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Chapter 3: Using Variables and Constants

Chapter 3: Using Variables and Constants. Previewing the Modified Play It Again Movies Application. Play It Again Movies application New screen for salesclerk’s name Sales tax added to order. Figure 3-1 Name Entry dialog box. Figure 3-2 Completed sales receipt. Lesson A Objectives.

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Chapter 3: Using Variables and Constants

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  1. Chapter 3: Using Variables and Constants

  2. Previewing the Modified Play It Again Movies Application • Play It Again Movies application • New screen for salesclerk’s name • Sales tax added to order Figure 3-1 Name Entry dialog box Figure 3-2 Completed sales receipt Programming with Microsoft Visual Basic 2012

  3. 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 • Explain the scope and lifetime of variables and named constants • Explain the purpose of Option Explicit, Option Infer, and Option Strict Programming with Microsoft Visual Basic 2012

  4. Using Variables to Store Information • Controls and variables temporarily store data • Variable • A temporary storage location in main memory • Specified by data type, name, scope, and lifetime • Reasons to use variables • To hold information that is not stored in a control on the form • To allow for more precise treatment of numeric data • To enable code to run more efficiently Programming with Microsoft Visual Basic 2012

  5. Selecting a Data Type for a Variable Data type Specifies the type of data a variable can store Provides a class template for creating variables Unicode A universal coding scheme for characters Assigns a unique numeric value to each character in the written languages of the world Using Variables to Store Information (cont.) Programming with Microsoft Visual Basic 2012

  6. Using Variables to Store Information (cont.) Figure 3-3 Basic data types in Visual Basic Programming with Microsoft Visual Basic 2012

  7. Using Variables to Store Information (cont.) • The textbook uses: • The Integer data type for all integers • Either the Decimal or Double data type for numbers containing decimal places or numbers used in calculations • The String data type for text or numbers not used in calculations • The Boolean data type for Boolean values Programming with Microsoft Visual Basic 2012

  8. Using Variables to Store Information (cont.) Selecting a Name for a Variable • Names must beginwith a letter or underscore • Names can contain only letters, numbers, or underscores • No punctuation, special characters, or spaces are allowed • The recommended length for a name variable is 32 characters • Variable names cannot be reserved words (such as Sub or Double) Figure 3-4 Three-character IDs and examples Programming with Microsoft Visual Basic 2012

  9. Using Variables to Store Information (cont.) • Valid names: • intFeb_Income • decSales2014 • dblEastRegion • strName • Invalid Names: • 4thQuarter • The name must begin with a letter or underscore • dblWestRegion • The name cannot contain a space • strFirst.Name • The name cannot contain punctuation • decSales$East • The name cannot contain a special character Programming with Microsoft Visual Basic 2012

  10. Using Variables to Store Information (cont.) Declaring a Variable • Declaration statement • Used to declare (create) a variable and reserve space in memory for it • If no initial value is given to a variable when declaring it, the computer stores a default value • Numeric variables are set to 0 • Boolean variables are set to False • Object and String variables are set to Nothing • Date variables are set to 1/1/0001 12:00:00AM Programming with Microsoft Visual Basic 2012

  11. Using Variables to Store Information (cont.) Figure 3-6 Syntax and examples of a variable declaration statement Programming with Microsoft Visual Basic 2012

  12. Assigning Data to an Existing Variable • Assignment statement • Assigns a value to a variable at run time • Syntax: variablename = expression • An expression may include literal constants, object properties, variables, keywords, and arithmetic operators • Literal constant • A data item whose value does not change while the application is running • Example: The string “Mary” • Literal type character • Forces a literal constant to change its data type Programming with Microsoft Visual Basic 2012

  13. Assigning Data to an Existing Variable (cont.) Figure 3-7 Syntax and examples of assigning a value to a variable during run time Programming with Microsoft Visual Basic 2012

  14. Assigning Data to an Existing Variable (cont.) The TryParse Method • Converts a string to a number • Is preferred over Val • Allows the programmer to specify the data type • Val only returns a Double number • Arguments • dataType:A numeric data type, such as Integer • String: A string to be converted • Variable:A variable that receives the numeric value Programming with Microsoft Visual Basic 2012

  15. Assigning Data to an Existing Variable (cont.) Figure 3-9 Basic syntax and examples of the TryParse method Programming with Microsoft Visual Basic 2012

