CHAPTER FOUR

1
CHAPTER FOUR

• Variables andArithmetic Operations

2
Objectives

• Create, modify, and program a TextBox object
• Use code to place data in the Text property of a
  Label object
• Use the AcceptButton and CancelButton properties
• Understand and declare String and Numeric
  variables
• Use assignments statements to place data in
  variables

3
Objectives

• Use literals and constants in coding statements
• Understand scope rules for variables
• Convert string and numeric data
• Understand and use arithmetic operators and
  arithmetic operations

4
Objectives

• Format and display numeric data as a string
• Create a form load event
• Create a concatenated string
• Debug a program

5
New Concepts

• Structures ltprimitive data typesgt
• Controls
  TextBox , MaskedTextBox
• Events form load
• Properties AcceptButton, CancelButton,
  Size, TextAlign, MultiLine
• Methods ToString, ToltTypegt, Clear,
  Focus
• Classes Convert, String, Object
• Miscellaneous
• Variables, Literals, Constants ( local and global
  )
• Operators ( arithmetic, concatenation )
• Enter key, Escape key
• Option Strict, Option Explicit
• Scope, Lifetime
• Exceptions ( format, division by 0, overflow )

6
Introduction

• Controls
• TextBox
• Label
• Button

7
TextBox Objects

• TextBox objects allow a user to input a value
  into an application.
• A TextBox is often paired with a label that
  describes what should be input. In a command-line
  oriented application, this would be a prompt.
• Characters typed into a TextBox become part of a
  String (regardless of the characters).

8
TextBox Objects
9
Sizing and Positioning a TextBox Object

• Select the TextBox object. Scroll in the
  Properties window until the Text property is
  visible and then click the right column for the
  Text property
• Type the maximum number of characters the user
  normally will enter into the text box and then
  press the ENTER key. When entering numbers, the
  digit 8 often is entered because it is wider than
  other digits. In this example, the value 888 is
  entered because three digits is the maximum
  number of digits the user normally will enter

10
Sizing and Positioning a TextBox Object

• Using the Font property in the Properties window,
  change the Font property to the correct font and
  font size. For this application, change the font
  to Times New Roman and change the font size to
  12. Then, drag the right edge of the TextBox
  object to resize the TextBox object so it is
  slightly wider than the 888 entry
• To horizontally align the text in the label and
  the text in the text box, drag the text box up
  until a red snap line indicates the bottoms of
  the text are aligned. Then, release the left
  mouse button

11
Sizing and Positioning a TextBox Object
12
Aligning Text in a TextBox Object

• Select the TextBox object. In the Properties
  window, scroll until the TextAlign property is
  visible.
• click the TextAlign property in the left column,
  and then click the list arrow in the right column
  of the TextAlign property
• Click Center in the TextAlign property list
• Because the TextBox object is sized properly,
  remove the digits in the TextBox object. Select
  the characters 888 in the Text property, press
  the DELETE key on your keyboard, and then press
  the ENTER key

13
Aligning Text in a TextBox Object
14
Creating a MultiLine Text Box

• Select the TextBox object, click the Action tag,
  and point to the MultiLine check box
• Click the MultiLine check box
• Not done for this application!

15
Creating a MaskedTextBox Object
16
Creating a MaskedTextBox Object not done for
this application

• Drag a MaskedTextBox .NET component from the
  Toolbox to the Windows Form object. Then, click
  the Action tag on the TextBox object and point to
  the Set Mask command
• Click Set Mask on the MaskedTextBox Tasks list
  and then click the Short date mask description in
  the Input Mask dialog box
• Click the OK button in the Input Mask dialog box
  and then click anywhere in the Windows Form object

17
Creating a MaskedTextBox Object
18
Label Objects

• Drag a Label object onto the Windows Form object
  to the correct location. Name the label
  lblTotalCostOfDownloads. Change the label to the
  appropriate font size (Times New Roman, 12
  point). In the Text property for the Label
  object, enter the maximum number of characters
  (888.88) that will appear in the label during
  execution of the program
• Drag the Label object up until the red snap line
  appears. Then release the left mouse button

