T U T O R I A L 2009 Pearson Education, Inc. All rights reserved. 1 23 Ticket Information Application Introducing Sequential-Access Files

T U T O R I A L

2009 Pearson Education, Inc. All rights reserved.

1

23Ticket Information

ApplicationIntroducing

Sequential-Access Files


2

Outline

23.1 Test-Driving the Ticket Information Application

23.2 Data Hierarchy

23.3 Files and Streams

23.4 Writing to a File—Creating the Write Event Application

23.5 Building the Ticket Information Application

23.6 Using LINQ and Class File to ExtractData from a Text File


3

In this tutorial you will learn: ■ 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.■ Use the OpenFileDialog component.■ Add and configure a MonthCalendar control.■ Use LINQ to query a sequential-access file.

Objectives


4

■ A file is a collection of data that is given a name, such as data.txt or Welcome.sln.

■ Data in files exists even after the application that created the data terminates—such data is called persistent data.

■ Sequential-access files contain information that is read from a file in the order in which it was originally written to the file.

■ This application allows the user to create or open a text file with information.

Introduction

Application Requirements


5

23.1 Test-Driving the Ticket Information Application

A local town has asked you to write an application that allows its residents to view community events for the current month. 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 one. When the user selects an event, the application must display its time and price and a brief description of the event. The community-event information is stored in a sequential-access file named calendar.txt.


6

■ Run the completed Ticket Information application (Fig. 23.1).

■ The MonthCalendar control should show theday and month on which you actually run the application.

Test-Driving the Ticket Information Application

Figure 23.1 | Ticket Information application’s GUI.

Arrow buttons allow userto scroll through months

MonthCalendar control

ComboBox lists any events

TextBox displays event details


7

■ Select the 13th in the MonthCalendar. Note that the ComboBox displays "- No Events -" (Fig. 23.2).

■ Select the 19th. Click the ComboBox to view the scheduled events, then select an event to view its information.

Test-Driving the Ticket InformationApplication (Cont.)

Figure 23.2 | Ticket Information application displaying event information.

13th day of the month selected

No events displayed

19th day of the month

selected

Event information

displayed


8

■ Data items processed by computers form a data hierarchy (Fig. 23.3).

■ Digits, letters and special symbols are referred to as characters.

■ The set of all characters representing data items on a particular computer is called that computer’s character set.

23.2 Data Hierarchy


9

Figure 23.3 | Data hierarchy.

23.2 Data Hierarchy (Cont.)


10

■ The smallest data item is called a bit—a digit that can hold only 0 or 1.

■ Computer circuitry performs various simple bit manipulations such as:

– examining the value of a bit

– setting the value of a bit

– reversing the value of a bit

■ Bytes are composed of eight bits.

■ Characters in Visual Basic are Unicode characters, which are composed of two bytes.



11

■ Fields are composed of characters and convey meaningful information.

■ A record is a collection of several related fields.– A payroll system, for example, would have a record for

each employee, with several information fields.

■ A group of related records is stored in a file. A payroll file for a company might contain thousands of employee records.



12

■ A record key identifies a record as belonging to a particular person or entity.

– The record key must be unique.

– In the payroll record just described, the employee identification number normally would be chosen as the record key.

■ The most common type of organization is called a sequential-access file, in which records are stored in order.

■ Sometimes a group of related files is called a database.



13

■ Files are viewed as sequential streams of bytes (Fig. 23.4).

■ When a file is opened, Visual Basic creates an object and associates a stream with that object:

– StreamReader takes text input from a file.

– StreamWriter outputs text to a file.

23.3 Files and Streams

Figure 23.4 | Visual Basic’s conceptual view of an n-byte file.


14

Common Programming Error

Attempting to open a file from multiple programs at once (or even the same program) is a logic error. A file can be opened by only one program at a time.


15

■ Open the WriteEvent template application (Fig. 23.5).

■ Add an OpenFileDialog component from the All Windows Forms category of the Toolbox.

