A-LevelComputer ScienceData types and structures9618/42 · May/June 2023 · Q228 Marks

A business sells a single product. Customers can purchase one or more of this product. Each sale has an ID and a quantity, for example "ABC" and 2 The business needs a program to store the data about the sales in a circular queue data structure.

(a)Write program code to declare a record structure, SaleData, to store the data about each sale.[2]
(b)Write program code to: • declare a global array, CircularQueue, of 5 items to store the sale records • declare the global pointers Head and Tail • declare the global variable NumberOfItems • initialise all elements of the array CircularQueue to an empty record, where the ID is null ("") and quantity is -1 • initialise Head, Tail and NumberOfItems to 0[4]
(c)The function Enqueue(): • takes a new record as a parameter • inserts the record in the circular queue at the element pointed to by Tail • updates pointers and other variables as required • returns -1 if the circular queue is full • returns 1 if the record is stored successfully. Write program code for the function Enqueue().[6]
(d)The function Dequeue(): • returns a null or empty record if the circular queue is empty • returns the first record in the queue if the circular queue is not empty • updates pointers and other variables as required. Write program code for the function Dequeue().[6]
(e)The procedure EnterRecord(): • takes an ID and quantity as input and creates a sale record • uses Enqueue () to insert the record in the circular queue • outputs "Full" if the record was not inserted in the circular queue • outputs "Stored" if the record was inserted in the circular queue. Write program code for the procedure EnterRecord().[5]
(f)(i)The following sale records need to be entered into the program: ID Quantity ADF 10 OOP 1 BXW 5 XXZ 22 HQR 6 LLP 3 Amend the main program to: • use EnterRecord() to input the six records in the table • use Dequeue () to remove one record • output either the ID and quantity of the removed record, or an error message if the circular queue is empty • use EnterRecord() to input the record with the ID "LLP" for a second time • output the ID and quantity for all the records currently stored in the array CircularQueue. Write program code to perform these tasks.[4]
(f)(ii)Test your program. Take a screenshot of the output.[1]

Mark scheme

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

[1] Record declaration named SaleData // class declaration (and end) named SaleData...
[1] ...SaleID declared as string, Quantity as integer in record // if a class then a constructor assigning attributes SaleID and Quantity
[1] Global array CircularQueue of 5 items of type SaleData
[1] Global variables Head, Tail and NumberOfItems all initialised to 0
[1] One record declared setting ID to "" and Quantity to -1
[1] ...stored in all 5 array elements
[1] Function Enqueue() header (and end) taking one parameter (type SaleData)
[1] Checks if queue is full ...
[1] and returns -1
[1] (otherwise) Inserts parameter to CircularQueue[Tail]
[1] increments Tail and resets to 0 if 5
[1] Increments number of items and returns 1
[1] Function header Dequeue() (and end where appropriate)
[1] Checking if queue is empty...
[1] ....and returning appropriate empty/null record/object/list element
[1] (Otherwise) returning the item at Head
[1] Incrementing Head and changing value 0 if it is 4/5
[1] Decrement number of items
[1] Procedure header EnterRecord (and end where appropriate) (ignore parameters)
[1] Takes as input an ID (string) and quantity (integer)
[1] Creates a record/object using inputs
[1] Calls Enqueue () with record as parameter and stores/uses return value
[1] Outputs "Full" and "Stored" in correct places
[1] Calling EnterRecord() 6 times before dequeue
[1] Calling Dequeue() and storing/using return value
[1] checking if an empty record is returned and outputting either the ID and quantity of returned record or outputting the error message if empty record
[1] Calling EnterRecord() again after dequeue
[1] Output the ID and quantity for all the records currently stored in CircularQueue
[1] 1 mark for screenshot showing: Data for 6 records input, 5 messages stating (e.g.) stored and 1 message stating (e.g.) full, 1 output of ADF 10 (dequeued), Repeat successful input of LLP 3, Output of the 5 records