1. Chapter 3: Using Variables and Constants

2. Previewing the Modified 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
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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
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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
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5. Using Variables to Store Information (cont.)
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
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6. Using Variables to Store Information (cont.)
Figure 3-3 Basic data types in Visual Basic
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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
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8. Using Variables to Store Information (cont.)
Selecting a Name for a Variable
Names must begin with 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
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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
dblWest Region
The name cannot contain a space
strFirst.Name
The name cannot contain punctuation
decSales$East
The name cannot contain a special character
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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/ :00:00AM
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11. Using Variables to Store Information (cont.)
Figure 3-6 Syntax and examples of a variable declaration statement
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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
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13. Assigning Data to an Existing Variable (cont.)
Figure 3-7 Syntax and examples of assigning a value to a variable during run time
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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
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15. Assigning Data to an Existing Variable (cont.)
Figure 3-9 Basic syntax and examples of the TryParse method
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16. Assigning Data to an Existing Variable (cont.)
Figure 3-10 Results of the TryParse method for the Double, Decimal and Integer data types
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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
TryParse is recommended for converting strings to numeric data types
Will not produce an error if the conversion fails
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18. Assigning Data to an Existing Variable (cont.)
Figure 3-11 Syntax and examples of the Convert class methods
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19. The Scope and Lifetime of a Variable
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
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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 Dim keyword
Most variables used in this course will be procedure-level variables
Figure 3-12 User Interface for the Discount Calculator application
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21. The Scope and Lifetime of a Variable (cont.)
Figure 3-13 Click event procedures using procedure-level variables
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22. The Scope and Lifetime of a Variable (cont.)
Figure 3-14 Code Editor window for the Discount Calculator application
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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 Private keyword
Figure 3-16 User Interface for the Total Scores application
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24. The Scope and Lifetime of a Variable (cont.)
Figure 3-17 Total Scores application code using a class-level variable
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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
Static keyword
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
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26. Static Variables (cont.)
Figure 3-19 Total Scores application’s code using a static variable
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27. Named Constants Named constant Const statement
A memory location inside the computer whose contents cannot be changed at run time
Const statement
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
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28. Named Constants (cont.)
Figure 3-20 Syntax and examples of the Const statement
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29. Named Constants (cont.)
Figure 3-21 User interface for the Area Calculator application
Figure 3-22 Calculate Area button’s Click event procedure
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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
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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
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32. Option Statements (cont.)
Figure 3-25 Option statements entered in the General Declarations section
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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
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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
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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
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36. Modifying the Play It Again Movies Application (cont.)
Figure 3-27 Modified user interface for the Play It Again Movies application
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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
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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
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39. Modifying the Calculate Button’s Code (cont.)
Figure Revised pseudocode and flowchart for the btnCalc control’s Click event procedure
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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
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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
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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
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43. Concatenating Strings (cont.)
Figure 3-36 String concatenation included in the assignment statement
Figure 3-37 Concatenated strings displayed in the lblMessage control
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44. The InputBox Function InputBox function Arguments
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
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45. The InputBox Function (cont.)
Figure 3-38 Example of an input dialog box
Figure 3-39 Basic syntax and examples of the InputBox function
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46. The InputBox Function (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
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47. The 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.NewLine in 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
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48. The ControlChars.Newline Constant (cont.)
Figure 3-43 Modified assignment statement
Figure 3-45 Salesclerk’s name shown on the sales receipt
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49. Designating a 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
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50. Using the ToString Method to Format Numbers
Formatting
Specifying decimal places and special characters to display
The ToString method 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
formatString must take the form Axx, where A is the format specifier and xx is the precision specifier
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51. Using the ToString Method to Format Numbers (Cont.)
Figure 3-46 Syntax and examples of the ToString method
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52. Using the ToString Method to Format Numbers (Cont.)
Figure 3-48 Play It Again Movies application’s code at the end of Lesson B (continues…)
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53. Using the ToString Method to Format Numbers (Cont.)
Figure 3-48 Play It Again Movies application’s code at the end of Lesson B
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54. Lesson B Summary
The concatenation operator (&) is used to link strings
The InputBox function displays an interactive dialog box
Use ControlChars.NewLine to move the insertion point to a new line
Set the default button in the form’s AcceptButton property
The ToString method formats a number for string output
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55. Lesson C Objectives After studying Lesson C, you should be able to:
Include a static variable in code
Code the TextChanged event procedure
Create a procedure that handles more than one event
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56. Modifying the Load and Click Event Procedures
Figure 3-51 Revised TOE Chart for the Play It Again Movies application in Lesson C
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57. Modifying the Load and Click Event Procedures (Cont.)
Figure 3-52 Revised pseudocode for the Calculate button in Lesson C
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58. Coding the TextChanged Event Procedure
Occurs when the Text property value of a control changes
Can occur when:
The user enters data into the control
The code assigns data to the control’s Text property
Example: A change is made to the number of items ordered
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59. Coding the TextChanged Event Procedure (Cont.)
Associating a Procedure with Different Objects and Events
Handles clause
Appears in an event procedure’s header
Indicates the object and event associated with the procedure
You can associate an event procedure with more than one object and/or event
In the Handles section of procedure header, list each object and event separated by commas
Procedure names are entered in Pascal case
Capitalize the first letter in the name and the first letter of each subsequent word in the name
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60. Coding the TextChanged Event Procedure (cont.)
Figure 3-53 TextChanged event procedure associated with the txtDvds control
Figure 3-54 Completed ClearLabels procedure
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61. Lesson C Summary
The TextChanged event procedure responds to a change in the value of a control’s Text Property
The Handles clause determines which objects and events are associated with the event procedure
To create a procedure for more than one object or event, list each object and event after the Handles keyword
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