■ Name it openFileDialog, and change its FileName property to calendar.txt.

■ Set property CheckFileExists to False so that if the user specifies a nonexistent file, that file is created.

Adding a Dialog to Open or Create a File


16

Figure 23.5 | openFileDialog added and renamed.

OpenFileDialog component

Adding a Dialog to Open or Create a File (Cont.)


17

Determining Whether a File Name Is Valid

■ Method CheckValidity (Fig. 23.6) receives a file name as a String.

■ If the file name is valid, the Function returns True.

Figure 23.6 | Method CheckValidity header.

CheckValidity Function procedure header


18

Determining Whether a File Name Is Valid (Cont.)

■ String method EndsWith (Fig. 23.7) returns False if the value of variable name does not end with .txt.

■ In this case, a MessageBox is displayed, informing the user that the application expects a text file.

Figure 23.7 | Displaying an error message indicating an invalid file name.

Displaying error message if incorrect file type is provided


19

Determining Whether a File Name Is Valid (Cont.)

■ After the user enters a file name, line 13 (Fig. 23.8) disables the Open File... Button.

■ Lines 14–15 enable the Enter and Close File Buttons.

Figure 23.8 | Changing the GUI if a valid file name is entered.

Enabling and disabling Buttons


20

Creating a StreamWriter Object

■ To access the classes that enable you to perform file processing, import namespace System.IO (Fig. 23.9).

Figure 23.9 | System.IO namespace imported into class WriteEventForm.

Importing namespace System.IO


21

Creating a StreamWriter Object (Cont.)

■ Namespace System.IO includes class StreamWriter, which is used to create objects for writing text to a file (Fig. 23.10).

Figure 23.10 | Declaring a StreamWriter variable.

Declaring StreamWriter variable


22

■ Double click the Open File... Button to create the empty event handler (Fig. 23.11).

■ The ShowDialog method displays the Open dialog to allow the user to open a file, and returns a value of type DialogResult.


Figure 23.11 | Displaying the Open dialog and retrieving the result.

Displaying Open dialog

If the user clicks Cancel, the

event handler exits without performing any actions


23


■ Property FileName of OpenFileDialog (Fig. 23.12) specifies the full path of the file the user selected.

Figure 23.12 | Retrieving the name and path of selected file.

Setting variable to user-specified file name


24

■ Method CheckValidity determines whether the specified file is a text file (that is, the file name ends with ".txt").

■ The StreamWriter constructor takes two arguments.

– The first indicates the name of the file to which you write information.

– The second is a Boolean value that determines whether the StreamWriter appends information to the end of the file.



25


Figure 23.13 | Validating the filename and initializing a StreamWriter object.

Check for valid filename

Create StreamWriter object


26

Common Programming Error

When you open an existing file by invoking the StreamWriter constructor with a False second argument, data previously contained in the file is lost.


27

Writing Information to a Sequential-Access File

■ Method ClearUserInput (Fig. 23.14) clears the TextBoxes and resets the NumericUpDown control’s value.

Figure 23.14 | Clearing user input.

Clearing user input


28

■ Double click the Enter Button to create its event handler (Fig. 23.15).

■ Write the user input to the file by using the Write method. – Each field is separated by a Tab character.

■ The WriteLine method writes its argument to the file, followed by a newline character.

Writing Information to aSequential-Access File (Cont.)

Figure 23.15 | StreamWriter writing to a file.

Writing information to a file


29

■ Double click the Close File Button to create its event handler (Fig. 23.16).

■ The StreamWriter’s Close method closes the stream.

– You should always close the file after you’ve finished processing it to ensure that you don’t lose any data.

Closing the StreamWriter

Figure 23.16 | Closing the StreamWriter.

Closing StreamWriter object


30

Writing Event Information to a File

■ Run your application (Fig. 23.17).

Figure 23.17 | Write Event application running.


31

■ Click the Open File... Button and open the existing calendar.txt file from:

– C:\Examples\Tutorial23\TemplateApplication\TicketInformation\TicketInformation\bin\Debug (Fig. 23.18).

