Microsoft Visual Basic 2010: Reloaded Fourth Edition

1. Microsoft Visual Basic 2010: Reloaded Fourth Edition Chapter Three Memory Locations and Calculations

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 Microsoft Visual Basic 2010: Reloaded, Fourth Edition 2

3. Objectives (cont'd.) 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 during run time Send the focus to a control during run time Explain the difference between syntax errors and logic errors Format an application’s numeric output Microsoft Visual Basic 2010: Reloaded, Fourth Edition 3

4. Internal Memory Internal memory: a component inside a computer comprised of memory locations Each memory location has a unique numeric address and can hold only one item at a time A programmer can reserve memory locations for a program by assigning each location a name, a data type, and an initial value Data type: indicates the type of data the memory location will store Two types of memory locations that a programmer can declare: variables and constants Microsoft Visual Basic 2010: Reloaded, Fourth Edition 4

5. Internal Memory (cont'd.) Microsoft Visual Basic 2010: Reloaded, Fourth Edition 5

6. Variables Variables: computer memory locations used to temporarily store data while an application is running Contents can change during run time 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 Microsoft Visual Basic 2010: Reloaded, Fourth Edition 6

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8. Variables (cont'd.) Each variable must be assigned a data type, which determines the memory location’s data type Each data type is a class Integer, Long, or Short data types can store integers (whole numbers) Decimal, Double, and Single data types: store real numbers (numbers with a decimal place) Char data type: stores one Unicode character String data type: stores multiple Unicode characters Microsoft Visual Basic 2010: Reloaded, Fourth Edition 8

9. Variables (cont'd.) Unicode: Universal coding scheme for characters that assigns a unique numeric value to each character Other data types Boolean data type: stores a value of True or False Date data type: stores date and time information Object data type: stores any type of data Computer must determine the data type at run time, making it more inefficient Microsoft Visual Basic 2010: Reloaded, Fourth Edition 9

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11. Microsoft Visual Basic 2010: Reloaded, Fourth Edition 11 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 and data type Initial value (optional) Initialization Numeric data types: automatically initialized to 0 String data type: automatically initialized to Nothing Boolean data type: initialized to False

12. Microsoft Visual Basic 2010: Reloaded, Fourth Edition 12 Declaring a Variable in Code (cont’d.)

13. Microsoft Visual Basic 2010: Reloaded, Fourth Edition 13 Declaring a Variable in Code (cont’d.)

14. Microsoft Visual Basic 2010: Reloaded, Fourth Edition 14 Assigning Data to an Existing Variable Assignment statement: Used to assign values to properties of controls Used to assign values to variables Assignment operator (=): Expression on the right of the = operator is assigned to the variable on the left of the = operator Expression: can contain literal constants, object properties, variables, keywords, or arithmetic operators

15. Assigning Data to an Existing Variable (cont'd.) 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 String literals are enclosed in quotation marks Microsoft Visual Basic 2010: Reloaded, Fourth Edition 15

16. Microsoft Visual Basic 2010: Reloaded, Fourth Edition 16 Assigning Data to an Existing Variable (cont'd.)

17. Microsoft Visual Basic 2010: Reloaded, Fourth Edition 17 Assigning Data to an Existing Variable (cont'd.)

18. 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 2010: Reloaded, Fourth Edition 18

19. Using the TryParse Method (cont'd.) 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 2010: Reloaded, Fourth Edition 19

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21. Microsoft Visual Basic 2010: Reloaded, Fourth Edition 21 Using the TryParse Method (cont'd.)

22. Using the Convert Class Methods Convert class: Contains methods for converting numeric values to specific data types Commonly used methods of the Convert class include: ToDouble ToDecimal ToInt32 ToString Microsoft Visual Basic 2010: Reloaded, Fourth Edition 22

23. Microsoft Visual Basic 2010: Reloaded, Fourth Edition 23 Using the Convert Class Methods (cont’d.)

24. Microsoft Visual Basic 2010: Reloaded, Fourth Edition 24 Including Variables in Arithmetic Expressions 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

25. Microsoft Visual Basic 2010: Reloaded, Fourth Edition 25 Including Variables in Arithmetic Expressions (cont'd.)

26. Microsoft Visual Basic 2010: Reloaded, Fourth Edition 26 Including Variables in Arithmetic Expressions (cont'd.)

27. Microsoft Visual Basic 2010: Reloaded, Fourth Edition 27 Including Variables in Arithmetic Expressions (cont'd.)

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29. Microsoft Visual Basic 2010: Reloaded, Fourth Edition 29 Including Variables in Arithmetic Expressions (cont'd.)

