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  Computing Notes
  => 1 Introduction To Computing
  => 1.1 Computer Hardware
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  => 1.3 The Human-Computer Interface
  => 1.4 Business Information Systems
  => 1.5 Batch Processing
  => 2 Word Processing
  => 2.1 Introduction
  => 2.2 Editing Facilities
  => 2.3 Bullets And Numbering
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  => 2.5 Headers And Footers
  => 2.6 Style Controls
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  => 2.11 Inserting Graphics
  => 2.12 Mail Merging
  => 2.13 Macros
  => 3 Programming In QuickBASIC
  => 3.1 Introduction
  => 3.2 Variables, Input And Output
  => 3.3 Arithmetic Operators
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  => 3.11 Jackson Structure Diagrams
  => 3.12 String Processing
  => 4 Data Representation
  => 4.1 Different Computer Codes
  => 4.2 Binary Integers
  => 4.3 Higher Number Bases
  => 4.4 Graphics, Sounds And Other Interpretations
  => 4.5 Fixed Point Binary Numbers
  => 4.6 Floating Point Binary Numbers
  => 4.7 Range And Accuracy
  => 5 Spreadsheets
  => 5.1 Introduction
  => 5.2 General Features
  => 5.3 "What If" Calculations
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  => 5.10 Goal Seeker
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  => 6 Files
  => 6.1 File Concepts
  => 6.2 Serial & Sequential Files
  => 6.3 Indexed Sequential Files
  => 6.4 Random Access Files
  => 6.5 Overview of File Processing
  => 7 Standard Algorithms
  => 7.1 Linear Searches
  => 7.2 Binary Search
  => 7.3 Internal Sorting
  => 7.4 External Sorting
  => 8 Legal Issues And Data Security
  => 8.1 The Computer Missuse Act 1990
  => 8.2 The Data Protection Act 1984
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  => 8.6 Security Of Data
  => 8.7 Data Integrity
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  => 9.1 Flat-file Databases
  => 9.2 Introduction To Relational Databases
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  => 10.1 Introduction
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  => 11.1 Introduction
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  => 12.1 Input Devices
  => 12.2 Output Devices
  => 12.3 Storage Devices
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  => 13.1 The Processor And Memory
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INDIA
6.5 Overview of File Processing
Section 6.5

6 Files

6.5 Overview of File Processing

In this final part we shall consider operations on files and their uses.

The following terms are often used:

Interrogating/Referencing - Searching to find a particular key.

Maintenance - Updating various records plus adding and deleting records.

Sorting - Changing the sequence of records.

6.5.1 Updating Files

Updating By Overlay

Records in indexed sequential files and random files can be accessed directly, modified and written back to their original locations.

Updating By Copying

This method involves copying the records one by one to a new file, making modifications as needed.

The result is two versions (or generations) of the file.

6.5.2 File Backup And Generations

Each time a master file is updated another, out of date, generation is left.

It is common to keep three generations:

  • Grandfather
  • Father
  • Son (Current version)

6.5.3 Choosing Between Serial And Direct Access Files

The choice of file organisation is a vital consideration. The following questions need to be answered:

  • What is the most suitable storage medium for the volume of data involved?
  • Must the information always be up-to-date>
  • Do users require immediate access to data>
  • Can requests for information be grouped together and be batched processed?
  • Are report required in a particular sequence?
  • What is the hit rate?
  • How volatile is the file?

6.5.4 Hit Rate

This is the measure of how many records are accessed out of the total number, usually expressed as a percentage.

Example

Updating a payroll master file.

During the process 190 out of 200 employee records are updated.

(190 / 200) × 100 = 95%

6.5.5 Volatility

This is the frequency at which records are added or deleted from a file.

If this frequency is high, then the file is said to be volatile.

6.5.6 Uses Of Different File Organisations

Serial Files

Serial file organisation is mainly used for transaction files. As events in the real world take place, relevant data records are written to a transaction file.

Mainly used in:

  • Sales in a shop.
  • Customer's withdrawing money from an ATM.
  • Postal orders arriving at a mail order company.

The transactions may be batched and the master file updated later. Alternatively, the master file may be updated as soon as each event occurs (in real-time). The transaction file is then kept as a record of what occured in case the master file corrupted and its father needs to be updated.

Sequential Files

Sequential file organisation is used for master files in high-hit rate applications.

May be used in:

  • Payroll
  • Direct mailing (a.k.a. Junk mailing)

Indexed Sequential Files

Indexed sequential files can be processed either sequentially or randomly. This is very useful because when most of the records need to be processed then they can be sequentially processed. When only a few need to be updated then they can be directly accessed.

May be used in:

  • Stock control

The stock file would be directly accessed when the customer makes a purchase. The master file would be accessed using a multi-level index to find the relevant record. The description and the price would then be printed on the receipt and the quantity in stock would be updated right away.

The file would be sequentially processed if a report of all the stock or sales is needed in stock code sequence. Processing the file this way is fast, but it is not as fast as processing a sequential file.

Random Files

Random files are used when extremly fast access is required to individual records. Becuase the hashing algorithm generates the record address when it's applied to the record's key no time is taken looking through various levels of index.

May be used in:

  • Utility programs to validate user names and passwords (on a network).
  • Airline booking systems.

If reports are needed containing all the records in key sequence, these will take a long time to generate.


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