© Copyright 1992-2004 by Deitel & Associates, Inc. and Pearson Education Inc. All Rights Reserved....

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© Copyright 1992-2004 by Deitel & Associates, Inc. and Pearson Education Inc. All Rights Reserved.

Outline

24.1 Test-Driving the Ticket Information Application24.2 Data Hierarchy24.3 Files and Streams24.4 Writing to a File: Creating the Write Event Application24.5 Building the Ticket Information Application24.6 Wrap-Up

Tutorial 24 – Ticket Information Application

Introducing Sequential-Access Files

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Objectives

• In this tutorial, you will learn to:– Create, read from, write to and update files.

– Understand a computer’s data hierarchy.

– Become familiar with sequential-access file processing.

– Use StreamReader and StreamWriter classes to read from, and write to, sequential-access files.

– Add and configure a MonthCalendar control.

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24.1 Test Driving the Ticket Information Application

Application Requirements A local town has asked you to write an application that allows its residents to view community events for the current month. Events taking place in the town include concerts, sporting events, movies and other forms of entertainment. When the user selects a date, the application must indicate whether there are events scheduled for that day. The application must list the scheduled events and allow the user to select a listed event. When the user selects an event, the application must display the time and price of the event and a brief description of the event. The community event information is stored in a sequential-access file named calendar.txt.

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Figure 24.1 Ticket Information application’s GUI.

Arrow buttons allow user to scroll through months

MonthCalendar control

ComboBox lists any events

TextBox displays event details

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18th day of the month selected

No events displayed

19th day of the month selected

Event information displayed

Figure 24.2 Ticket Information application displaying event information.

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24.2 Data Hierarchy

• Data– Data has many forms, such as letters– Letters, numbers, and some special symbols are characters

– The set of all characters that represent data is known as the character set

– Data is broken down by a computer into bits, or binary digits

– Binary digits can either be 1 or 0

– Visual Basic .NET uses Unicode characters, which means every character is 2 bytes (16 bits)

– Fields are composed of characters– Records are groups of related fields– Files are groups of related records

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24.2 Data Hierarchy

• Data (continued)– Record keys distinguish a given record from all other

records– Records are often stored in order of their record keys in a file

known as a sequential file

– Related files are often stored in databases

– Databases are managed by programs in a database management system (DBMS)

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24.2 Data HierarchyFigure 24.3 Data hierarchy.

Sally

Tom

Judy

Iris

Randy

Black

Blue

Green

Orange

Red

Judy Green

J u d y Field

01001010

Byte (character J

(character J )

1 Bit

Record

File

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24.3 Files and Streams

0 1 2 3 4 5 6 7 8 9 n-1...

... end-of-file marker

Figure 24.4 Visual Basic .NET’s conceptual view of an n-byte file.

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24.4 Writing to a File: Creating the Write Event Application

• Adding a dialog to open or create a file– Select OpenFileDialog in the Windows Forms tab of the

Toolbox

– The property FileName is the file the application reads information from

– If CheckFileExists is False, the user is allowed to specify a file name

– A new file is created and opened if a user specifies a file name that does not exist

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OpenFileDialog control

Figure 24.5 OpenFileDialog added and renamed.

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• Determining if a file name is valid– Method CheckValidity receives a file name and returns True if it is valid

– Method EndsWith receives a filename and returns True if it ends with a .txt extension

– If the file name is valid, the Enter and Close File Buttons will be enabled

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CheckValidity Function procedure

header

Figure 24.6 Method CheckValidity header.

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Displaying error message if incorrect file type is provided

Figure 24.7 Displaying an error message indicating an invalid file name.

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Figure 24.8 Changing the GUI’s appearance if a valid file name is entered.

Enabling and disabling Buttons

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• StreamWriter– Used to create objects for writing text to a file– DialogResult determines if Cancel Button was clicked

and the method should therefore exit. – StreamWriter takes two arguments

• Name of the file to write to

• A Boolean value to determine if StreamWriter will append information on the end of the file

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Figure 24.9 System.IO namespace imported into FrmWriteEvent class.

Importing namespace System.IO

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Figure 24.10 Declaring a StreamWriter object.

Declaring StreamWriter

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Figure 24.11 Write Event application Form in design view.

