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Microsoft Visual Basic 2008: Reloaded Third Edition

Microsoft Visual Basic 2008: Reloaded, Third Edition. 2. Objectives. After studying this chapter, you should be able to:Declare variables and named constantsAssign data to an existing variableConvert data to the appropriate type using the TryParse method and the Convert class methodsWrite arithm

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Microsoft Visual Basic 2008: Reloaded Third Edition

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    1. Microsoft Visual Basic 2008: Reloaded Third Edition Chapter Three Variables, Constants, and Arithmetic Operators

    2. Microsoft Visual Basic 2008: Reloaded, Third Edition 2 Objectives After studying this chapter, you should be able to: Declare variables and named constants Assign data to an existing variable Convert data to the appropriate type using the TryParse method and the Convert class methods Write arithmetic expressions Understand the scope and lifetime of variables and named constants

    3. Microsoft Visual Basic 2008: Reloaded, Third Edition 3 Objectives (continued) Understand the purpose of the Option statements Use a TOE chart, pseudocode, and a flowchart to code an application Clear the contents of a control’s Text property while an application is running Send the focus to a control while the application is running Explain the difference between syntax errors and logic errors Format an application’s numeric output

    4. Microsoft Visual Basic 2008: Reloaded, Third Edition 4 Variables Variables: computer memory locations used to store data while an application is running Use a meaningful variable name that reflects the purpose of the variable Use camel casing for variable identifiers Variable names should conform to naming rules

    5. Variables (continued) Microsoft Visual Basic 2008: Reloaded, Third Edition 5

    6. Variables (continued) Each variable must be assigned a data type Data type: the type of data the variable can store Each data type is a class Unicode: Universal coding scheme for characters Assigns a unique numeric value to each character Microsoft Visual Basic 2008: Reloaded, Third Edition 6

    7. Microsoft Visual Basic 2008: Reloaded, Third Edition 7

    8. Microsoft Visual Basic 2008: Reloaded, Third Edition 8 Declaring a Variable in Code Declaration statement: used to declare, or create, a variable Declaration statement includes Scope keyword: Dim, Private, or Static Name of the variable Data type Initial value (optional) Numeric data types are automatically initialized to 0 String data type is automatically initialized to “”

    9. Microsoft Visual Basic 2008: Reloaded, Third Edition 9

    10. Microsoft Visual Basic 2008: Reloaded, Third Edition 10 Assigning Data to an Existing Variable Assignment statement: Used to assign values to properties of controls Used to assign values to variables Assignment operator: (=) Value on the right of the = operator is assigned to the variable on the left of the = operator

    11. Assigning Data to an Existing Variable (continued) String: group of characters enclosed in quotation marks Literal constant: An item of data whose value does not change while the application is running Can be a numeric or a string literal constant A numeric literal without a decimal place is treated as an integer A numeric literal with a decimal place is treated as a Double type Microsoft Visual Basic 2008: Reloaded, Third Edition 11

    12. Microsoft Visual Basic 2008: Reloaded, Third Edition 12

    13. Using the TryParse Method Method: a specific portion of a class’s instructions that performs a task for the class TryParse method: Part of every numeric data type’s class Used to convert a string to that numeric data type Argument: a value that is provided to a method Basic syntax of TryParse method has two arguments: String: string value to be converted Variable: location to store the result Microsoft Visual Basic 2008: Reloaded, Third Edition 13

    14. Using the TryParse Method (continued) If TryParse conversion is successful, the method stores the value in the variable If unsuccessful, a 0 is stored in the numeric variable Microsoft Visual Basic 2008: Reloaded, Third Edition 14

    15. Microsoft Visual Basic 2008: Reloaded, Third Edition 15

    16. Using the Convert Class Convert class: Contains methods for converting numeric values to specific data types Use the dot member access operator to separate the class name from the method name Commonly used methods of the Convert class include: ToDouble ToDecimal ToInt32 ToString Microsoft Visual Basic 2008: Reloaded, Third Edition 16

    17. Microsoft Visual Basic 2008: Reloaded, Third Edition 17

    18. Option Explicit, Option Infer, and Option Strict Undeclared variable: a variable that does not appear in a declaration statement (such as Dim) Is assigned a data type of Object Misspelling a variable name can result in an undeclared variable unless Option Explicit is on Option Explicit On statement Appears in the General Declarations section of the Code Editor window (above Public Class statement) Enforces that all variables must be declared before being used Microsoft Visual Basic 2008: Reloaded, Third Edition 18