  16. Assigning Data to an Existing Variable (cont.) Figure 3-10 Results of the TryParse method for the Double, Decimal and Integer data types Programming with Microsoft Visual Basic 2012

  17. Assigning Data to an Existing Variable (cont.) The Convert Class • Can be used to convert a number from one type to another • Methods include ToDecimal, ToDouble, ToInt32, and ToString • TryParseis recommended for converting strings to numeric data types • Will not produce an error if the conversion fails Programming with Microsoft Visual Basic 2012

  18. Assigning Data to an Existing Variable (cont.) Figure 3-11 Syntax and examples of the Convert class methods Programming with Microsoft Visual Basic 2012

  19. The Scope and Lifetime of a Variable • Scope • Indicates where a variable can be used • Lifetime • How long a variable remains in memory • Scope and lifetime are determined by where a variable is declared: either the General Declarations section or the form’s Declaration section • Three types of scope: • Class: The variable can be used by all procedures in a form • Procedure: The variable can be used within a procedure • Block: The variable can be used within a specific code block Programming with Microsoft Visual Basic 2012

  20. The Scope and Lifetime of a Variable (cont.) • Variables with Procedure Scope • Can be used only by that procedure • Declared at the beginning of the procedure • Removed from memory when the procedure ends • Declared using the Dimkeyword • Most variables used in this course will be procedure-level variables Figure 3-12 User Interface for the Discount Calculator application Programming with Microsoft Visual Basic 2012

  21. The Scope and Lifetime of a Variable (cont.) Figure 3-13 Click event procedures using procedure-level variables Programming with Microsoft Visual Basic 2012

  22. The Scope and Lifetime of a Variable (cont.) Figure 3-14 Code Editor window for the Discount Calculator application Programming with Microsoft Visual Basic 2012

  23. The Scope and Lifetime of a Variable (cont.) • Variables with Class Scope • Can be used by all procedures in the form • Declared in the form’s Declarations section • Will remain in memory until the application ends • Declared using the Privatekeyword Figure 3-16 User Interface for the Total Scores application Programming with Microsoft Visual Basic 2012

  24. The Scope and Lifetime of a Variable (cont.) Figure 3-17 Total Scores application code using a class-level variable Programming with Microsoft Visual Basic 2012

  25. Static Variables • Static variable • A procedure-level variable with an extended lifetime • Remains in memory between procedure calls • Retains its value even when the procedure ends • Statickeyword • Used to declare a static variable • Static variables act like class-level variables but have narrower scope • They can only be used by the procedure in which they are declared Programming with Microsoft Visual Basic 2012

  26. Static Variables (cont.) Figure 3-19 Total Scores application’s code using a static variable Programming with Microsoft Visual Basic 2012

  27. Named Constants • Named constant • A memory location inside the computer whose contents cannot be changed at run time • Conststatement • Creates named constant • Stores value of expression in a named constant • expression: Can be a literal constant, another named constant, or an arithmetic operator • Cannot contain a variable or method Programming with Microsoft Visual Basic 2012

  28. Named Constants (cont.) Figure 3-20 Syntax and examples of the Const statement Programming with Microsoft Visual Basic 2012

  29. Named Constants (cont.) Figure 3-21 User interface for the Area Calculator application Figure 3-22 Calculate Area button’s Click event procedure Programming with Microsoft Visual Basic 2012

  30. Option Statements Option Explicit and Option Infer • Prevent you from using undeclared variables Option Strict • Implicit type conversion • Converts the right-side value to the data type on the left side • Promotion • Data is converted to a greater precision number (e.g., Integer to Decimal) • Demotion • Data is truncated (e.g., Decimal to Integer) • Data loss can occur when demotion occurs • Infer • Ensures that every variable is declared with a data type Programming with Microsoft Visual Basic 2012

  31. Option Statements (cont.) Option Strict (cont.) • Disallows implicit conversions • Type conversion rules are applied when this option is on Figure 3-24 Rules and examples of type conversions Programming with Microsoft Visual Basic 2012

  32. Option Statements (cont.) Figure 3-25 Option statements entered in the General Declarations section Programming with Microsoft Visual Basic 2012

  33. Lesson A Summary • Declare a variable using {Dim | Private | Static} • An assignment statement assigns a value to a variable • Three levels of scope: block, procedure, class • The TryParse method 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 2012