19
Label Objects
20
Accept Button in Form Properties

• The program will carry out the event handler
  processing associated with the AcceptButton if
  the user clicks the button or if the user presses
  the ENTER key
• Click a blank area in the Windows Form object to
  select it. Scroll in the Properties window until
  the AcceptButton property is visible. Click the
  AcceptButton property name in the left column and
  then click the AcceptButton property list arrow
  in the right column
• Click btnCalculateCost in the AcceptButton
  property list

21
Cancel Button in Form Properties

• When the user presses the ESC key, the event
  handler processing for the button identified as
  the Cancel button will be executed
• Click a blank area in the Windows Form object to
  select it
• Click the CancelButton property name in the left
  column in the Properties window for the Windows
  Form object, and then click the CancelButton list
  arrow
• Click the button name (btnClear) in the
  CancelButton property list

22
Visual Studio Preparation for Code Entry

• With the Toolbox visible, click the Toolbox Close
  button. The Toolbox closes and the work area
  expands in size. To reshow the Toolbox after it
  has been closed, click the Toolbox button on the
  Standard toolbar

23
String Data Type

• A String data type allows every character
  available on the computer to be stored in it
• The data type for the value the user enters in a
  TextBox object and that is stored in the Text
  property of the TextBox object is string
• A variable is a named location in RAM where data
  is stored
• A String variable is a named location in RAM that
  can store a string value

24
String Data Type
Variable Declaration in Visual Basic
25
Assignment Statements

• One method to place data in the variable is to
  use an assignment statement

26
Assignment Statements

• With Visual Studio displaying the code editing
  window and the insertion point located in the
  desired column, type Dim followed by a space.
  Then, type the name of the String variable you
  want to define, strNumberOfSongs on your
  keyboard
• Press the SPACEBAR, type the word As and then
  press the SPACEBAR again
• Because the entry should be String, type str on
  your keyboard
• Press the ENTER key

27
Assignment Statements

• To begin the assignment statement, press
  CTRLSPACEBAR in order to display the
  IntelliSense list of allowable entries. Then,
  type strn to highlight the strNumberOfSongs
  variable name in the IntelliSense list
• Press the SPACEBAR, press the EQUAL SIGN key, and
  then press the SPACEBAR
• Type me. to display the IntelliSense list, and
  then type txt to identify the txtNumberOfSongs
  TextBox object in the IntelliSense list
• Press the PERIOD key and then, if necessary, type
  te to highlight the Text entry in the
  IntelliSense list
• Press the ENTER key

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Assignment Statements
29
Numeric Data Types

• A numeric data type must be used in arithmetic
  operations

30
Numeric Data Types
31
Other Data Types
32
Other Data Types
33
Miscellaneous Data Types
34
Variable Prefixes
35
Literals

• A value is called a literal when the value being
  used in the assignment statement is literally the
  value that is required

36
Forced Literal Types

• Sometimes you might want a literal to be a
  different data type than the Visual Basic default

37
Constants

• A constant variable will contain one permanent
  value throughout the execution of the program

38
Constants

• The declaration of a constant variable begins
  with the letters Const, not the letters Dim
• You must assign the value to be contained in the
  constant on the same line as the definition of
  the constant.
• You cannot attempt to change the value in the
  constant variable anywhere in the program. If you
  attempt this, you will produce a compiler error
• The letter c often is placed before the prefix of
  the constant variable name to identify throughout
  the program that it is a constant variable and
  cannot be changed
• Other than the letter c constant variable names
  are formed using the same rules and techniques as
  nonconstant names

39
Referencing a Variable

• When a variable is declared, it will be
  underlined with a green squiggly line until it is
  referenced in a statement
• It is mandatory when using a variable in a
  program that the variable is defined prior to
  using the variable name in a statement

40
Scope of Variables

• The scope of a variable specifies where within
  the program the variable can be referenced in a
  Visual Basic statement
• A variable can be referenced only within the
  region of the program where it is defined
• The code between the Sub statement and the End
  Sub statement is a procedure
• A variable that can only be referenced within the
  region of the program where it is defined is
  called a local variable
• Local variables have a certain lifetime in the
  program
• Global variables can be used in multiple regions
  of a program

