Chapter 17 Files and Streams

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Chapter 17 Files and Streams
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Outline
17.1 Introduction17.2   Data Hierarchy17.3   Fil
es and Streams17.4   Classes File and
Directory17.5   Creating a Sequential-Access
File17.6   Reading Data from a Sequential-Access
File17.7   Random-Access Files17.8   Creating a
Random-Access File17.9   Writing Data Randomly
to a Random-Access File17.10   Reading Data
Sequentially from a Random-Access
File17.11   Case Study A Transaction-Processing
Program
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17.1 Introduction

• Variables and arrays
• Only temporary
• Variable goes out of scope
• Program terminates
• Files
• Long term storage
• Persistent data in Secondary Storage Devices
• In this chapter, we explain how to create, update
  and process data files in Visual Basic programs.
• We consider both sequential-access files and
  random-access files.

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17.2   Data Hierarchy

• Data Hierarchy
• Bit (Binary Digit)
• Either zero or one.
• All data reduced to combinations of bits for
  processing.
• Byte
• Eight bits
• Character
• Two bytes
• Character set
• Set of all characters used to program and
  represent data on a particular computer (Digits,
  letters and special symbols)

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17.2   Data Hierarchy

• Field
• Group of characters that convey a meaning.
• For example, a field consisting of uppercase and
  lowercase letters can represent a persons name.
• Record
• Group of several, related fields.
• In a payroll system, for example, a record for a
  particular employee might include the following
  fields (Employee ID, Name, Address, Hourly pay
  rate).
• File
• Group of related records
• Record key
• Identifies record to a particular entity.
• Sequential file
• Records stored in order of record-key.

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17.2   Data Hierarchy
Fig. 17.1 Data hierarchy.
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17.3   Files and Streams

• To perform file processing in Visual Basic,
  namespace System.IO must be referenced.
• This namespace includes definitions for stream
  classes such as
• FileStream we use it to read data to, and write
  data from, sequential-access and random-access
  files.
• StreamReader reads from a text file.
• StreamWriter writes to a text file.
• BinaryReader reads from a binary file.
• BinaryWriter writes to a binary file.
• Files are opened by creating objects of these
  stream classes.

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17.4   Classes File and Directory

• Directory
• Used to organize files
• The \ separator character
• Used to separate directories and files in a path
• C\VisualBasic\MyFile
• Class File
• Used to manipulate and determine information
  about files
• Cannot write to or read from files directly.
• Class Directory
• Used to manipulate directories

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17.4   Classes File and Directory
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17.4   Classes File and Directory
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• 1 ' Fig 17.5 FileTest.vb
• 2 ' Using classes File and Directory.
• 3
• 4 Imports System.IO
• 5 Imports System.Windows.Forms
• 6
• 7 Public Class FrmFileTest
• 8 Inherits Form
• 19 ' invoked when user presses key
• 20 Protected Sub txtInput_KeyDown(ByVal
  sender As Object, _
• 21 ByVal e As System.Windows.Forms.KeyEven
  tArgs) Handles _
• 22 txtInput.KeyDown
• 23
• 24 ' determine whether user pressed Enter
  key
• 25 If e.KeyCode Keys.Enter Then
• 26
• 27 Dim fileName As String ' name of
  file or directory
• 28
• 29 ' get user-specified file or
  directory

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• 35 ' get file's creation date,
  modification date, etc.
• 36 txtOutput.Text
  GetInformation(fileName)
• 37
• 38 ' display file contents through
  StreamReader
• 39 Try
• 40
• 41 ' obtain reader and file
  contents
• 42 Dim stream As StreamReader
• 43 stream New
  StreamReader(fileName)
• 44 txtOutput.Text
  stream.ReadToEnd()
• 45
• 46 ' handle exception if
  StreamReader is unavailable
• 47 Catch exceptionCatch As
  IOException
• 48
• 49 ' display error
• 50 MessageBox.Show("FILE ERROR",
  "FILE ERROR", _
• 51 MessageBoxButtons.OK,
  MessageBoxIcon.Error)
• 52
• 53 End Try

