A stack is to be set up using the information in the table. Identifier BasePointer TopPointer Stack Data type INTEGER INTEGER REAL Description points to the bottom of the stack points to the top of the stack 1D array to implement the stack A constant, with identifier Capacity, limits the size of the stack to 25 items.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2023). The mark or mark type is shown in brackets.

1. [1] MP1 Correct constant declaration
2. [1] MP2 Two correct variable declarations
3. [1] MP3 Correct array declaration Example answer: CONSTANT Capacity = 25 DECLARE BasePointer : INTEGER DECLARE TopPointer : INTEGER DECLARE Stack : ARRAY[1:25] OF REAL
4. [1] IF TopPointer >= BasePointer THEN
5. [1] Item ← Stack[TopPointer]
6. [1] TopPointer ← TopPointer – 1
7. [1] ELSE
8. [1] RETURN Item
9. [1] MP1 A queue is a first in first out / FIFO data structure and a stack is a first in last out / FILO / LIFO data structure // Data is removed from a queue in the order it is received and removed from a stack in the reverse order to which it is received
10. [1] MP2 Both ADTs can vary in size / are of indeterminate length
11. [1] MP3 Data is popped and pushed (onto/from a stack) at the same end but it is enqueued and dequeued (to/from a queue) at different/opposite ends // a queue has two accessible ends and a stack has only one
12. [1] MP4 A stack has only one moveable pointer whereas a queue has two.