30. Arithmetic Assignment Operators Arithmetic assignment operators: abbreviate an assignment statement that contains an arithmetic operator for specific cases Statement must be of the form: variableName = variableName arithmeticOperator value Microsoft Visual Basic 2010: Reloaded, Fourth Edition 30

31. Microsoft Visual Basic 2010: Reloaded, Fourth Edition 31 Arithmetic Assignment Operators (cont’d.)

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33. 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 A variable’s scope and lifetime are determined by where you declare the variable Variables declared in the form’s Declarations section have class scope Variables declared within a procedure have either procedure scope or block scope Microsoft Visual Basic 2010: Reloaded, Fourth Edition 33

34. Variables with 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 2010: Reloaded, Fourth Edition 34

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36. Variables with Procedure Scope (cont’d.) Microsoft Visual Basic 2010: Reloaded, Fourth Edition 36

37. Variables with Class Scope Microsoft Visual Basic 2010: Reloaded, Fourth Edition 37

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41. 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 2010: Reloaded, Fourth Edition 41

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44. 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 2010: Reloaded, Fourth Edition 44

45. Option Explicit, Option Infer, and Option Strict (cont'd.) 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 2010: Reloaded, Fourth Edition 45

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47. Option Explicit, Option Infer, and Option Strict (cont'd.) 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 2010: Reloaded, Fourth Edition 47

48. Option Explicit, Option Infer, and Option Strict (cont'd.) Microsoft Visual Basic 2010: Reloaded, Fourth Edition 48

49. Coding the Sunshine Cellular Application Microsoft Visual Basic 2010: Reloaded, Fourth Edition 49

50. Coding the Sunshine Cellular Application (cont'd.) Microsoft Visual Basic 2010: Reloaded, Fourth Edition 50

51. 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 2010: Reloaded, Fourth Edition 51

52. Using Pseudocode to Plan a Procedure (cont’d.) Microsoft Visual Basic 2010: Reloaded, Fourth Edition 52

53. 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 2010: Reloaded, Fourth Edition 53

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55. Microsoft Visual Basic 2010: Reloaded, Fourth Edition 55 Coding the Calculate Order Button’s Click Event Procedure

56. Coding the Calculate Order Button’s Click Event Procedure (cont'd.) Microsoft Visual Basic 2010: Reloaded, Fourth Edition 56

57. Coding the Calculate Order Button’s Click Event Procedure (cont'd.) Microsoft Visual Basic 2010: Reloaded, Fourth Edition 57

58. Coding the Calculate Order Button’s Click Event Procedure (cont'd.) Microsoft Visual Basic 2010: Reloaded, Fourth Edition 58

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60. Completing the Sunshine Cellular Application Microsoft Visual Basic 2010: Reloaded, Fourth Edition 60

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63. Testing and Debugging the Application To test an application: Select a set of sample data Manually compute the expected output Run the application and compare its output with the expected output Bug: an error in the program code Syntax error: an error that violates the programming language’s syntax Usually caused by mistyping Microsoft Visual Basic 2010: Reloaded, Fourth Edition 63

64. Testing and Debugging the Application (cont'd.) Logic error: when the application does not perform as expected Valid data: data that the application is expecting Invalid data: data that is unexpected Test a program with both valid and invalid data Microsoft Visual Basic 2010: Reloaded, Fourth Edition 64

65. Microsoft Visual Basic 2010: Reloaded, Fourth Edition 65 Testing and Debugging the Application (cont'd.)

66. Microsoft Visual Basic 2010: Reloaded, Fourth Edition 66 Testing and Debugging the Application (cont'd.)

67. Microsoft Visual Basic 2010: Reloaded, Fourth Edition 67 Testing and Debugging the Application (cont'd.)

68. Microsoft Visual Basic 2010: Reloaded, Fourth Edition 68 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 FormatString argument: specifies the type of formatting to use Precision specifier: controls the number of significant digits or zeros to the right of the decimal point

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71. Microsoft Visual Basic 2010: Reloaded, Fourth Edition 71 Formatting Numeric Output (cont'd.)

72. Microsoft Visual Basic 2010: Reloaded, Fourth Edition 72 Programming Tutorial 1

73. Microsoft Visual Basic 2010: Reloaded, Fourth Edition 73 Programming Tutorial 2

74. Microsoft Visual Basic 2010: Reloaded, Fourth Edition 74 Programming Example

75. Microsoft Visual Basic 2010: Reloaded, Fourth Edition 75 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 class level

76. Summary (cont'd.) Assignment statement is used to assign values to an existing variable during run time Literals are constant items of data that do not change during run time 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 2010: Reloaded, Fourth Edition 76

77. Summary (cont'd.) A procedure-level variable is usable only by the procedure in which it is declared A class-level variable is usable by all procedures in the form A static variable is a procedure-level variable that retains its value even when the procedure ends Use comments to document your code Use Const to declare a named constant Option Explicit On forces declaration of all variables before use Microsoft Visual Basic 2010: Reloaded, Fourth Edition 77

78. Microsoft Visual Basic 2010: Reloaded, Fourth Edition 78 Summary (cont'd.) 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 Pseudocode or a flowchart is used to plan a procedure’s code 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

79. Summary (cont'd.) Test a program with both valid and invalid data Use the ToString method to format a program’s numeric output with special characters, such as for currency, percentages, and number of decimal places Microsoft Visual Basic 2010: Reloaded, Fourth Edition 79