OxfordAQA A Level Computer Science: Records, Queues and Stacks — Practice Questions

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: Records, Queues and Stacks study guide · Records, Queues and Stacks revision notes

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What do these questions practise? Sub-topics 3.2.2 (Records), 3.2.3 (Queues) and 3.2.4 (Stacks) of OxfordAQA International AS and A-level Computer Science 9645: definitions and operations, appropriate uses, array implementations with empty and full tests, traces and library use.

Section A

1. Explain what is meant by a record and a field. [2]

2. State whether a queue and a stack are FIFO or LIFO structures. [2]

3. State the difference between the stack operations pop and peek. [2]

4. Give one situation where a queue is appropriate and one where a stack is appropriate, explaining each. [4]

Section B

5. A stack of size 5 is implemented in a one-dimensional array with pointer top, initially −1. The following operations are carried out: push 4, push 7, pop, push 1, push 3, pop, peek. (a) State the value returned by each pop and by the peek. [3] (b) State the final value of top. [1]

6. A circular queue of size 4 uses front, rear and count, initially 0, −1 and 0. The operations are: enqueue 10, 20, 30; dequeue; dequeue; enqueue 40, 50, 60. (a) Give the contents of the array and the values of front, rear and count at the end. [4] (b) State whether the queue is full. [1]

7. Explain why a linear queue implemented in an array may report that it is full when there are unused spaces in the array, and describe how a circular queue overcomes this. [4]

8. Write Python code for the push method of a stack class that stores items in self.items, has capacity self.size and pointer self.top. It should return False if the stack is full. [4]

9. A program stores details of products: a product code (string), a price (real) and a quantity in stock (integer). Describe how a record could be used, and how many products could be stored. [3]

Section C

10. Describe how a stack could be used to reverse the string “QUEUE”, and state the output. [3]

11. Evaluate whether a programmer should implement their own queue in an array or use a language library queue. [6]

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Answers

1. A record is a collection of related data items treated as a single entity [1]; each data item in it is a field, which has a name and may have its own data type [1].

2. Queue: FIFO (first in, first out) [1]. Stack: LIFO (last in, first out) [1].

3. Pop returns the top item and removes it from the stack, decrementing top [1]; peek returns the top item without removing it, so top is unchanged [1].

4. Queue, any one with reason: print queue, so documents print in the order sent [2]; keyboard buffer, so keys are processed in order [2]. Stack, any one with reason: undo, because the most recent action must be reversed first [2]; subroutine calls, because the most recent call returns first [2].

5. (a) First pop returns 7 [1]; second pop returns 3 [1]; peek returns 1 [1]. (b) top = 1 [1].

6. (a) Array [50, 60, 30, 40] [1]; front = 2 [1]; rear = 1 [1]; count = 4 [1]. (b) Yes, the queue is full because count equals the size [1].

7. In a linear queue rear only moves forward, so once it reaches the last index the queue is treated as full [1], even though dequeued items at the front have left free spaces [1]. A circular queue lets the pointers wrap around to index 0 using (pointer + 1) MOD size [1], so freed spaces at the start of the array can be reused without moving any items [1].

8. Example answer:

def push(self, item):
    if self.top == self.size - 1:   # full test [1]
        return False                # [1]
    self.top += 1                   # increment pointer [1]
    self.items[self.top] = item     # store at new top [1]
    return True

9. Define a record type, for example a data class Product, with fields code: str, price: float and quantity: int [1]; each product is one record, so its fields can be passed and updated together [1]; store the records in an array or list, so the number of products is limited only by the array size, or is unlimited for a dynamic list [1].

10. Push each character in order: Q, U, E, U, E [1]. Then pop repeatedly until the stack is empty, appending each popped character to the output [1]. Output: “EUEUQ” [1].

11. Own implementation, for: shows understanding of how the structure works [1]; full control over size and behaviour, such as a fixed-size buffer [1]; useful where no library exists [1]. Against: takes time to write and test [1]; risk of errors in pointer handling [1]. Library, for: already tested, efficient and documented, such as Python’s collections.deque [1]; less code to write [1]. Against: less control over memory and may hide behaviour [1]. A justified conclusion, for example that libraries are preferable in production code but own implementations aid learning or special requirements, is needed for full marks. Maximum 6.

Frequently asked questions

Will I have to write code in the exam? Programming questions can ask you to write or complete code in your chosen language; practise the empty and full tests until they are automatic.

Should my code clear the array slot after a pop? It is not required; moving the pointer is what removes the item from the stack’s logical contents.

Based on the OxfordAQA International AS and A-level Computer Science (9645) specification, Version 1.1, sub-topics 3.2.2 to 3.2.4.