Writing Event Information to a File (Cont.)

Figure 23.18 | Open dialog displaying contents of the templateTicket Information application’s Debug folder.


32

■ Enter event information, then click Enter to add an event to the calendar.txt file.

■ When you have entered all the events you wish, click the Close File Button.

■ Use the IDE to open calendar.txt (Fig. 23.19).

Figure 23.19 | Sequential-access file generated by Write Event application.

Writing Event Information to a File (Cont.)

Day and time of event,

ticket price, eventname and description


33

1 Imports System.IO

2

3 Public Class WriteEventForm

4

5 Private output As StreamWriter

6

7 ' determine validity of file type

8 Private Function CheckValidity(ByVal name As String) As Boolean

9 ' show error if user specified invalid file

10 If name.EndsWith(".txt") = False Then

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

12 "Invalid File Type", _

13 MessageBoxButtons.OK, MessageBoxIcon.Error)

14 Return False ' to indicate invalid file type

15 ' change status of Buttons if file name is valid

16 Else

17 openFileButton.Enabled = False

18 enterButton.Enabled = True

19 closeFileButton.Enabled = True

20 Return True ' to indicate text file specified

21 End If

22 End Function ' CheckValidity

■ Figure 23.20 presents the source codefor the Write Event application.

Outline

System.IO

(1 of 4 )

Importing namespace System.IO

StreamWriter used towrite text to a file


34

23

24 ' handles Open File... Button's Click event

25 Private Sub openFileButton_Click(ByVal sender As System.Object, _

26 ByVal e As System.EventArgs) Handles openFileButton.Click

27

28 ' display Open dialog

29 Dim result As DialogResult = openFileDialog.ShowDialog()

30

31 ' open specified file if user did not click Cancel Button

32 If result <> Windows.Forms.DialogResult.Cancel Then

33 ' get specified filename

34 Dim fileName As String = openFileDialog.FileName

35

36 ' validate file name

37 If CheckValidity(fileName) = True Then

38 ' enable user to append data to file via StreamWriter

39 output = New StreamWriter(fileName, True)

40 End If

41 End If

42 End Sub ' openFileButton_Click

Outline

System.IO

(2 of 4 )

Retrieve user input from Open dialog

Storing filename entered by user

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


35

43

44 ' clear TextBoxes and reset NumericUpDown control

45 Private Sub ClearUserInput()

46 dayUpDown.Value = 1

47 priceTextBox.Clear()

48 eventTextBox.Clear()

49 descriptionTextBox.Clear()

50 End Sub ' ClearUserInput

51

52 ' handles Enter Button’s Click event

53 Private Sub enterButton_Click(ByVal sender As System.Object, _

54 ByVal e As System.EventArgs) Handles enterButton.Click

55

56 ' using StreamWriter to write data to file

57 output.Write(dayUpDown.Value & ControlChars.Tab)

58 output.Write(timeDateTimePicker.Text & ControlChars.Tab)

59 output.Write(priceTextBox.Text & ControlChars.Tab)

60 output.Write(eventTextBox.Text & ControlChars.Tab)

61 output.WriteLine(descriptionTextBox.Text)

62 ClearUserInput() ' prepare GUI for more user input

63 End Sub ' enterButton_Click

Outline

System.IO

(3 of 4 )

Append data to end of file

Append data to end of file


36

64

65 ' handles Close File Button’s Click event

66 Private Sub closeFileButton_Click(ByVal sender As System.Object, _

67 ByVal e As System.EventArgs) Handles closeFileButton.Click

68

69 output.Close() ' close StreamWriter

70

71 ' allow user to open another file

72 openFileButton.Enabled = True

73 enterButton.Enabled = False

74 closeFileButton.Enabled = False

75 End Sub ' closeFileButton_Click

76 End Class ' WriteEventForm

Outline

System.IO

(4 of 4 )

Closing the file’s associated stream


37

When the Form loads:

Display the current day’s events

When the user selects a date on the calendar:

Display the selected day’s events

When the user selects an event from the Pick an event: ComboBox:

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

Display event information in the Description: TextBox

When procedure CreateEventList is called:

Extract data for the selected day from calendar.txt

Clear the Pick an event: ComboBox

23.5 Building the Ticket Information Application


38

If events are scheduled for that day

Add each event to the Pick an event: ComboBox

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

Display "Pick an event." in the Description: TextBox

Else

Display "- No Events -" in the Pick an event: ComboBox

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

When procedure ExtractData is called:Clear the community events collection

Open calendar.txt file for reading

Until there are no events left in the file

Read the next line of the file

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

Store the event information

23.5 Building the Ticket InformationApplication (Cont.)


39

Action Control Event/Method

Label the application’s controls dateLabel, eventLabel, descriptionLabel

Application is run

TicketInfor-mationForm

Load

Display the current day’s events eventComboBox

dateMonthCalendar DateChanged

Display the selected day’s events eventComboBox, descriptionTextBox

■ Use an ACE table to convert the pseudocode into Visual Basic (Figure 23.21).

Figure 23.21 | ACE table for the Ticket Information application. (Part 1 of 3.)

Action/Control/Event (ACE) Table forthe Ticket Information Application


40


eventComboBox Selected-IndexChanged

Retrieve index of selected item in the

Pick an event: ComboBox

eventComboBox

Display event information in the

Description: TextBox

descriptionTextBox

CreateEventList

Extract data for the selected day from

calendar.txt

dateMonthCalendar

Clear the Pick an event: ComboBox eventComboBox

I f events are scheduled for that day

Add each event to the Pick an event:

ComboBox

eventComboBox, communityEvents

Display "- Events - " in the Pick an

event: ComboBox

eventComboBox


Action/Control/Event (ACE) Table forthe Ticket Information Application (Cont.)


41


Display "Pick an event." in the


descriptionTextBox CreateEventList

Else

Display "- No Events - " in the Pick an

event: ComboBox

eventComboBox

Display "No events today." in the


descriptionTextBox

ExtractData

Clear the community events collection communityEvents

Open calendar.txt file for reading input

Until there are no events left in the file input

Read the next line of the file input

I f the current event is for the day

selected by the user

Store the event information

communityEvents


Action/Control/Event (ACE) Table forthe Ticket Information Application (Cont.)


42

■ Open the TicketInformation template application and add the MonthCalendar control (Fig. 23.22).

■ Change its Name to dateMonthCalendar.

Adding a MonthCalendar Control

Figure 23.22 | Ticket Information template application’s Form.


43

Good Programming Practice

Append the MonthCalendar suffix to MonthCalendar control names.


44

■ Import namespace System.IO, which allows the application to use class StreamReader (Fig. 23.23).

Figure 23.23 | Importing the System.IO namespace.

Beginning to Build the TicketInformation Application


45

■ Store the event information read from the file in a List(Of CommunityEvent) named communityEvents (Fig. 23.24).

■ In the Solution Explorer, right click the TicketInformation project.

– Select Add > Existing Item...

– Select the CommunityEvent.vb file and click Add.

Beginning to Build the TicketInformation Application (Cont.)

Figure 23.24 | List declared to hold event information.

Creating a List of CommunityEvents


46

■ Double click the Form to generate its event handler (Fig. 23.25).

Figure 23.25 | Load event handler calling method CreateEventList.

Handling the Form’s Load Event

You add code to the CreateEventList

procedure later


47

■ Double click the MonthCalendar to generate the empty DateChanged event handler (Fig. 23.26).

Figure 23.26 | dateMonthCalendar’s DateChanged event handler.

Handling the MonthCalendar’sDateChanged Event

Calling method CreateEventList


48

■ The CreateEventList method (Fig. 23.27) declares currentEvent to iterate through the events.

– The currently selected date is specified by the MonthCalendar control’s SelectionStart property, which is the first date in any range selected.