Open File… Button

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Figure 24.12 Displaying the Open dialog and retrieving the result.

Displaying Open dialog

If the user clicks Cancel the event handler exits

Setting variable to user-specified file name

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Figure 24.13 Validating the file name and initializing a StreamWriter object.

Check for valid file name

Create StreamWriter object

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Figure 24.14 Clearing user input.

Clearing user input

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Figure 24.15 StreamWriter writing to a file.

Writing information to file

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Figure 24.16 Closing the StreamWriter.

Closing StreamWriter object

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Figure 24.17 Write Event application executing.

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Figure 24.18 Open dialog displaying contents of the template Ticket Information application’s bin folder.

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Figure 24.19 Sequential-access file generated by Write Event application.

Day and time of event, ticket price,

event name and description

Outline28


1 Imports System.IO

2

3 Public Class FrmWriteEvent

4 Inherits System.Windows.Forms.Form

5

6 Private m_objOutput As StreamWriter

7

8 ' Windows Form Designer generated code

9

10 ' determine validity of file type

11 Private Function CheckValidity(ByVal strName As String) _

12 As Boolean

13

14 ' show error if user specified invalid file

15 If strName.EndsWith(".txt") = False Then

16 MessageBox.Show("File name must end with .txt", _

17 "Invalid File Type", _

18 MessageBoxButtons.OK, MessageBoxIcon.Error)

19 Return False ' to indicate invalid file type

20

WriteEvent.vb(1 of 4)

Importing a namespace System.IO

StreamWriter that will write text to a file

Outline29


21 ' change status of Buttons if file name is valid

22 Else

23 btnOpenFile.Enabled = False

24 btnEnter.Enabled = True

25 btnCloseFile.Enabled = True

26 Return True ' to indicate text file specified

27 End If

28

29 End Function ' CheckValidity

30

31 ' handles Open File... Button’s Click event

32 Private Sub btnOpenFile_Click(ByVal sender As System.Object, _

33 ByVal e As System.EventArgs) Handles btnOpenFile.Click

34

35 ' display Open dialog

36 Dim result As DialogResult = objOpenFileDialog.ShowDialog

37

38 ' exit event handler if user clicked Cancel Button

39 If result = DialogResult.Cancel Then

40 Return

41 End If

42

43 ' get specified filename

44 Dim strFileName As String = objOpenFileDialog.FileName

45


Retrieve user input from Open dialog

Storing filename entered by user

Outline30


46 ' show error if user specified invalid file

47 If CheckValidity(strFileName) = True Then

48

49 ' append data to file via StreamWriter

50 m_objOutput = New StreamWriter(strFileName, True)

51 End If

52

53 End Sub ' btnOpenFile_Click

54

55 ' clear TextBoxes and reset NumericUpDown control

56 Private Sub ClearUserInput()

57 updDay.Value = 1

58 txtPrice.Clear()

59 txtEvent.Clear()

60 txtDescription.Clear()

61 End Sub ' ClearUserInput

62

63 ' handles Enter Button’s Click event

64 Private Sub btnEnter_Click(ByVal sender As System.Object, _

65 ByVal e As System.EventArgs) Handles btnEnter.Click

66


Create StreamWriter object to associate a stream with the user-specified text file

Outline31


67 ' using StreamWriter to write data to file

68 m_objOutput.WriteLine(updDay.Text)

69 m_objOutput.WriteLine(dtpTime.Text)

70 m_objOutput.WriteLine(txtPrice.Text)

71 m_objOutput.WriteLine(txtEvent.Text)

72 m_objOutput.WriteLine(txtDescription.Text)

73 ClearUserInput() ' prepare GUI for more user input

74 End Sub ' btnEnter_Click

75

76 ' handles Close File Button’s Click event

77 Private Sub btnCloseFile_Click(ByVal sender As System.Object, _

78 ByVal e As System.EventArgs) Handles btnCloseFile.Click

79

80 m_objOutput.Close() ' close StreamWriter

81

82 ' allow user to open another file

83 btnOpenFile.Enabled = True

84 btnEnter.Enabled = False

85 btnCloseFile.Enabled = False

86 End Sub ' btnCloseFile_Click

87

88 End Class ' FrmWriteEvent


Append data to end of file

Closing the file’s associated stream

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24.5 Building the Ticket Information Application

