A Level Computer Science: File Organisation and Access — Practice Questions (Cambridge 9618)

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Data Representation (A Level) study guide · Data Representation (A Level) revision notes

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What do these questions practise? Section 13.2 File organisation and access of the Cambridge International AS & A Level Computer Science 9618 syllabus (2027–2029): serial, sequential and random file organisation; sequential and direct access; choosing an organisation and access method for a problem; and hashing algorithms to read and write records in random files.

Section A

1. Describe how records are organised in (a) a serial file, (b) a sequential file and (c) a random file. [3]

2. State which access method(s) can be used with each of a serial, sequential and random file. [3]

3. Explain why a new record is simply appended to the end of a serial file, but not to a sequential file. [2]

Section B

4. A hashing algorithm calculates the address of a record as key MOD 50. Records with keys 1234, 2084, 3317 and 1284 are written to the file in that order. (a) Calculate the address for each key. [2] (b) Identify which records cause a collision. [1] (c) Describe two methods of dealing with a collision. [4]

5. Explain how a record is found in a random file when the next free location method (linear probing) has been used for collisions. [3]

6. Describe how a record is found in a sequential file using sequential access, and explain why the search can stop before the end of the file if the record is not present. [3]

7. Explain how direct access can be used with a sequential file. [2]

8. For each scenario, choose the most suitable file organisation and justify your choice: (a) a log of temperature readings added every minute and processed in bulk once a day; [2] (b) a payroll master file of employees, processed in employee-number order once a month; [2] (c) a hotel booking system where any guest’s record must be retrieved immediately. [2]

Section C

9. Write pseudocode for a function GetAddress(Key : INTEGER) RETURNS INTEGER that returns the hash address Key MOD 100 plus 1, so that addresses run from 1 to 100. [2]

10. Explain why a good hashing algorithm should spread records evenly across the file, and what happens to access time if many collisions occur. [3]

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Answers

1. (a) Serial: records stored one after another in the order they were added, with no sorting [1]. (b) Sequential: records stored in order of a key field, such as customer ID [1]. (c) Random: records stored at an address calculated from the record key by a hashing algorithm, with no overall order [1].

2. Serial: sequential access only [1]. Sequential: sequential access, or direct access using an index [1]. Random: direct access (using the hash of the key) [1].

3. A serial file has no order, so the new record can go at the end [1]; a sequential file must remain in key order, so the record must be inserted in the correct position, usually by copying the file to a new file with the record inserted [1].

4. (a) 1234 → 34; 2084 → 34; 3317 → 17; 1284 → 34 [2 for all four; 1 for two or three]. (b) 2084 and 1284 collide with 1234 at address 34 [1]. (c) Any two described: open hashing / next free location: store the record in the next available address after the calculated one, wrapping round to the start of the file if necessary [2]; overflow area: store colliding records in a separate overflow area, searched serially when the record is not at its home address [2]; chaining: each address holds a pointer to a linked list of records with that hash [2].

5. Calculate the hash address from the key [1]; read the record at that address and compare its key with the one sought [1]; if it does not match, read the following addresses one by one until the record is found or an empty location is reached, which means the record is not in the file [1].

6. Start at the first record and read each record in turn, comparing its key with the key sought [1–2]. Because the file is in key order, once a record with a key greater than the target is read, the target cannot appear later, so the search can stop [1].

7. An index of key values and their record positions is kept [1]; the index is searched for the key and the record is read directly from that position without reading earlier records [1].

8. (a) Serial [1]: readings are simply appended in time order and all processed together, so no order by key or direct access is needed [1]. (b) Sequential [1]: every record is processed in employee-number order, so a file sorted on that key is efficient for batch processing [1]. (c) Random [1]: the hashing algorithm gives direct access to any record without reading others, so retrieval is fast [1].

9. Example answer:

FUNCTION GetAddress(Key : INTEGER) RETURNS INTEGER
    RETURN (Key MOD 100) + 1
ENDFUNCTION

[1 for MOD 100, 1 for adding 1 and returning the value]

10. If records are spread evenly, most can be found at their home address with one read [1]. If the algorithm maps many keys to the same addresses, there are many collisions [1], so records must be searched for in following locations or an overflow area, making access slower and less predictable [1].

Frequently asked questions

Is a sequential file the same as sequential access? No. Sequential is a way of organising the file (in key order); sequential access is a way of reading it (one record after another). A sequential file can also be accessed directly through an index.

Which collision method should I use in the exam? Use the one the question describes. If none is given, describe either next free location or an overflow area clearly.

Based on the Cambridge International AS & A Level Computer Science 9618 syllabus for 2027, 2028 and 2029, section 13.2 File organisation and access.