File Operations

     The file operations deal with the movement of data in and out of files. The file management system manages the independent files and acts as the interface between the user and the file operation. Before going in the detail of file organization let us describe some basic file operations that can be performed on any kind of file.

                         The basic file operations that can be performed on files are

Creation of file

Reading of file

Updation of file

Insertion in the file

Deletion from the file

Creation of File

          Before creating a file we have to collect the data validate the data and process the data to give it a record like structure. Then choose the name for the file and open the file on secondary storage device and store the collected data on it.

Reading of file

         After creating a file it is needed at any time we have to read a file. Either we have to read a particular record or the whole file. But for both types of read operations firstly, we have to check whether a file which has to be read is either on the disks or not. If the file exists then open the file for reading and according to your need either display a particular record of file on the output device.

Updation of File

Updation of file means that we have to perform some changes on the contents of file. For updating a file we have to update then search for the record which has to be changed in the file. If the record is present in the file then replace with the new record then overwrite the file.

Insertion in the File

    Sometime it is required to insert a new record in the file. If was have to inset a new record in the end of a file then the insertion is called appending. If we have to insert a new record after a particular record, then we have to find the position of insertion then place the new record at that position of shifting succeeding records.

Deletion from the file

       When some record is not required in the file then we delete that record. For example if some employee leaves the organization then the particular of the employee is not required. We delete the record of that employee from the file of the organization. For deleting a record we have to search the record for deletion in the file and if it exists delete that record from the file.

The Structure of Sequential Files

    In a sequential file records are stored one after the other on a storage device. Of course a sequential type of storage representations is not new to us. Previously we discussed the sequential representation of characters in a string of arrays and of certain linear and nonlinear lists. Because sequential allocation is conceptually simple. Yet flexible enough to cope with many of the problems associated with handing large volumes of data a sequential file been the most popular basic file structure used in the data processing industry.

      All types of external storage devices support a sequential file organization. Some devices by their physical nature can only support sequential files. For example information is stored on a magnetic tape as continuous series of records along the length of the tape. Accessing a particular record requires the accessing of all previous records in the file other devices which are strictly sequential in nature are paper tape readers card readers tape cassettes and line printers.

Indexed Sequential Files

     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 manner called indexes sequential organization.

DIRECT FILE ORGANISATION

    It offers an effective way to organize data when there is a need to access individual records directly.

       To access a record directly (or random access) a relationship is used to translate the key value into a physical address. This is called the mapping function R R. (key value) address

     Direct files are stored on DASD (Direct access storage Device)

     A calculation is performed on the key value to get an address. This address calculation technique is often termed as hashing. The calculation applied is called a hash function.

   Here we discuss a very commonly used hash function called Division Remainder.

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