Defining the CreateEventList Method

Figure 23.27 | CreateEventList calls method ExtractData andclears the ComboBox.

You add code to the ExtractData procedure

in the next box


49

■ The CreateEventList method (Fig. 23.28) has a For Each...Next statement.

– The statement iterates through List communityEvents and adds the name of each event to the ComboBox.

Defining the CreateEventList Method (Cont.)

Figure 23.28 | Displaying the events scheduled for the specified day.

Extracting event name from currentEvent and displaying

it in the ComboBox

Indicating that events are scheduled for the day

Indicating that no events are scheduled for the day


50

■ The Date selected in the MonthCalendar control is passed to the ExtractData method (Fig. 23.29).

■ Variable chosenDay is the selected day returned by the Day property of the currentDate parameter.

■ The eventInfo variable is an array of Strings used to store the event information retrieved from the file.

Reading a Sequential-Access File

Figure 23.29 | ExtractData method’s variable declarations.


51

■ ExtractData creates a new StreamReader object (Fig. 23.30), passing the name of the file to be read.

Figure 23.30 | Initializing the StreamReader used to read data froma sequential-access file.

Creating a StreamReader object to read the calendar.txt file

Reading a Sequential-Access File (Cont.)


52

■ The Do Until...Loop statement (Fig. 23.31) determines whether the end of the file has been reached.

■ The ReadLine method reads one line of text from the specified stream.

■ String method Split, splits the String at each Tab character, and assigns the resulting array of Strings to array eventInfo.

– This character is called a delimiter and is used to mark the boundaries between fields.



53

Figure 23.31 | Extracting the day from an event entry in the file.

Verify that the end of thefile has not been reached


Read a line of text from the file

Split the line of text into an array of Strings containing

each field in the record


54

■ If the day of the event read from the file and the specified day are the same, then the event information is stored in a CommunityEvent object (Fig. 23.32).


Figure 23.32 | Storing event information in the List of CommunityEvents.

Store event information in a CommunityEvent object

Add the event to the List


55

■ Double click the Pick an event: ComboBox to generate its event handler (Fig. 23.33).

– The SelectedIndex property of the ComboBox returns the index number of the selected event.

Handling the SelectedIndexChanged Event

Displaying event information in the Description: TextBox

Figure 23.33 | Display information for selected event.


56

■ Rewrite the ExtractData method (Fig. 23.34).

Figure 23.34 | Removed Do Until...Loop and local variables.

Using LINQ to Select Events From a Text File


57

■ Variable eventQuery stores the LINQ query (Fig. 23.35) used to search the data from calendar.txt.

■ The From clause specifies the range variable and the data source.

■ File method ReadAllLines returns an array of Strings in which each element is a line from the text file passed as an argument.

Using LINQ to Select Events From a Text File (Cont.)


58

Figure 23.35 | Specifying the data source.


Specifying the query’sdata source


59

■ A Let clause (Fig. 23.36) allows you to create and initialize a variable in a LINQ query.

– Line 47 declares an array of Strings named eventInfo.

■ The Where clause uses the array to determine whether the day of the event matches the selected day.

Declaring variable eventInfo inside the LINQ query

Figure 23.36 | Declaring a variable in a LINQ query.



60

■ As a LINQ query executes, it can create new objects which are returned as the query’s result (Fig. 23.37).

– The With keyword specifies that the property names used are from the new CommunityEvent object.


Figure 23.37 | Creating CommunityEvent objects in the Select clause.

Creates a new CommunityEvent object


61

■ Interface IEnumerable’s ToList method (Fig. 23.38) assigns the selected events to the List object communityEvents.

■ Method ToList returns a List of the items selected by the LINQ query.

Figure 23.38 | Assigning the query result to the List.



