COMPUTER FILES

FILES

A computer file is a block of arbitrary information, or resource for storing information, which is available to a computer program and is usually based on some kind of durable storage. Computer files can be considered as the modern counterpart of paper documents which traditionally are kept in offices and libraries’ files, and this is the source of the term.

Files are collection of data in a permanent storage media such as hard drive, CD or DVD ROM, floppy disk, memory card, flash drive etc.

FUNCTIONS OF FILE

(i) It provides machine executable code which is used to run application programs and the operating system.

(ii) It stores application programs or operating system configuration information.

(iii) It stores used data by the user such as Ms Word.

ORGANIZING THE DATA IN A FILE

Information in a computer file can consist of smaller packets of information (often called “records” or “lines”) that are individually different but share some trait in common. The way information is grouped into file is entirely up to the person designing the file. Most computer files are used by computer programs. These programs create, modify and delete files for their own use on an as – needed basis.

The content of the document file are arrange d in a way that word – processing program understands, but the user chooses the name and location of the file and provides the bulk of the information (such as words and text) that will be stored in the file. Many applications pack all their data files into single file, using internal marker to discern the different types of information contained within.

BYTE – ORIENTED FILE SYSTEM

In this type of file – system, a file is considered as a sequence of bytes, the operating system supplies routines that can read/write a specific number of bytes. To have any structure in a file, system and application programs accessing the file must adopt some convention that has to be respected by all programs.

RECORD – ORINTED FILE SYSTEM

In these file – systems, a file is a sequence of records of the same type. A record is a sequence of data bytes together with control information about the record’s size and may be some attributes. This structure is imposed by consistently interpreting the data and control information kept in the records, by system routine you call in order to perform file input or output.

FIELD

This is a single piece of information about an object. Data that has several parts can be divided into fields. A collection of fields make up a record. A field also define a unit of data on a source document, screen or report. Examples of field are Name, Address, Quantity, Amount etc. The field is the common denominator between the user and the computer. When you interactively query and update your database, you reference you data by field name.

DATA ELEMENT AND DATA ITEMS

Data elements describe the logical unit of data, while data items are the individual instances of the data element.

S/N	Product	Product description
1	Mail	PP 004
2	Cloth	CC456
3	Chair	TT454
4	Appliances	HH4454
5	Ware	DD565

From the above table, Product and Product description is called Data element while items listed under them is Data items.

FILE ORGANISATION TERMS

(i) Block (Physical Record): is the physical unit of transfer between the backing store and the main store. This is the smallest amount of information that can be transferred.

(ii) Bucket: This is the logical unit of transfer between the backing storage and main memory. Bucket size is limited by the size of the main memory buffer available for input/ output.

(iii) Hit: In a processing run, a record that is required is a hit. Hit rate = No of records / Total records in file.

 FILE STRUCTURE ORGANISATION

TYPES OF FILE ORGANISATION

1. SERIAL: Records in a series are inserted as they arrive. The file is usually read serially from beginning to end. Searching individual records is difficult because in each case the entire file must be read. Serial files are usually used for temporary storage of transaction data.

2. SEQUENCIAL FILES: Records are arranged in key value order, usually ascending but it can equally be descending .The file is usually read in order in which it is written, from beginning to the end. The great strength of sequential files is that two sequential files which share a key field and are ordered in the same way can be merged very efficiently.

3. INDEXED SEQUENTIAL FILES: Many applications required a mixture of low and high hit rate. In these cases, neither sequential nor direct organisation is adequate. Indexed sequential files are idea for such applications. Because the records are organised sequentially, high hit rate processing is well supported. Direct access of individual records for low hit rate application is provided by an index.

4. RANDOM OR DIRECT FILES: With direct or hashed access, a portion of disk space is reserved and a “hashing” algorithm computes the record address. So there is additional space required for this kind of file in the store. Records are placed randomly throughout the file and are accessed by addresses that specify their disc location.