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WEEK 3
TOPIC: CONCEPT OF COMPUTER FILES (cont’d)
CONTENT:
- Methods of accessing files.
- File classification.
- Criteria for classifying files.
Sub-Topic 1: METHODS OF ACCESSING FILES
An access method defines the way the computer process the read and write files. Each file organization can be accessed or processed in different ways, the methods of accessing files include the following:
- Serial Access
- Sequential Access
- Random Access
Serial Access
This is a method of storing or retrieving data or information that requires the program to start writing or reading at the beginning and continues until it finds the desired data. The only way to access a serially organized file is serially, for instance, the tape.
Sequential Access
The method of access used is still SERIAL but of course in an organized way, that is, in the order in which they are physically arranged. It is important to note that to process (e.g update) a sequential master tape file, the transaction file must also be in the sequence of the master file. Also, if the record required is the twentieth record on the file, in order to get it into storage to process it the computer will first have to read in all nineteen preceding records.
This is possible with files organized sequentially or indexed-sequentially.
Random Access:
This is a method of retrieval or storage of data that does not require any other stored data to be read first, that is, the storage locations can be accessed (read or written to) in any order, with the help of the record key. It is also called ‘direct access’. This type of access is possible on a file with indexed-sequential or random organization.
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