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• 60
• 61 ' get directory's creation date,
  modification date, etc
• 62 txtOutput.Text
  GetInformation(fileName)
• 63
• 64 ' obtain directory list of
  specified directory
• 65 directoryList
  Directory.GetDirectories(fileName)
• 66
• 67 txtOutput.Text vbCrLf
  vbCrLf _
• 68 "Directory contents"
  vbCrLf
• 69
• 70 ' output directoryList contents
• 71 For i 0 To directoryList.Length
  - 1
• 72 txtOutput.Text
  directoryList(i) vbCrLf
• 73 Next
• 74
• 75 ' notify user that neither file nor
  directory exists
• 76 Else
• 77 MessageBox.Show(txtInput.Text
  " does not exist", _
• 78 "FILE ERROR",
  MessageBoxButtons.OK, _

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• 86 ' get information on file or directory
• 87 Private Function GetInformation(ByRef
  fileName As String) _
• 88 As String
• 89
• 90 Dim information As String
• 91
• 92 ' output that file or directory exists
• 93 information fileName " exists"
  vbCrLf vbCrLf
• 94
• 95 ' output when file or directory was
  created
• 96 information "Created " _
• 97 File.GetCreationTime(fileName)
  vbCrLf
• 98
• 99 ' output when file or directory was
  last modified
• 100 information "Last modified " _
• 101 File.GetLastWriteTime(fileName)
  vbCrLf
• 102
• 103 ' output when file or directory was
  last accessed
• 104 information "Last accessed " _

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Demo
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17.5 Creating a Sequential-Access File

• To support file processing, the .NET Framework
  provides the System.IO namespace that contains
  many different classes to handle almost any type
  of file operation you may need to perform.
• The parent class of file processing is
  FileStream. With FileStream, you can use a
  FileStream object to get a stream ready for
  processing.
• A stream is an abstraction of a sequence of
  bytes, such as a file.

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17.5 Creating a Sequential-Access File

• Using FileStream Class
• The FileStream class gives the user the
  capability to read from, write to, open, and
  close files on a file system.
• This class provides access to standard input and
  output files.
• As one of the most complete classes of file
  processing of the .NET Framework, FileStream is
  equipped with all necessary properties and
  methods. To use it, you must first declare a
  variable of it.

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17.5 Creating a Sequential-Access File

• FileStream class has 9 constructors the one we
  will the most is
• FileStream(String, FileMode, FileAccess)
  
• Initializes a new instance of the FileStream
  class with the specified path, creation mode,
  read/write permission.
• We use the members of FileAccess and FileMode
  Enumerations with the constructors of this class
  to create or open a file.

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17.5 Creating a Sequential-Access File

• FileMode
• The members of the FileMode Enumerator are
• FileMode.Append If the file already exists, the
  new data will be added to its end. If the file
  doesn't exist, it will be created and the new
  data will be added to it
• FileMode.Create If the file already exists, it
  will be deleted and a new file with the same name
  will be created. If the file doesn't exist, then
  it will be created
• FileMode.CreateNew If the file already exists,
  the compiler will throw an error. If the file
  doesn't exist, it will be created
• FileMode.Open If the file exists, it will be
  opened. If the file doesn't exist, an error would
  be thrown
• FileMode.OpenOrCreate If the file already
  exists, it will be opened. If the file doesn't
  exist, it will be created
• FileMode.Truncate If the file already exists,
  its contents will be deleted completely but the
  file will be kept, allowing you to write new data
  to it. If the file doesn't exist, an error would
  be thrown.

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17.5 Creating a Sequential-Access File

• FileAccess
• In order to perform an operation on a file, you
  must specify to the operating system how to
  proceed. One of the options you have is to
  indicate the type of access that will be granted
  on the file. This access is specified using the
  FileAccess enumerator.
• The members of the FileAccess enumerator are
• FileAccess.Write New data can be written to the
  file.
• FileAccess.Read Existing data can be read from
  the file.
• FileAccess.ReadWrite Existing data can be read
  from the file and new data be written to the
  file.

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17.5 Creating a Sequential-Access File

• After a file is opened it's FileStream object can
  be passed to a BinaryReader, BinaryWriter,
  StreamReader and StreamWriter classes to work
  with the data in the file.
• StreamReader is useful for reading lines of
  information from standard text file and
  StreamWriter is useful for writing to standard
  text files.
• BinaryWriter class is used to write binary data
  to a file. The BinaryReader is useful to us for
  reading the data from the BinaryFile.

