File Handling

Advanced Higher Programming

What is a file? Up until now, any stored data

within a program is lost when the program closes.

A file is a permanent way to store data

File HandlingThree types of file can be used for storing data

SequentialSequential RandomRandom BinaryBinary

Sequential FilesSequential files are useful for:

Storing text Easy implementation in programs Where real-time editing of file(s) is

not required

Random FilesRandom file structures are useful for

Files that require real-time editing Storing records

Binary FilesBinary Files are useful for

Storing numbers, programs and images

Where no defined file structure is present

They will not be used in this course

Sequential Files Have a universal standard format

and are used in text editors such as windows notepad

Numerical data is stored as a string e.g., 5.32 would be stored as “5.32”

They are read from start to finish and so cannot be read and written to simultaneously

Sequential Files Data is ALWAYS written and retrieved

as CHARACTERS. Hence, any number written in this

mode will result in the ASCII Value of thenumber being stored.

For Example, The Number 17 is stored as two separate characters "1" and "7".Which means that 17 is stored as [ 49 55 ] and not as [ 17 ].

Sequential Files Are like a one dimensional array

The text, ONE DAT might be stored as:

“ D TENO CR”A EOF

Sequential FilesFiles are manipulated in 3 stages:

File Open Process File Close File

Sequential FilesFile OpenFile Open

If the file does not exist it is created and then openedcreated and then opened by the operating system.

A portion of memory (RAM)memory (RAM) is reserved by the Operating System.

Sequential FilesProcessing a FileProcessing a File

When a file is open it can be written to or readwritten to or read from. (both in the case of random and binary files)

Writing to a file will save it to backing store.backing store.

Sequential FilesSequential FilesClosing a fileClosing a file

When a file has been opened and processed it must then be must then be closedclosed.

The Operating system will then release the memory.release the memory.

Visual Basic VB supports all three file types, but

you are only likely to use two of them Text files Random Access files

Text Files

Sequential/Text Files

Input File opened for read-only access.

Output File opened for output which is only write-to or create

Append The file is opened for adding new data to an existing file. This is the default setting.

Random

The file is open for random access. This is writing or reading one record at a time.

Binary The file is opened in binary mode

Using the OpenFileDialog controlDim Filename as StringOpenFileDialog1.ShowDialog()Filename=

OpenFileDialog1.FilenamelblFilename.Text = Filename

Opening FilesOpening FilesFileOpen(1, Filename, OpenMode.Input) ‘to read from the file

FileOpen(1, Filename, OpenMode.Output) ‘to write to the file

FileOpen(1, Filename, OpenMode.Append) ‘to write to the end of the fileNote – 1 assigns the file the number 1. All files are

identified by a number, not by their name. If you have two or more files open at once they must have different numbers.

Opening Files The FileOpen statement opens a file

if it exists. When you open a file to read from it, an error results if it does not exists. When you open a file to write to it, if it doesn’t exist FileOpen first creates it and opens it.

Filename contains the name and path of the file

Opening (creating) a Sequential FileThis algorithm would achieve this:1.1. Enter FilenameEnter Filename2.2. Open File for writingOpen File for writing3.3. Input InformationInput Information4.4. Save to fileSave to file5.5. Close fileClose file

Opening (creating) a Sequential FileThis algorithm would achieve this: Enter FilenameEnter Filename Open File for writingOpen File for writing Input InformationInput Information Save to fileSave to file Close fileClose file

Filename= OpenFileDialog1.FileName

Opening (creating) a Sequential FileThis algorithm would achieve this: Enter FilenameEnter Filename Open File for writingOpen File for writing Input InformationInput Information Save to fileSave to file Close fileClose file

FileOpen(1, Filename, OpenMode.Output)

Writeline(1, DataToBeWritten)

Opening (creating) a Sequential FileThis algorithm would achieve this: Enter FilenameEnter Filename Open File for writingOpen File for writing Input InformationInput Information Save to fileSave to file Close fileClose file

Opening (creating) a Sequential FileThis algorithm would achieve this: Enter FilenameEnter Filename Open File for writingOpen File for writing Input InformationInput Information Save to fileSave to file Close fileClose file

FileClose (1)

Opening a sequential fileOpening a sequential fileThe final code would look like:Dim Filename as string‘File manipulation Program‘Create FilePrivate sub cmdCreateFile_Click()

OpenFileDialog1.ShowDialog()Filename = FileDialog1.FileNameFileOpen(1,Filename, OpenMode.Output)WriteLine(1,DataToBeWritten)FileClose (1)

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