41
Converting Variable Data

• Visual Basic includes several procedures that
  allow you to convert one data type to another
  data type
• A method to convert a String data type to an
  Integer data type is named ToInt32
• The procedure is found in the Convert class,
  which is available in a Visual Studio 2005 class
  library

42
Using a Procedure

• A procedure that performs its task but does not
  return a value is called a Sub procedure
• A procedure that returns a value is called a
  Function procedure, or a function
• An argument identifies a value required by a
  procedure parameter / argument
• Every procedure is part of a class ( or module )

43
Using a Procedure
44
Option Strict On

• Visual Basic will, by default, automatically
  convert data types if the data type on the right
  side of the equal sign in an assignment statement
  is different from the data type on the left side
  of the equal sign
• To prevent automatic conversion of values, the
  developer must insert the Option Strict On
  statement in the program prior to any event
  handler code in the program project -gt
  compile settings
• The Option Strict On statement explicitly
  disallows any default data type conversions in
  which data loss would occur and any conversion
  between numeric types and strings

45
Option Strict On
46
Arithmetic Operations
47
Arithmetic Operators
48
Multiple Operations

• A single assignment statement can contain
  multiple arithmetic operations
• Hierarchy of Operations
• Exponentiation () is performed first
• Multiplication () and division (/) are performed
  next
• Integer division (\) is next
• MOD then occurs
• Addition (1) and subtraction (2) are performed
  last
• Within these five steps, calculations are
  performed left to right

49
Displaying Numeric Output Data
Every object has a ToString method
type structures / classes
50
Format Specifications for the ToString Function
method

• Use the format specifier to identify the format
  for the numeric data to be returned by the
  ToString function

51
Precision Specifier

• The precision specifier is a number that is
  included within the quotation marks in the
  function call to identify the number of positions
  to the right of the decimal point that should be
  returned

52
Clear Procedure

• The Clear procedure clears any data currently
  placed in the Text property of a TextBox object

53
Clearing the Text Property of a Label

• The Clear procedure cannot be used with a Label
  object
• You must write an assignment statement that
  assigns a null length string to the Text property
  of a Label object

54
Setting the Focus

• When the focus is on a TextBox object, the
  insertion point is located in the text box

55
Form Load Event

• A form load event occurs when the program starts
  and the Digital Downloads form is loaded
• This event handler completes the following tasks
• Displays the cost per download heading
• Clears the placeholder from the
  lblTotalCostOfDownloads Text property
• Sets the focus on the txtNumberOfSongs text box

56
Concatenation

• The process of joining two different values into
  a single string is called concatenation
• The values being concatenated must be String data
  types

57
Concatenation
58
Class Scope

• When a variable is referenced in two different
  event handling procedures, it must be defined at
  the class level instead of the procedure (event
  handler) level

59
Debugging Your Program

• A Format Exception occurs when the user enters
  data that a statement within the program cannot
  process properly

60
Debugging Your Program

• An Overflow Exception occurs when the user enters
  a value greater than the maximum value that can
  be processed by the statement
• It is not possible to divide by zero, so if your
  program contains a division operation and the
  divisor is equal to zero, the Divide By Zero
  Exception will occur

61
Program Design
62
Program Design
63
Event Planning Document
64
Summary

• Create, modify, and program a TextBox object
• Use code to place data in the Text property of a
  Label object change property at run time
• Use the AcceptButton and CancelButton properties
• Understand and declare String and Numeric
  variables
• Use assignment statements to place data in
  variables

65
Summary

• Use literals and constants in coding statements
• Understand scope rules for variables
• Convert string and numeric data
• Understand and use arithmetic operators and
  arithmetic operations

66
Summary

• Format and display numeric data as a string
• Create a form load event
• Create a concatenated string
• Debug a program

67
CHAPTER FOUR COMPLETE

• Variables andArithmetic Operations