BACS 287

BACS 287

File-Based Programming

BACS 287

Data Hierarchy

Database - Collection of files, relationships, integrity information, etc

Files - All records about the entity class Records - Collection of fields about an entity Fields - Named group of bytes associated with a

characteristic of interest Bytes - 8 bits, can store 1 character Bits - Binary Digits (1,0)

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File Processing

File processing manages data from bits up to files. (Database requires separate database management package).

File processing is most closely associated with the traditional approach to programming (i.e., procedural).

VB .NET can support the traditional approach as well as more modern approaches.

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File Processing Basics

Data files are used as a way to make sure that the results of your program are available after the program finishes.

Permanent files are stored on disks. VB directly supports 3 basic file organizations:

– Sequential – text files in continuous blocks– Random – for fixed length files (text or binary)– Binary – for arbitrarily structured files

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Sequential Data Files

Sequential data files store data in the order that it arrives.

New data is added to the ‘back’ of the file. If you want to find a specific record, you must

read all records from the start of the file. This is efficient for some applications.

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Sequential File Structure

. . . .5050505050

You must read throughthe 1st four records toget to 5th

1st 2nd 3rd 4th 5th

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StreamReaders & StreamWriters

Sequential files can be processed as a “stream” of data

StreamReaders are VB objects that read data from a stream

StreamWriters are VB objects that write data to a stream

Neither can be used before the file is opened

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Example StreamReader Statements

Creating and reading from StreamReaderDim stVar as IO.StreamReaderstVar = IO.File.OpenText(filespec)Dim StrVar = srVar.ReadLinesrVar.EndOfStreamSrVar.Close()

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StreamReaders Use

strVar = srVar.ReadLine reads the line pointed to, assigns the line to the string variable strVar, and moves the pointer to the next line of the file.

The value of srVar.EndOfStream will be True after the entire file has been read.

The statement srVar.Close() terminates communication with the file.

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StreamReader Use

If sr is a variable of type StreamReader, an entire text file can be read with a loop of the formDo while not sr.EndOfStream strVar = srVar.ReadLine . .Loop

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Example StreamWriter Statements

Creating and writing to StreamWriterDim swVar as IO.StreamWriterswVar = IO.File.CreateText(filespec)swVar.WriteLIne(strData)swVar.Close()

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StreamWriter Use

swVar.WriteLine(info) initally places the information into the first line of the file.

Subsequent statements of that form place information into lines at the end of the file.

The statement swVar.Close() terminates communication with the file.

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Text File Modes

OpenText – open for input CreateText – open for output AppendText – open for append A file should not be opened in two different

modes at the same time.

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Example StreamWriter Statements

Adding to existing file via StreamWriterIf (IO.File.Exists(filespec))…Dim swVar as IO.StreamWriterswVar = IO.File.AppendText(filespec)swVar.WriteLIne(strData)swVar.Close()

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StreamWriter Use

1. Execute the statement Dim swVar As IO.StreamWriter = _ IO.File.AppendText(filespec) where filespec identifies the file. 2. Add lines of data to the end of the file with

the WriteLine method.3. After all the data have been written into the

file, close the file with swVar.Close().Note: If the file does not exist, the AppendText method will create it.

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Avoiding Errors

Attempting to open a non-existent file for input brings up a message box titled:

FileNotFoundException There is a method to determine if a file exists

before attempting to open it: IO.File.Exists(filespec)

will return True if the file exists.

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Test for Existence of File

Dim sr As IO.StreamReaderIf IO.File.Exists(filespec) Then sr = IO.File.OpenText(filespec)Else message = "Either no file has yet been " message &= "created or the file named" message &= filespec & " is not found." MessageBox.Show(message, "File Not Found")End If

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Deleting Data from a Sequential File

An individual item of a sequential file cannot be changed or deleted directly.

A new file must be created by reading each item from the original file and recording it, with the single item changed or deleted, into the new file.

The old file is then erased, and the new file renamed with the name of the original file.

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Delete and Move Methods

Delete method: IO.File.Delete(filespec) Move method (to change the filespec of a

file): IO.File.Move(oldfilespec, newfilespec)

Note: The IO.File.Delete and IO.File.Move methods cannot be used with open files.

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Random Data Files

Random data files are stored so you can access the record of interest directly (thus, random files support direct access).

Random access files have records of fixed length. The records are defined using a special VB

command called the ‘Type Statement’.

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Random File Structure

. . . .5050505050

5th recordstarts atthe 201stbyte

1st 2nd 3rd 4th 5th

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VB Type Statement

TYPE structurenamefieldname1 AS datatypefieldname2 AS datatype...

END TYPE

TYPE employeeName as string * 30Address as string * 50Salary as integer

END TYPE

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VB File-Based ExamplePublic Function Open_Data_File()Rem*******************************************************************Rem Routine to open a new data file on the user disk. lngRecLengthRem is used to define record length to open statement.Rem******************************************************************* Dim lngRecLength As Long ' determine the length of recRestaurant records lngRecLength = Len(recRestaurant) ' open a random access data file Open "d:\287\projects\p2\p2_test\p2data.fil" For Random As #1 Len =

lngRecLength ' find the position of the last record in the data file lngLastRecord = LOF(1) / lngRecLength End Function

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Random File Commands

There are a few commands that are needed to use random files in VB.– Type statement– Open– Close– Get– Put

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Opening/Closing Random Files

You must open a random file before you can access its data

Open data.fil for Random as #1 Len = 50

Likewise, you close it after you are finishedClose #1

While it is open, you can add and modify records using the manipulation commands

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Random File Manipulation

2 commands allow you to read file contents (get) and write contents (put).

You need 3 pieces of information for both the Get and Put commands– File number– Record position– Record work area

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Random File Manipulation

GET statement:GET filenumber, record position, work area

PUT statement:PUT filenumber, record position, work area

Examples:Get #1, Position, wrkRecordPut #1, Position, wrkRecord

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Random File Manipulation

RAM

CPU

Disk Drive(contains the

file)Disk Controller

BUS