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17.5 Creating a Sequential-Access File

• Stream Writing
• A streaming operation is typically used to
  create a stream. Once the stream is ready, you
  can write data to it. The writing operation is
  performed through various classes such as
  StreamWriter, BinaryWriter.
• Stream Reading
• As opposed to writing to a stream, you may
  want to read existing data from it. The Reading
  operation is performed through various classes
  such as StreamReader, BinaryReader.

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17.5 Creating a Sequential-Access File

• Stream Closing
• When you use a stream, it requests resources from
  the operating system and uses them while the
  stream is available.
• When you are not using the stream anymore, you
  should free the resources and make them available
  again to the operating system so that other
  services can use them.
• This is done by closing the stream. To close a
  stream, you can call the Close() method of the
  class(es) you were using.
• Before calling Close(), call the class' Flush()
  method to clear the memory areas that the object
  was using when performing its operations.

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StreamWriter Class

• 1. Declare FileStream object and create a text
  file named file.txt with access mode of writing
• Dim fs As New FileStream("file.txt",
  FileMode.Create, FileAccess.Write)
• 2. Create a new StreamWriter and pass the
  filestream object fs as argument
• Dim sw As New StreamWriter(fs)
• 3. Write text to the created file using the
  WriteLine or Write methods
• sw.WriteLine("This is an example of using
  file handling in VB .NET.")
• sw.WriteLine("This concept is
  interesting.")
• 4. Close the file
• sw.Close() 'closing the file
• fs.Close() closing the stream

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StreamWriter Class

• The default location where the files we create
  are saved is the bin directory of the Windows
  Application with which we are working.

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StreamReader

• 1. Declare FileStream object to open the file
  named file.txt with access mode of reading
• Dim fs As New FileStream("file.txt",
  FileMode.Open, FileAccess.Read)
• 2. Create a new StreamReader and pass the
  filestream object fs as argument
• Dim sr As New StreamReader(fs)
• 3. Peek method of StreamReader object tells how
  much more data is left in the file
• While sr.Peek() -1
• 'displaying text from text file in a
  RichTextBox for instance
• rtbFile.Text sr.ReadLine()
• End While
• 4. Close the file
• sr.Close() 'closing the file
• fs.Close() closing the stream

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17.6  Reading Data from a Sequential-Access File

• The StreamReader class supports several methods
  for reading text files and offers a way of
  determining whether you are at the end of the
  file
• Line-by-line ? using ReadLine method.
• An entire file? using ReadToEnd method.
• One character at a time ? using Read method.

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Example Working with sequential access file
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Demo
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17.6  Reading Data from a Sequential-Access File

• Line-by-line
• We can read each line with a ReadLine method.
• To determine whether we have reached the end of
  the file, we call the Peek method of the
  StreamReader object.
• The Peek method reads the next character in the
  file without changing the place that we are
  currently reading. If we have reached the end of
  the file, Peek returns -1.

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17.6  Reading Data from a Sequential-Access File

• An entire file
• You can also read an entire text file from
  the current position to the end of the file by
  using the ReadToEnd method.
• One character at a time If you need to read the
  file a character at a time, you can use the Read
  method. This method returns the integer character
  value of each character read. Following example
  demonstrates how to use the Read method

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17.6  Reading Data from a Sequential-Access File

• Dim intSingleChar as Integer Dim
  cSingleChar as String While sr.Peek
  -1  intSingleChar sr.Read()  ' Convert the
  integer value into a character  cSingleChar
  Chr(intSingleChar) End While

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17.6  Reading Data from a Sequential-Access File

• To retrieve data sequentially from a file,
  programs normally start from the beginning of the
  file, reading data consecutively until the
  desired data are found.
• A FileStream object can reposition its
  file-position pointer (which contains the byte
  number of the next byte to be read from or
  written to the file) to any position in the
  filewe show this feature when we introduce
  random-access file-processing applications.
• When a FileStream object is opened, its
  file-position pointer is set to zero (i.e., the
  beginning of the file)