1. Programming with Microsoft Visual Basic 2010 5th Edition
Chapter Three
Using Variables and Constants

2. Previewing the Modified Playtime Cellular Application
Previewing the Playtime Cellular application
Access Run command on Start menu
Browse to VB2010\Chap03 folder
Click the Playtime Cellular (Playtime Cellular.exe) file
View order form
Enter customer information from pages
Completed application resembles Chapter 2 version

3. Figure 3-2 Completed order form

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

5. Lesson A Objectives (cont’d.)
Explain the scope and lifetime of variables and named constants
Explain the purpose of the Option Explicit, Option Infer, and Option Strict

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

7. Using Variables to Store Information (cont’d.)
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

8. Basic data types in Visual Basic
Figure 3-3
Basic data types in Visual Basic

9. Using Variables to Store Information (cont’d.)
For this course:
Use Integer data type for all integers
Use either Decimal or Double data type for 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

10. Using Variables to Store Information (cont’d.)
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

11. Figure 3-4 Variable naming rules and examples

12. Using Variables to Store Information (cont’d.)
Declaring a variable
Declaration statement: Used to declare (create) a variable and reserve space in memory for it
Syntax shown in Figure 3-5 on next slide
If no initial value is given to variable when declaring it, computer stores 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/ :00:00AM

13. Figure 3-5 Syntax and examples of a variable declaration statement

14. Assigning Data to an Existing Variable
Assignment statement: Assigns value to variable at run time
Syntax: variablename = expression
Expression may include literal constants, object properties, variables, keywords, arithmetic operators
Literal constant
Data item whose value does not change
Example: The string “Mary”
Literal type character
Forces literal constant to change data type

15. Figure 3-6 Assignment statements in which the value’s data type matches the variable’s data type

16. Assigning Data to an Existing Variable (cont’d.)
TryParse method: Converts string to number
TryParse is preferred over Val
Allows programmer to specify data type
Val only returns a type Double value
Syntax shown in Figure 3-7 on next slide
dataType: Numeric data type, such as Integer
string : String to be converted
variable : Variable that receives the numeric value

17. Figure 3-7 Basic syntax and examples of the TryParse method

18. Assigning Data to an Existing Variable (cont’d.)
Convert class: Can be used to convert a number from one type to another
Syntax shown in Figure 3-9 on next slide
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

19. Figure 3-9 Syntax and examples of the Convert class methods

20. The Scope and Lifetime of a Variable
Scope: Indicates where variable can be used
Lifetime: How long variable remains in memory
Scope and lifetime are determined by where variable is declared
Three types of scope
Class: 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

21. The Scope and Lifetime of a Variable (cont’d.)
Variables with procedure scope
Can be used only by that procedure
Declared at beginning of procedure
Removed from memory when procedure ends
Declared using Dim keyword
Most variables used in this course will be procedure-level variables
Sales tax example UI and code given on following slides illustrate use of procedure variables

22. The Scope and Lifetime of a Variable (cont’d.)
Figure 3-10 User interface for the Sales Tax Calculator application

23. Click event procedures using procedure-level variables
Figure 3-11
Click event procedures using procedure-level variables

24. The Scope and Lifetime of a Variable (cont’d.)
Variables with class scope
Can be used by all procedures in a form
Declared in form’s Declarations section
Remain in memory until application ends
Declared using Private keyword
Total Sales example UI and code given on following slides illustrate use of class-level variables

25. The Scope and Lifetime of a Variable (cont’d.)
Figure 3-12 User interface for the Total Sales application

26. Figure 3-13 Code using a class-level variable

27. 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 class-level variables but have narrower scope
Can only be used within procedure where declared

28. Figure 3-14 Code using a static variable

29. Named Constants Named constant Const statement: Creates named constant
Memory location inside computer whose contents cannot be changed at run time
Const statement: Creates named constant
Stores value of expression in named constant
expression: Can be literal constant, another named constant, or an arithmetic operator
Cannot contain a variable or method
Syntax and examples shown in Figure 3-15 on next slide

30. Figure 3-15 Syntax and examples of the Const statement

31. Figure 3-16 User interface for the Area Calculator application
Figure 3-17 Calculate Area button’s Click event procedure

32. 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
Data converted to greater precision number
e.g., Integer to Decimal
Demotion
Data truncated
e.g., Decimal to Integer
Data loss can occur when demotion occurs

33. Option Explicit, Option Infer, and Option Strict (cont’d.)
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
Figure 3-18 on following slide contains examples

34. Figure 3-18 Rules and examples of type conversions

35. Option Explicit, Option Infer, and Option Strict (cont’d.)
Figure 3-19 Option statements entered in the General Declarations section

36. Lesson A Summary Declare a variable using {Dim | Private | Static}
Assignment statement: Assigns value to a variable
Three levels of scope: Block, procedure, class
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