62

1 Imports System.IO

2

3 Public Class TicketInformationForm

4

5 ' stores events

6 Private communityEvents As _

7 New List(Of CommunityEvent)()

8

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

10 Private Sub CreateEventList()

11

12 Dim currentEvent As CommunityEvent ' control variable

13

14 ' stores event information in List for selected day

15 ExtractData(dateMonthCalendar.SelectionStart)

16

17 ' remove any items in ComboBox

18 eventComboBox.Items.Clear()

19

■ Figure 23.39 presents the sourcecode for the application.

Outline

System.IO

(1 of 5 )Importing namespace System.IO


63

20 ' add each new event name to ComboBox

21 If communityEvents.Count > 0 Then

22 For Each currentEvent In communityEvents

23 ' extract and display event name

24 eventComboBox.Items.Add(currentEvent.Name)

25 Next

26

27 ' inform user that events are scheduled

28 eventComboBox.Text = " - Events - "

29 descriptionTextBox.Text = "Pick an event."

30

31 Else ' inform user that no events are scheduled

32 eventComboBox.Text = " - No Events - "

33 descriptionTextBox.Text = "No events today."

34 End If

35 End Sub ' CreateEventList

36

Outline

System.IO

(2 of 5 )


64

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

38 Private Sub ExtractData(ByVal currentDate As Date)

39 ' set to selected date in MonthCalendar control

40 Dim chosenDay As Integer = currentDate.Day

41

42 communityEvents.Clear() ' clear the List of events

43

44 ' select the events from the file scheduled for selected day

45 Dim eventQuery = _

46 From line In File.ReadAllLines("calendar.txt") _

47 Let eventInfo As String() = line.Split(ControlChars.Tab) _

48 Where Convert.ToInt32(eventInfo(0)) = chosenDay _

49 Select New CommunityEvent With _

50 { _

51 .Day = chosenDay, _

52 .Time = eventInfo(1), _

53 .Price = Convert.ToDecimal(eventInfo(2)), _

54 .Name = eventInfo(3), _

55 .Description = eventInfo(4) _

56 } ' end LINQ query that creates CommunityEvent objects

Outline

System.IO

(3 of 5 )

Retrieve an array containing each line of calendar.txt

Split line into arrayof Strings

Check whether chosenDay is the day from the line being processed

Creating a CommunityEvent object with an object initializer


65

57

58 ' assign the selected events to the List

59 communityEvents = eventQuery.ToList()

60 End Sub ' ExtractData

61

62 ' handles Form’s Load event

63 Private Sub TicketInformationForm_Load( _

64 ByVal sender As System.Object,ByVal e As System.EventArgs) _

65 Handles MyBase.Load

66

67 ' display any events scheduled for today in ComboBox

68 CreateEventList()

69 End Sub ' TicketInformationForm_Load

70

71 ' handles MonthCalendar’s DateChanged event

72 Private Sub dateMonthCalendar_DateChanged( _

73 ByVal sender As System.Object, _

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

75 Handles dateMonthCalendar.DateChanged

76

Outline

System.IO

(4 of 5 )

Retrieve a List containing each CommunityEvent object in the query result


66

77 ' display any events for the specified date in ComboBox

78 CreateEventList()

79 End Sub ' dateMonthCalendar_DateChanged

80

81 ' handles ComboBox’s SelectedIndexChanged event

82 Private Sub eventComboBox_SelectedIndexChanged(ByVal sender As _

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

84 Handles eventComboBox.SelectedIndexChanged

85

86 ' get the event selected in the ComboBox

87 Dim selectedEvent As CommunityEvent = _

88 communityEvents(eventComboBox.SelectedIndex)

89

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

91 descriptionTextBox.Text =

92 selectedEvent.Time & ControlChars.CrLf & _

93 "Price: $" & selectedEvent.Price & ControlChars.CrLf & _

94 selectedEvent.Description

95 End Sub ' eventComboBox_SelectedIndexChanged

96 End Class ' TicketInformationForm

Outline

System.IO

(5 of 5 )

Documents

T U T O R I A L 2009 Pearson Education, Inc. All rights reserved. 1 23 Ticket Information Application Introducing Sequential-Access Files