Action Control Event/Method Label the application’s controls lblDay,

lblTime, lblPrice, lblEvent, lblDescription

Application is run

FrmEvents Load

Display the current day’s events

mvwDate DateChanged

Display the selected day’s events

cboEvent SelectedIndexChanged

Retrieve index of selected item in the Pick an event: ComboBox

cboEvent

Display event information in the Description: TextBox

txtDescription

CreateEventList

Extract data for the current day

Clear the Pick an event: ComboBox cboEvent

Figure 24.21 ACE table for the Ticket Information application.

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Action Control Event/Method If events are scheduled for that day Add each event to the Pick an event: ComboBox

cboEvent, m_strData

DIsplay "- Events -" in the Pick an event: ComboBox

cboEvent

Display "No events today." in the Description: TextBox

txtDescription

Else DIsplay "- No Events -" in the Pick an event: ComboBox

cboEvent

Display "No events today." in the Description: TextBox

txtDescription

ExtractData

Retrieve the selected date from the calendar

mvwDate

Open calendar.txt file for reading objInput

Read the first line of the file objInput


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Action Control Event/Method While there are events left in the file and the number of events is less than 10

objInput

If the current event is for the day selected by the user Store the event information

m_strData, strLine, objInput

Increment the number of events for the selected day

Else Move to the beginning of the next record in the file

objInput

Read the next line of the file objInput


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Figure 24.22 MonthCalendar template application’s Form.

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Figure 24.23 System.IO namespace imported to FrmEvents.

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• Building the Ticket Information application– Array m_strData stores the event information read from

the file– m_intNumberOfEvents stores the number of events for a

given day

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Figure 24.24 Instance variables declared in the Ticket Information application.

Creating an array of Strings

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Figure 24.25 Load event handler calling method CreateEventList.

You will add code to CreateEventList later

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Figure 24.26 MonthCalendar’s DateChanged event handler.

Calling the CreateEventList

method

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• CreateEventList– ExtractData is passed the selected date to do two actions

• Store event information in array m_strData• assign the number of events scheduled for the specified date to m_intNumberOfEvents

– Informs user whether there are events scheduled for a given day

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Figure 24.27 CreateEventList modified to call method ExtractData and clear the ComboBox.

You will add code to ExtractData in the

next box

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Figure 24.28 Displaying the events scheduled for the specified day.

Extracting event name from array and displaying it in the

ComboBox

Indicating that events are scheduled

for the day

Indicating that no events are scheduled

for the day

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• ExtractData used to read from a sequential file– A Do While…Loop is used to ensure that there are still

characters on the file– For every entry in the file, the date is read by StreamReader and then converted to the Integer intFileDay

– If the date read from the file is the same as the specified date, then the rest of the event information will be read

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Figure 24.29 ExtractData method’s variable declarations.

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Figure 24.30 Using StreamReader to read data from a sequential-access file.

Creating a StreamReader

object to read the calendar.txt file

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Figure 24.31 Extracting the day from an event entry in the file.

Verify that end of file has not been reached

and less than 10 events are stored in the array

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Figure 24.32 Sequentially reading event entries from the file.

Store event information in a row of the array

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Figure 24.33 cboEvent_SelectedIndexChanged defined to display event information.

Displaying event information

in the TextBox.

Outline50


1 Imports System.IO

2

3 Public Class FrmEvents

4 Inherits System.Windows.Forms.Form

5

6 ' stores information for up to 10 events

7 Private m_strData As String(,) = New String(9, 4) {}

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9 ' number of events on a given day

10 Private m_intNumberOfEvents As Integer

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12 ' Windows Form Designer generated code

13

14 ' populates ComboBox with current day’s events (if any)

15 Private Sub CreateEventList()

16

17 Dim intCount As Integer ' counter

18

19 ' stores event information in array m_strData

20 ' and assigns number of events to m_intNumberOfEvents

21 ExtractData(mvwDate.SelectionStart)

22

23 ' remove any items in ComboBox

24 cboEvent.Items.Clear()

25

TicketInformation-.vb(1 of 6)

Importing namespace System.IO

Outline51



26 ' add each new event name to ComboBox

27 If m_intNumberOfEvents > 0 Then

28

29 For intCount = 0 To m_intNumberOfEvents - 1

30

31 ' extract and display event name

32 cboEvent.Items.Add(m_strData(intCount, 3))

33 Next

34

35 ' inform user that events are scheduled

36 cboEvent.Text = " - Events - "

37 txtDescription.Text = "Pick an event."

