File Access (Cambridge International A-Level Computer Science)

What are file access methods?

• Once a file is organised, a file access method is how records are read from or located in it
• The access methods available depend on the file organisation used

Which access method works with which organisation?

• Sequential access works on serial and sequential files; direct access works on sequential and random files

Exam Hint

• This matrix is a common exam trap.
• Learn it exactly.

File access methods

Sequential access

• Sequential access reads records one after the other, starting at the physical start of the file, and continues until the record is found or the end of the file is reached

On serial files

• Records are in chronological order, so every record is checked until the target is found or all records have been checked

On sequential files

• Records are in order of a key field (or index), and it is the key field that is compared
• Checking can stop early: as soon as the current record's key is greater than the target key, the target cannot be present
• Example: reading student results in candidate-number order

Direct access

• Direct access locates a specific record without reading the records before it
• The target is found using the key field of the record

On random files

• A hashing algorithm is applied to the record's key field to calculate the address of the location where the record should be
• If the record is not at that location (a collision has occurred), a resolution method such as linear probing or an overflow area is used (see Hashing)
• Example: accessing a bank account record by account number

On sequential files

• An index of all key fields is kept
• The index is searched for the address of the file location where the target record is stored, and the record is then read directly

Exam Hint

• Watch the direction of the pairing: direct access on a sequential file uses an index, but direct access on a random file uses a hashing algorithm. Questions often ask you to explain both in the same part.
• "Sequential access" and "sequential organisation" are not the same thing. Serial files also use sequential access.