  34. Lesson B Objectives After studying Lesson B, you should be able to: • Include procedure-level and class-level variables 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 2012

  35. Modifying the Play It Again Movies Application • Modifications needed • Calculate and display • the sales tax • Display salesperson’s name • Revise the TOE chart to reflect the new tasks • You must modify btnCalc button’s Click event and the form’s Load event Figure 3-26 Revised TOE chart for the Play It Again Movies application Programming with Microsoft Visual Basic 2012

  36. Modifying the Play It Again Movies Application (cont.) Figure 3-27 Modified user interface for the Play It Again Movies application Programming with Microsoft Visual Basic 2012

  37. Modifying the Calculate Button’s Code • General strategy • Remove the existing code from the Click event procedure • Recode the procedure using variables in equations • Use the Option Explicit On statement • Enforces full variable declaration • Use the Option Infer Off statement • Ensures that variables are declared with data types • Use the Option Strict On statement • Suppresses implicit type conversions Programming with Microsoft Visual Basic 2012

  38. Modifying the Calculate Button’s Code (cont.) Figure 3-28 A jagged blue line indicates a syntax error Figure 3-29 Lines to delete from the procedure Programming with Microsoft Visual Basic 2012

  39. Modifying the Calculate Button’s Code (cont.) Figure 3-30 Revised pseudocode and flowchart for the btnCalc control’s Click event procedure Programming with Microsoft Visual Basic 2012

  40. Modifying the Calculate Button’s Code (cont.) Figure 3-31 List of named constants and variables Figure 3-32 Const and Dim statements entered in the procedure Programming with Microsoft Visual Basic 2012

  41. Modifying the Calculate Button’s Code (cont.) Figure 3-34 Calculated amounts shown in the interface Figure 3-33 Code entered in the btnCalc control’s Click event procedure Programming with Microsoft Visual Basic 2012

  42. Concatenating Strings • Concatenate • To connect strings together • Concatenation operator • The ampersand (&) • Include a space before and after the ampersand • Numeric values used with the & operator are converted to strings Figure 3-35 Examples of string concatenation Programming with Microsoft Visual Basic 2012

  43. Concatenating Strings (cont.) Figure 3-36 String concatenation included in the assignment statement Figure 3-37 Concatenated strings displayed in the lblMessage control Programming with Microsoft Visual Basic 2012

  44. The InputBox Function InputBox function • Displays an input dialog box and retrieves user input • Arguments • prompt: Contains the message to display inside the dialog box • title: Controls the text that appears in the dialog box’s title bar • defaultResponse: Controls the text that appears in the input field • The returned value most often assigned to String variable • The syntax is shown in Figure 3-39 on the next slide Programming with Microsoft Visual Basic 2012

  45. The InputBoxFunction (cont.) Figure 3-38 Example of an input dialog box Figure 3-39 Basic syntax and examples of the InputBox function Programming with Microsoft Visual Basic 2012

  46. The InputBoxFunction (cont.) Figure 3-42 Dialog box created by the InputBox function Figure 3-40 Class-level variable declared in the form’s Declarations section Figure 3-41 frmMain Load event procedure Programming with Microsoft Visual Basic 2012

  47. The ControlChars.Newline Constant ControlChars.NewLine constant • Advances the insertion point to the next line in a control • Also used to advance the insertion point in file or on the printer • To use, type ControlChars.NewLinein the appropriate location • Can be used with string concatenation • Line continuation character (_) • Used to break up a long line of code into two or more lines Programming with Microsoft Visual Basic 2012

  48. The ControlChars.NewlineConstant (cont.) Figure 3-43 Modified assignment statement Figure 3-45 Salesclerk’s name shown on the sales receipt Programming with Microsoft Visual Basic 2012

  49. Designating a Default Button Default button • Activated by pressing the Enter key • Not required to have the focus • Only one per form • The default button should be the button used most often by the user • Except if button’s task is destructive and irreversible, such as deleting data • Set the form’s AcceptButton property to the button name Programming with Microsoft Visual Basic 2012

  50. Using the ToString Method to Format Numbers • Formatting • Specifying decimal places and special characters to display • The ToStringmethod replaces the Format function • Syntax: variablename.ToString(formatString) • variablename: The name of a numeric variable • formatString: The string specifying the format you want to use • formatStringmust take the form Axx, where A is the format specifier and xx is the precision specifier Programming with Microsoft Visual Basic 2012

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