    19. Option Explicit, Option Infer, and Option Strict (continued) Option Infer Off statement: ensures that every variable is declared with a data type Implicit type conversion: occurs when you attempt to assign data of one type to a variable of another type without explicitly attempting to convert it If converted to a data type that can store larger numbers, the value is said to be promoted If converted to a data type that can store only smaller numbers, the value is said to be demoted Can cause truncation and loss of precision Microsoft Visual Basic 2008: Reloaded, Third Edition 19

    20. Option Explicit, Option Infer, and Option Strict (continued) Option Strict On statement: ensures that values cannot be converted from one data type to a narrower data type, resulting in lost precision Microsoft Visual Basic 2008: Reloaded, Third Edition 20

    21. Microsoft Visual Basic 2008: Reloaded, Third Edition 21

    22. Microsoft Visual Basic 2008: Reloaded, Third Edition 22 Using a Variable in an Arithmetic Expression Arithmetic operators: used to perform calculations Precedence number: indicates the order in which an operation in an expression is performed If an expression has two operators with the same precedence, they are evaluated from left to right Use parentheses to change the order of evaluation Integer division operator (\): divides two integers and returns an integer value Modulus arithmetic operator (Mod): divides two numbers and returns the remainder

    23. Microsoft Visual Basic 2008: Reloaded, Third Edition 23 Using a Variable in an Arithmetic Expression (continued)

    24. Microsoft Visual Basic 2008: Reloaded, Third Edition 24

    25. Using a Variable in an Arithmetic Expression (continued) Line continuation character (_): Used to break up a long instruction into two or more physical lines Underscore must be preceded by a space and must appear at the end of a physical line A variable can store only one value at a time A second assignment statement on the same variable will replace its current value with the new value Microsoft Visual Basic 2008: Reloaded, Third Edition 25

    26. The Scope and Lifetime of a Variable Scope: indicates where the variable can be used Lifetime: indicates how long the variable remains in memory Variables can have module scope, procedure scope, or block scope Module scope: variable is declared in the form’s Declarations section Variables declared within a procedure have either procedure scope or block scope Microsoft Visual Basic 2008: Reloaded, Third Edition 26

    27. Using Variables Having Procedure Scope Procedure-level variable: declared within a procedure Use the Dim keyword in the declaration Procedure scope: only the procedure can use the variable With procedure-level scope, two procedures can each use the same variable names Comments: Used to internally document the procedure Are ignored by the compiler Appear in green in the Code Editor Microsoft Visual Basic 2008: Reloaded, Third Edition 27

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    29. Using a Variable Having Module Scope (continued) Microsoft Visual Basic 2008: Reloaded, Third Edition 29

    30. Microsoft Visual Basic 2008: Reloaded, Third Edition 30

    31. Microsoft Visual Basic 2008: Reloaded, Third Edition 31

    32. Microsoft Visual Basic 2008: Reloaded, Third Edition 32

    33. Named Constants Named constant: memory location whose value cannot be changed while the application is running Declared using the Const keyword Good programming practice to specify the data type as well Many programmers use Pascal case for named constants Literal type character: forces a literal constant to assume a specific data type Named constants help to document the program code Microsoft Visual Basic 2008: Reloaded, Third Edition 33

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    35. Microsoft Visual Basic 2008: Reloaded, Third Edition 35

    36. Coding the Sunshine Cellular Application Microsoft Visual Basic 2008: Reloaded, Third Edition 36

    37. Coding the Sunshine Cellular Application (continued) Microsoft Visual Basic 2008: Reloaded, Third Edition 37

    38. Using Pseudocode to Plan a Procedure Pseudocode: short phrases that describe the steps a procedure needs to take to accomplish its goal Microsoft Visual Basic 2008: Reloaded, Third Edition 38

    39. Using a Flowchart to Plan a Procedure Flowchart: uses standardized symbols to show the steps a procedure must take to accomplish its goal Can be used in place of pseudocode for planning Three symbols: Start/stop symbol (oval): indicates start and stop points Process symbol (rectangle): represents tasks Input/output symbol (parallelogram): represents input or output tasks Flowlines: connect the symbols to show the direction Microsoft Visual Basic 2008: Reloaded, Third Edition 39

