Data Files

1
Data Files
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Introduction

• A file is an organised collection of data,
  usually on backing store.
• A data file usually consists of parts called
  records. The data in a record consists of items,
  each stored in a different field.
• Fields and records can be variable or fixed in
  length.
• Variable length records waste less space but need
  separators
• A Variable length field can store an item however
  long it is.
• The value in a key field is unique to that
  record. This means that the keys must all be
  different from one another.

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Introduction - cont.

• Expressed briefly
• To create a file organise data into a file
• To write data transfer it to backing store
• To read data transfer it from backing store
  to the main store
• To save a file copy it from the main store on
  to backing store
• To load a file read it from backing store
  into the main store
• To open a file prepare it for access
• To close a file procedure necessary when a
  file is finished with.

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Directories

• A directory means an area of a disk which can be
  split into sub-directories.
• A catalogue is a simple list of the files in a
  directory.
• The operating system allows users to
• create and access sub-directories
• to obtain a list of a given directory
• save, load, delete, rename, and copy files.

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Files

• A serial access file has data stored in the order
  it was written. Records are read by reading
  through all preceding records first.
• A sequential access file is similar but records
  are in order of the key field.
• Any record of a direct ( or random ) access file
  can be accessed without having to access other
  records first.

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Updating

• Updating can involve
• insertion - adding new records to file
• deletion - removing existing records from the
  file
• amendment - changing the items within the
  existing records.

7
File operations

• A master file is used as a reference file and is
  updated when necessary.
• A transaction file is temporary and is prepared
  in order to update a master file.
• A file can be searched to retrieve specified
  information, sorted to put it into given order
  and / or merged with another file.

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CD ROMS

• All types of compact disks can be read by a
  computer including music CDs, photo CDs and
  CD-ROMs
• CD-ROMs are exchangeable and easy to transport
• CD-ROMs store more data than floppy disks and are
  also faster
• CD-ROMs are Read Only. The data is written
  during manufacture.