File Organisation

Introduction

• Computers are used to access vast amounts of data and to present it as useful information.

• Millions of people expect to be able to retrieve the information they need in a useful form when they ask for it.

• This information is all stored as data in files, everything from bank statements to movie collections.

• In order to be able to find data efficiently it needs to be organised.

• Data of all types is stored as records in files.

• These files can be organised using different methods.

# File organisation
- Serial
- Sequential
- Random

Serial file organisation

• The serial file organisation method physically stores records of data in a file, one after another, in the order they were added to the file.

• New records are appended to the end of the file.

• Serial file organisation is often used for temporary files storing transactions to be made to more permanent files.

Sequential file organisation

• The sequential file organisation method physically stores records of data in a file, one after another, in a given order.

• The order is usually based on the key field of the records as this is a unique identifier.

• For example, a file could be used by a supplier to store customer records for gas or electricity in order to send regular bills to each customer.

• All records are stored in ascending customer number order, where the customer number is the key field that uniquely identifies each record.

Random file organisation

• The random file organisation method physically stores records of data in a file in any available position.

• The location of any record in the file is found by using a hashing algorithm on the key field of a record.

Customer 6 record not found

File access

• There are different methods of file access (the method used to physically find a record in the file).

• We will consider two of them: sequential access and direct access.

# File access

- Sequential

- Direct(hash)

Sequential access

• The sequential access method searches for records one after another from the physical start of the file until the required record is found, or a new record can be added to the file.

• This method is used for serial and sequential files.

• Sequential access is efficient when every record in the file needs to be processed, for example, a monthly billing or payroll system.

• These files have a high hit rate during the processing as nearly every record is used when the program is run.

Direct access

• The direct access method can physically find a record in a file without other records being physically read.

• Both sequential and random files can use direct access.

• This allows specific records to be found more quickly than using sequential access.

• Direct access is required when an individual record from a file needs to be processed.

• For example, when a single customer record needs to be updated when the customer’s phone number is changed.

• Here, the file being processed has a low hit rate as only one of the records in the file is used.

Hashing algorithms

• In the context of storing and accessing data in a file, a hashing algorithm is a mathematical formula used to perform a calculation on the key field of the record.

• The result of the calculation gives the address where the record should be found.

• More complex hashing algorithms are used in the encryption of data.