    40. Microsoft Visual Basic 2008: Reloaded, Third Edition 40

    41. Microsoft Visual Basic 2008: Reloaded, Third Edition 41 Coding the Calculate Order Button’s Click Event Procedure

    42. Coding the Calculate Order Button’s Click Event Procedure (continued) Microsoft Visual Basic 2008: Reloaded, Third Edition 42

    43. Coding the Calculate Order Button’s Click Event Procedure (continued) Microsoft Visual Basic 2008: Reloaded, Third Edition 43

    44. Coding the Calculate Order Button’s Click Event Procedure (continued) Microsoft Visual Basic 2008: Reloaded, Third Edition 44

    45. Coding the Calculate Order Button’s Click Event Procedure (continued) Microsoft Visual Basic 2008: Reloaded, Third Edition 45

    46. Completing the Sunshine Cellular Application Microsoft Visual Basic 2008: Reloaded, Third Edition 46

    47. Microsoft Visual Basic 2008: Reloaded, Third Edition 47

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    49. Testing and Debugging the Application Bug: an error in the program code Valid data: data that the application is expecting Invalid data: data that is unexpected Debugging: process of locating and correcting errors in a program Microsoft Visual Basic 2008: Reloaded, Third Edition 49

    50. Testing and Debugging the Application (continued) Syntax error: an error that violates the programming language’s syntax Usually caused by mistyping Logic error: occurs when you enter an instruction that does not give the expected results Test a program with both valid and invalid data Microsoft Visual Basic 2008: Reloaded, Third Edition 50

    51. Microsoft Visual Basic 2008: Reloaded, Third Edition 51 Testing and Debugging the Application (continued)

    52. Microsoft Visual Basic 2008: Reloaded, Third Edition 52 Testing and Debugging the Application (continued)

    53. Microsoft Visual Basic 2008: Reloaded, Third Edition 53 Formatting Numeric Output Formatting: specifying the number of decimal places and any special characters to display ToString method of a variable can be used to format a number Format specifier: specifies the type of formatting to use Precision specifier: controls the number of significant digits or zeros to the right of the decimal point

    54. Microsoft Visual Basic 2008: Reloaded, Third Edition 54

    55. Microsoft Visual Basic 2008: Reloaded, Third Edition 55 Formatting Numeric Output (continued)

    56. Microsoft Visual Basic 2008: Reloaded, Third Edition 56 Formatting Numeric Output (continued)

    57. Microsoft Visual Basic 2008: Reloaded, Third Edition 57 Programming Tutorial

    58. Microsoft Visual Basic 2008: Reloaded, Third Edition 58 Programming Example

    59. Microsoft Visual Basic 2008: Reloaded, Third Edition 59 Summary Variables and named constants are memory locations that store data Variables can change value, but constants cannot Variables and constants have a name, data type, initial value, scope, and lifetime Use Dim or Static to declare a variable at block or procedure level Use Private to declare a variable at module level

    60. Summary (continued) Assignment statement is used to assign values to an existing variable Literals are constant items of data that do not change String literal constants are enclosed in quotation marks Use the TryParse method to convert a string to a number The Convert class contains methods to convert values to a specified data type Microsoft Visual Basic 2008: Reloaded, Third Edition 60

    61. Microsoft Visual Basic 2008: Reloaded, Third Edition 61 Summary (continued) Option Explicit On forces declaration of all variables before use Option Infer Off warns if a variable declaration does not include a data type Option Strict On disallows any implicit type conversions that may cause a loss of data Integer division operator divides two integers and returns the result Modulus operator divides two numbers and returns the remainder

    62. Summary (continued) A procedure-level variable is usable only by the procedure in which it is declared A module-level variable is usable by all procedures in the form Use comments to document your code A static variable is a procedure-level variable that retains its value even when the procedure ends Pseudocode or a flowchart is used to plan a procedure’s code Microsoft Visual Basic 2008: Reloaded, Third Edition 62

    63. Summary (continued) You can clear the contents of a text box or label control by assigning an empty string or String.Empty value The Focus method moves the focus to a control Test a program with both valid and invalid data You can format a program’s numeric output with special characters, such as for currency, percentages, and number of decimal places Microsoft Visual Basic 2008: Reloaded, Third Edition 63

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