File Organisation

Question 1

Give examples of different tile types

Answer 1

Word processing, spreadsheet files and so on

Question 2

How is the content stored in a file?

Answer 2

Defined binary code that allows the file to be used in the way intended

Question 3

What do computers, that store files for a specific purpose, store them as?

Answer 3

Binary File or

Text File

Question 4

What is meant by a text file?

Answer 4

It contains data stored by a defined character code (ASCII).

Question 5

How can you create a text file?

Answer 5

Using a text editor

Question 6

How does a binary file store data?

Answer 6

In its internal representation i.e. an integer value might be stored in 2 bytes in 2s complement representation

Question 7

How is a binary file created?

Answer 7

Via a specific program

Question 8

How are binary files organised?

Answer 8

Based on a record

Question 9

Explain what is meant by a record?

Answer 9

A file contains a record, and each record contains fields. Each field consists of a value

Question 10

What is meant by a serial file?

Answer 10

It contains records which have no defined order.

Question 11

How is a serial file used?

Answer 11

For banks to record transactions involving customer accounts. A program would be running. Each time there was a withdrawal or a deposit the program would receive the details as data input and would record these in a transaction file. The records would enter the file in chronological order but otherwise the file would have no ordering of the records.

Question 12

What type of file is a text file?

Answer 12

Serial but it the file has repeating lines which are defined by an end of line character or characters. There is no end of record character.

Question 13

Why must must a record in a serial file have a defined format?

Answer 13

To allow correct input and output

Question 14

What is meant by a sequential file?

Answer 14

Records that are ordered

Question 15

Why use sequential files?

Answer 15

For long term storage of data

Question 16

What would be an alternative to a sequential file?

Answer 16

Database

Question 17

What is the difference between a sequential file and a serial file?

Answer 17

In banking, a sequential file could be used as a master file for an individual customer account. Periodically, the transaction file would be read and all affected customer account master files would be updated

Question 18

How are sequential files ordered?

Answer 18

Using a key field where the values are unique and sequential but not always consecutive

Question 19

What is the difference between a key field and a database primary key?

Answer 19

Primary key values are required to be unique but not sequentia

Question 20

How are records in a sequential file found?

Answer 20

By sequentially reading the value of the key field until the required value is found.

Question 21

State what is meant by direct access files?

Answer 21

It is known as random-access files where the randomness is only that the access is not defined by a sequential reading of the file.

Question 22

Where is direct access files used?

Answer 22

For large files

Question 23

Why is direct access used for large files?

Answer 23

It saves time compared to searching through a sequential file

Question 24

How is data stored in a direct access file?

Answer 24

In an identifiable record where finding it may involve an initial direct access to a nearby record followed by a limited serial search

Question 25

How is the chosen position for a record, when searching direct access, selected?

Answer 25

Using hashing algorithm

Question 26

How does a hashing algorithm work?

Answer 26

1. Take the key field value as an input
2. Outputs a value for the position of the record relative to the start of the file
3. It will take into account the length of the file (number of records)

Question 27

Give an example of a hashing algorithm in action

Answer 27

1.If the key field has a numeric value then divide the value by a suitably large number
2. use the remainder from the division to define the position
3.The method will not create a unique position.
4.If a hash position is calculated that duplicates one already calculated by a different key, the next position in the file is used.
This is why a search will involve a direct access followed by limited serial search.

Question 28

State what is meant by file access

Answer 28

How is stored data in a file going to be used

Question 29

How should a serial file read an individual data item?

Answer 29

Successively read record by record until the required data is read.

Question 30

How should a sequential file read an individual data item?

Answer 30

Process is similar to serial but only the value in they key field has to be read.

Question 31

How should direct-access read an individual data item?

Answer 31

The value in they key field is submitted to the hashing algorithm which then provides the same value for the position in the file that was provided when the algorithm was used at the time of data input.

Question 32

How are files deleted or editted?

Answer 32

Sequential Files -

1. create a new version of the file
2. Data is copied from the old file to the new file until the record is reached which needs deleting or editing
3. if deletion is needed, reading and copying of the old file continues form the next record
4. If a record has changed, an edited version of the record is written to the new file
5. remaining records are copied to the new file

Direct-Access:

1. No need to create new file (unless file is full).
2. A deleted record has a flag set so in the next reading process, the record is skipped over.

Question 33

Where is serial file organisation best used?

Answer 33

Batch processing or backing up data on magnetic tape

Question 34

Where is direct-access file organisation best used?

Answer 34

If a problem needs a file in which individual data items might be read, updated, or deleted then direct-access file organisation is most suitable -> serial least suitable