9618 · 13.2
File organisation and access flashcards
Revision flashcards for Cambridge 9618 File organisation and access (syllabus 13.2). Flip, recall, then mark a real past-paper question.
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What is serial file organisation?
Records are stored one after another in the order they are added. There is no inherent sorting or ordering of the records themselves. It's like a simple diary or log file.
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What is sequential file organisation?
Records are stored in order based on a specific key field (e.g., Customer ID, surname). The file is sorted, which is the key difference from serial organisation.
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What is random file organisation?
Records are stored at a physical address that is calculated directly from the record's key field, typically using a hashing algorithm. This allows for direct access.
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What is sequential file access?
A method of accessing records by reading the file from the beginning, one record at a time, until the desired record is found. It is the only method for serial files.
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What is direct file access?
A method of accessing a record by calculating its physical address and jumping straight to it, without reading any other records. This requires random file organisation.
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What is a hashing algorithm in the context of file organisation?
A function that takes a record's key field as input and computes a physical disk address where the record should be stored or found. Example: `Address = Key MOD N`.
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What is a collision (or synonym) in random file organisation?
When a hashing algorithm produces the same address for two or more different keys. This requires a strategy to handle the 'colliding' record, such as using an overflow area.
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What is an overflow area?
A separate part of a file or storage medium used to store records that cannot be placed at their calculated hash address due to a collision.
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Can a sequentially organised file be accessed directly?
No, not in the true sense. While you could potentially calculate a record's approximate position if records are fixed-length, true direct access requires a random organisation where the key maps to a physical address. Sequential files are designed for sequential access.
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When is sequential organisation most suitable?
For batch processing applications where a large proportion of the records need to be processed in order, such as a monthly payroll system or generating utility bills.
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When is random organisation most suitable?
For real-time or transaction processing applications where rapid access to individual records is critical, such as an airline booking system or an ATM.
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What is the main drawback of adding a new record to a sorted sequential file?
The entire file may need to be rewritten to insert the new record in the correct sorted position to maintain the file's integrity. This is very inefficient.