control’s Click event procedure

40. Modifying the Calculate Order Button’s Code (continued) Programming with Microsoft Visual Basic 2008, Fourth Edition40 Figure 3-25: Named constants and variables for the btnCalc control’s Click event procedure

41. Modifying the Calculate Order Button’s Code (continued) Programming with Microsoft Visual Basic 2008, Fourth Edition41 Figure 3-26: Const and Dim statements entered in the procedure

42. 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 Programming with Microsoft Visual Basic 2008, Fourth Edition42

43. Programming with Microsoft Visual Basic 2008, Fourth Edition43 Figure 3-29: Examples of string concatenation Concatenating Strings (continued)

44. The InputBox Function InputBox function: Displays dialog box and retrieves user input Syntax: InputBox(prompt[,title] [,defaultResponse]) –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 Arguments are String literals, constants, or variables Programming with Microsoft Visual Basic 2008, Fourth Edition44

45. The InputBox Function (continued) Programming with Microsoft Visual Basic 2008, Fourth Edition45 Figure 3-32: Example of a dialog box created by the InputBox function

46. Programming with Microsoft Visual Basic 2008, Fourth Edition46 Figure 3-34: Module-level variable declared in the form’s Declarations section The InputBox Function (continued)

47. Programming with Microsoft Visual Basic 2008, Fourth Edition47 Figure 3-35: frmMain Load event procedure The InputBox Function (continued)

48. The ControlChars.Newline Constant ControlChars.NewLine constant: Issues carriage return followed by a line feed –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 Programming with Microsoft Visual Basic 2008, Fourth Edition48

49. The ControlChars.Newline Constant (continued) Programming with Microsoft Visual Basic 2008, Fourth Edition49 Figure 3-37: Modified assignment statement

50. 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 desired button to specify the default button Programming with Microsoft Visual Basic 2008, Fourth Edition50

51. 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 Programming with Microsoft Visual Basic 2008, Fourth Edition51

52. Programming with Microsoft Visual Basic 2008, Fourth Edition52 Figure 3-40: Syntax and examples of the ToString method Using the ToString Method to Format Numbers (continued)

53. 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 Programming with Microsoft Visual Basic 2008, Fourth Edition53

54. Lesson C Objectives After 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 Programming with Microsoft Visual Basic 2008, Fourth Edition54

55. 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 Programming with Microsoft Visual Basic 2008, Fourth Edition55

56. Programming with Microsoft Visual Basic 2008, Fourth Edition56 Figure 3-45: Revised TOE chart Modifying the Load and Click Event Procedures (continued)

57. Programming with Microsoft Visual Basic 2008, Fourth Edition57 Figure 3-46: Revised Pseudocode for the Calculate Order button

58. 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 Programming with Microsoft Visual Basic 2008, Fourth Edition58

59. Associating a Procedure with Different Objects and Events Handles keyword: 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 Programming with Microsoft Visual Basic 2008, Fourth Edition59

60. Programming with Microsoft Visual Basic 2008, Fourth Edition60 Figure 3-48: txtBlue control’s TextChanged event procedure Associating a Procedure with Different Objects and Events (continued)

61. Programming with Microsoft Visual Basic 2008, Fourth Edition61 Figure 3-49: Completed ClearLabels procedure Associating a Procedure with Different Objects and Events (continued)

62. 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 Programming with Microsoft Visual Basic 2008, Fourth Edition62