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Programming with Microsoft Visual Basic 2008 Fourth Edition

Programming with Microsoft Visual Basic 2008 Fourth Edition Chapter Three Using Variables and Constants Previewing the Playtime Cellular Application Previewing the Playtime Cellular application Access Run command on Start menu Browse to VB2008Chap03 folder

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Programming with Microsoft Visual Basic 2008 Fourth Edition

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  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 Edition

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

  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 Edition

  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 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: • 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 Edition

  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 Edition

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

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

  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 Edition

  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 Edition

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

  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 Edition

  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 Edition

  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 Edition

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

  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 Edition

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

  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 Edition

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

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

  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 Edition

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

  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 Edition

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

  26. Named Constants (continued) Figure 3-17: Calculate Area button’s Click event procedure Programming with Microsoft Visual Basic 2008, Fourth Edition

  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 Edition

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

  29. Option Explicit, Option Infer, and Option Strict (continued) • 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 Edition

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

  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 Edition

  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 Edition

  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 Edition

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

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

  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 Edition

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

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

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

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

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

  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 Edition

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

  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 Edition

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

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

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

  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 Edition

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

  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 Edition

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