The Fun That Is File Structures

Pages 256-261By: Christine Zeitschel

File Structure Basics:

• A file is a collection of related information into a unit whose primary purpose is to store data.

• Files are stored in auxiliary or secondary storage devices.

• Files store data in a more permanent form than primary memory.

FILES- Secondary storage devices

• Two most common forms- disk & tape

• These files can be both read and written

• Therefore- A file is a group of data with records of more than one field.

Access Methods

• Determine how records are retrieved, in an order or randomly.

• Use a sequential file to obtain data one record after another

• Use random access to pick one specific file.

• Two types include: indexed & hashed files

Sequential Files• A file which

records can only be selected one after another, in other words it’s like reading every page of a book to find one quote, but instead that quote is record for a computer.

More sequential files• The only thing known by the operating

system for sequential files is that records come one after another and that the end-of file (EOF) is the end marker for records.

Processing Sequential Files• Sequential access is more effective

when a person needs to retrieve each record like a paycheck at the end of the month for printing.

• This method would not work as well for a school trying to find one persons record and going through everyone’s record to get to yours.

• Updating becomes difficult as each file must be checked in order for updates.

Four Types of Files for Updating• New Master file- has most current data• Old Master file- permanent file which

needs updating• Transaction file- has changes which are

needed to be made to the master file, where you can add, delete or change transactions—a key is need like an individual ID for a field to process transactions

• Error Report file- contains a list of all errors during update process

A Deeper Look at Keys- all files must have the same keys

1) If trans key is less than master file key, add them

2) If equal either change the master file with a revision (R) or delete (D)

3) If trans key is greater put old master file key as new master file key, or add (A) new code

Indexed Files• A file made up of a data

file, with only 2 fields- the key and the address

• Index files act just as an index would for a history book, they allow you to do a search for a subject like WWI, but instead using binary and then they take you to that file.

Index File

KEY35423543354435453546

ADDRESSJames DeanMarilyn MonroeJohn KennedyBob DoleRobert Kennedy

Hashed Files

• Uses a function to map files

• The user provides a key the function maps the key to the address and then to the operating system to be retrieved

KEY- Address = Hash Function (Key) Address

The work of an index is eliminated by the use of a function to do the work.