38

39 ' inform user that no events are scheduled

40 Else

41 cboEvent.Text = " - No Events - "

42 txtDescription.Text = "No events today."

43 End If

44

45 End Sub ' CreateEventList

46

47 ' extracts event data for a specified day from calendar.txt

48 ' and returns number of events during that day

49 Private Sub ExtractData(ByVal dtmDay As Date)

50

Outline52


51 ' set to selected date in MonthCalendar control

52 Dim intChosenDay As Integer = dtmDay.Day

53 Dim intFileDay As Integer ' day of event from file

54 Dim intLineNumbers As Integer ' counts lines to skip

55

56 m_intNumberOfEvents = 0 ' set number of events to 0

57

58 ' initialize StreamReader to read lines from file

59 Dim objInput As StreamReader = New StreamReader("calendar.txt")

60

61 ' read first line before entering loop

62 Dim strLine As String = objInput.ReadLine

63

64 ' loop through lines in file

65 Do While objInput.Peek > -1 AndAlso m_intNumberOfEvents < 10

66

67 intFileDay = Convert.ToInt32(strLine) ' extract day

68


Creating StreamReader object

Using method ReadLine to read the first line of the file

Ensuring that the end of the file has not been reached

Outline53


69 ' if event scheduled for specified day, store information

70 If intFileDay = intChosenDay Then

71 m_strData(m_intNumberOfEvents, 0) = strLine

72 m_strData(m_intNumberOfEvents, 1) = objInput.ReadLine




76 m_intNumberOfEvents += 1

77 Else

78

79 ' skip to next event in file

80 For intLineNumbers = 0 To 3

81 strLine = objInput.ReadLine

82 Next

83

84 End If

85

86 ' read day of next event in file

87 strLine = objInput.ReadLine

88 Loop ' End Do While

89

90 End Sub ' ExtractData


Reading information from a file and storing the data in an array

Using method ReadLine to skip to the next event in the file

Using method ReadLine to read the day of the next event

Outline54


91

92 ' handles Form’s Load event

93 Private Sub FrmEvents_Load(ByVal sender As System.Object, _

94 ByVal e As System.EventArgs) Handles MyBase.Load

95

96 ' display any events scheduled for today in ComboBox

97 CreateEventList()

98 End Sub ' FrmEvents_Load

99

100 ' handles MonthCalendar’s DateChanged event

101 Private Sub mvwDate_DateChanged(ByVal sender As System.Object, _

102 ByVal e As System.Windows.Forms.DateRangeEventArgs) _

103 Handles mvwDate.DateChanged

104

105 ' display any events for the specified date in ComboBox

106 CreateEventList()

107 End Sub ' mvwDate_DateChanged

108

109 ' handles ComboBox’s SelectedIndexChanged event

110 Private Sub cboEvent_SelectedIndexChanged(ByVal sender As _

111 System.Object, ByVal e As System.EventArgs) _

112 Handles cboEvent.SelectedIndexChanged


Outline55


113

114 ' place time, price and description of event in TextBox

115 txtDescription.Text = m_strData(cboEvent.SelectedIndex, 1)

116 txtDescription.Text &= ControlChars.CrLf

117 txtDescription.Text &= "Price: $" & _

118 m_strData(cboEvent.SelectedIndex, 2)

119 txtDescription.Text &= ControlChars.CrLf

120 txtDescription.Text &= m_strData(cboEvent.SelectedIndex, 4)

121 End Sub ' cboEvent_SelectedIndexChanged

122

123 End Class ' FrmEvents


© Copyright 1992-2004 by Deitel & Associates, Inc. and Pearson Education Inc. All Rights Reserved....

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Transcript of © Copyright 1992-2004 by Deitel & Associates, Inc. and Pearson Education Inc. All Rights Reserved....