CONCEPT OF COMPUTER FILES

Many tasks in an information oriented society require people to deal with voluminous data and to use computers to deal with this data efficiently and speedily.

FILE TERMINOLOGIES

a) Files deal with storage and retrieval of data in a computer. Programming languages incorporate statements and structures which allow users to write programs to access and use the data in the file.

b) Record is a collection of related fields that can be treated as a unit from an applications point of view.

c) Field is an elementary data item characterized by its size, length and type.

d) Data item refers to a single unit of values. e.g. Roll Number, Name, Date of Birth, Age etc. The three types of data item are numeric, alphabet and alpha – numeric.

There are two kinds of data items. This are:

• Group Items: Data items that can be divided into sub-items are called ‘Group Items’. E.g. the name of a person is a group item because it can be divided into three sub-items: First Name, Middle Initial and Last Name. Other examples are: Address, Date, Qualification, Salary, etc.
• Elementary Items: Data items that cannot be divided into sub-items are called ‘Elementary Items’. E.g. the roll number of a student is an elementary item because it cannot be divided further into sub-items. Other examples are: Marks, Age, etc.

 

FILE ORGANIZATION

File organization refers to the logical relationships among the various records that constitute the file, particularly with respect to the means of identification and access to any specific record. File structure refers to the format of the label and data blocks and of any logical record control information. Files stored on magnetic media can be organised in a number of ways, just as in a manual system. There are advantages and disadvantages to each type of file organisation, and the method chosen will depend on several factors such as:

• how the file is to be used;
• how many records are processed each time the file is updated;
• whether individual records need to be quickly accessible.

 

Types of file organization

The available methods include:

1. Serial File Organization: A serial file is one in which the information or files stored in it have been added in the order in which they occurred or have been saved. An analogy to a serial file is a grocery list; you add items to the list in the order that you need them or think of them, so the items are not in any particular order. Serial files can be stored on a disc or in your computer’s internal memory. A Serial file is a sequence of varying length records. Like INDEXED files, the records are numbered starting with zero. When a serial file is created it contains no records. As data is written to the file, records are added at the end of the file. The records are normally accessed sequentially but they may also be accessed randomly by record number. Random access might be slow.
2. Sequential File Organization: A sequential file contains records organized by the order in which they were entered. The order of the records is fixed. Records in sequential files can be read or written only sequentially. A file that contains records or other elements that are stored in a chronological order based on account number or some other identifying data. In order to locate the desired data, sequential files must be read starting at the beginning of the file. A sequential file may be stored on a sequential access device such as magnetic tape or on a direct access device such as magnetic disk.
3. Indexed sequential File Organization: Records in indexed sequential files are stored in the order that they are written to the disk. Records may be retrieved in sequential order or in random order using a numeric index to represent the record number in the file. The record size, specified when the file is created, may range from 1 to 8000 bytes. When there is need to access records sequentially by some key value and also to access records directly by the same key value, the collection of records may be organized in an effective manned called Indexes Sequential Organization.
4. Random File Organization: Random files are record-based files with an internal structure that supports “direct access” by record number. This means that your program can read from or write to a specific record in a random access file, say the 50th record, without reading through the previous 49 records. Compare that to reading or writing a sequential file, where to get to a specific record, you must read through all preceding records.

 

METHODS OF ACCESSING FILES

• Serial Access.
• Random Access.
• Sequential Access.

 

FILE CLASSIFICATION 

Most large companies have hundreds or even thousands of files that store data pertaining to the business. Some of the files can be classified as either transaction files, master files or reference files.

1. Transaction Files: Transaction files contain details of all transactions that have occurred in the last period. A period may be the time that has elapsed since business started that day, or it may be a day, a week, a month or more. For example a sales transaction file may contain details of all sales made that day.
2. Master Files: Master files are permanent files kept up-to-date by applying the transactions that occur during the operation of the business.

 

CRITERIA FOR CLASSIFYING FILES

The criteria for classifying files are:

• Nature of content in the file.
• Organisation Method.
• Storage medium.