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File structures






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    File structures File structures Presentation Transcript

    • PRIMARY STORAGE SECONDARY STORAGE 1)Faster Access Slower Access 2)Expensive Cheaper 3)Lesser storage capacity Greater Storage capacity 4)Temporary Storage Permanent Storage
      • Secondary Storage Structure
      • Used for permanent storage
      • It’s a collection of records or a stream of bytes
      • Every Record is a collection of fields
      • A particular field is chosen as a Key
      • Records are organised in file by using the key. Primary and secondary keys.
      • Consider a student database.
      • Every student has a unique record
      • Record has details of student-i.e name, Student ID etc. These are the fields.
      • The unique key can be the Student ID. The records can be organised in the file on basis of student ID.
      • Sometimes data is too large to be stored in main memory.
      • Maintaining permanent record is possible only by using a secondary storage. Hence files.
      • Physical Files:
      • A collection of bits stored in the secondary storage device
      • Logical File:
      • A channel that connects he program to the physical file(Stream).
      • An example
      • FILE* out
      • out=fopen(“sample.txt”,”w”);
      • Here out is the logical file and sample.txt is the physical file.
      • Opening a File:
      • A logical file is associated with the physical file
      • Closing a File:
      • The logical file associated with the physical file is freed.
      • fclose(file pointer);
      • Readin g from file:
      • Data present in physical file is read by using the logical file
      • Writing to a File
      • Data can be written to physical file by using the logical file
      • Every logical file has a file position pointer.
      • When we open a new stream the position pointer is set to beginning of the file.
      • As data is read or written the file position pointer is moved accordingly.
      • To move file pointer to required position.
      • fseek(file pointer,offset, position); 
      • To display current location of pointer.
      • long position=ftell(pointer);
      • To check for end of file
      • while(!feof(pointer));
      • Sequential File
      • Stored in the order entered
      • Random Access Files
      • An record is accessed using an index.(Hashing).
      • Direct Access Files:
      • The records are stored based on their relative position with respect to first record.
      • Record with key 50 is placed at location 50
      • Records are stored in the order entered
      • Used when all the records have to be processed.
      • Complexity for searching O(n)
      • An record is accessed using an index.
      • The index of record position in file has to be maintained in the main memory.
      • The Index can be created using hashing.
      • Search complexity is less. Complexity of Indexing method used.
      • Disadvantage: While handling very large databases its not possible to maintain an index in the main memory.
      • The records are stored based on their relative position with respect to first record.
      • Record with key 50 is placed at location 50
      • The search complexity is O(1)
      • Disadvantage is a lot of memory is wasted.
      • For example if no record has key 100 the position 100 is wasted.