Cambridge A Level Computer Science 9618 — 2023 May/June Paper 2 · Variant 3

1 The following pseudocode represents part of the algorithm for a program. Line numbers are for reference only. 10 DECLARE Sheet4 : ARRAY[1:2, 1:50] OF INTEGER … 100 FOR PCount 0 TO 49 101 Sheet4[1, PCount] 0 102 Sheet4[2, PCount] 47 103 NEXT PCount (a) The pseudocode contains references to an array. Complete the table by writing the answer for each row. Answer The dimension of the array The name of the variable used as an array index The number of elements in the array [3] (b) The pseudocode contains two errors. One error is that variable PCount has not been declared. Identify the other error and state the line number where it occurs. Error .......................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... Line number .............................................................................................................................. [2] (c) The pseudocode does not include a declaration for PCount. State the data type that should be used in the declaration. ............................................................................................................................................. [1] (d) The pseudocode statements given in the following table are used in other parts of the algorithm. Complete the table by placing one or more ticks (✓) in each row. The first row has already been completed. Pseudocode statement Input Process Output INPUT MyChoice ✓ OUTPUT FirstName & LastName WRITEFILE YourFile, TextLine READFILE MyFile, TextLine Result SQRT(NextNum) [4]

Mark scheme: Question Answer Marks 1(a) 3 Answer The dimension of the array 2 The name of the variable used as an array index PCount The number of elements in the array 100 1(b) One mark per point: 2  The (second dimension/index of the) array is declared from 1 to 50 but the loop runs from 0 to 49  Line number: 10 / 100 / 101 / 102 1(c) Integer 1 1(d) One mark for each of rows 2 - 5 4 Pseudocode statement Input Process Output INPUT MyChoice  OUTPUT FirstName & LastName   WRITEFILE OutputFile, TextLine  READFILE MyFile, TextLine  Result  SQRT(NextNum) 

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2 A program stores a date of birth for a student using a variable, MyDOB, of type DATE. (a) MyDOB has been assigned a valid value corresponding to Kevin’s date of birth. Complete the pseudocode statement to test whether Kevin was born on a Thursday. IF ........................................................................................................................ THEN [2] (b) A function CheckDate()will take three integer parameters representing a day, month and year of a given date. The function will validate the date of birth for a student that the parameters passed to it represent. For a date to be valid, a student must be at least 18 in year 2020. (i) Two of the parameter values can be checked without reference to the third parameter. Describe these two checks. Check 1 ............................................................................................................................. ........................................................................................................................................... ........................................................................................................................................... Check 2 ............................................................................................................................. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... [2] (ii) Several values of the parameter representing the day can only be checked completely by referring to the value of one other parameter. Describe this check. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2]

Mark scheme: 2(a) One mark for each underlined part 2 IF DAYINDEX(MyDOB) = 5 THEN 2(b)(i) MP1 Value for month is between 1 and 12 (inclusive) 2 MP2 Value of year is <= 2002 2(b)(ii) MP1 Reference to month and day 2 MP2 Clear description for a check that the day number matches with a relevant month (Either day matches with month // month matches with day)

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3 A program processes data using a stack. The data is copied to a text file before the program ends. (a) The following diagram shows the current state of the stack. The operation of this stack may be summarised as follows: • The TopOfStack pointer points to the last item added to the stack. • The BottomOfStack pointer points to the first item on the stack. • The stack grows upwards when items are added. Stack Pointer Memory location Value 506 505 WWW TopOfStack 504 YYY 503 XXX 502 ZZZ 501 NNN 500 PPP BottomOfStack (i) An error will be generated if an attempt is made to POP a value when the stack is empty. State the maximum number of consecutive POP operations that could be performed on the stack shown above before an error is generated. ..................................................................................................................................... [1] (ii) The following operations are performed: 1. POP and store value in variable Data1 2. POP and store value in variable Data2 3. PUSH value AAA 4. PUSH value BBB 5. POP and discard value 6. POP and store value in variable Data2 Complete the diagram to show the state of the stack and the variables after the given operations have been performed. Stack Pointer Memory location Value 506 505 504 503 502 Variable Value 501 Data1 500 Data2 [4] (b) The data is copied to a text file before the program ends. (i) State an advantage of writing the data from the stack to a text file before the program ends. ........................................................................................................................................... ..................................................................................................................................... [1] (ii) A module SaveStack() will write the data from the stack to a text file. Express an algorithm for SaveStack() as five steps that could be used to produce pseudocode. Write the five steps. Step 1 ................................................................................................................................ ........................................................................................................................................... Step 2 ................................................................................................................................ ........................................................................................................................................... Step 3 ................................................................................................................................ ........................................................................................................................................... Step 4 ................................................................................................................................ ........................................................................................................................................... Step 5 ................................................................................................................................ ...........................................................................................................................................

Mark scheme: 3(a)(i) 6 1 3(a)(ii) 4 One mark for: MP1 Values 'BBB' and 'AAA' MP2 Values 'XXX' to 'PPP' (unchanged) MP3 Both pointers and labelled22 MP4 Values of both variables 3(b)(i) So that the data may be recovered / restored (the next time the program is 1 run) // the data is permanently saved / data is not lost when the program terminates 3(b)(ii) Max 5 marks 5 MP1 Open the text file in WRITE mode MP2 Check there is a value on the stack MP3 POP value …. MP4 Write value to the text file MP5 Repeat from Step 2 // loop referencing the stack items Alternative solution: Not using POP primitive MP1 Open the text file in WRITE mode MP2 Check there is a value on the stack MP3 Read value from ToS location MP4 Write the value to the text file – Must some attempt at ‘the value’ NOT ‘all the values’ MP5 Decrement ToS MP6 Repeat from step 2 // loop referencing the stack items

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4 A function MakeString() will: 1. take two parameters: • a count as an integer • a character 2. generate a string of length equal to the count, made up of the character 3. return the string generated, or return "ERROR" if the count is less than 1. For example, the function call: MakeString(3, 'Z') will return the string "ZZZ" Write pseudocode for function MakeString(). .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .................................................................................................................................................... [6]

Mark scheme: 4 FUNCTION MakeString(Count : INTEGER, AChar : CHAR) 6 RETURNS STRING DECLARE MyString : STRING DECLARE Index : INTEGER IF Count < 1 THEN MyString  "ERROR" ELSE MyString  "" FOR Index  1 TO Count MyString  MyString & AChar NEXT Index ENDIF RETURN MyString ENDFUNCTION MP1 Function heading and end including parameters and return type MP2 Declaration of locals Index and MyString MP3 Test for Count < 1 and if true, assign "ERROR" to MyString / Immediate RETURN MP4 Loop for Count iterations MP5 Use of concatenate – must have been initialised in a loop MP6 Return STRING (correctly in both cases)

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5 A program is designed, coded and compiled without errors. The compiled code is sent for testing. (a) The program will be tested using the walkthrough method. Additional information will be needed before this method can be used. Identify this additional information and explain why it is needed. Additional information ............................................................................................................... ................................................................................................................................................... Explanation ............................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... [3] (b) Testing is completed and the program is made available to users. Some time later, changes are made to the program to improve the speed of response. State the type of maintenance that has been applied to the program. ............................................................................................................................................. [1]

Mark scheme: 5(a) Max 3 marks 3 Additional Information: MP1 The (program/source) code/specification MP2 test plan // inputs/test data and expected outputs Explanation: MP3 The structure / design / algorithm of the program of the program needs to be known MP4 …. so that all paths through the program can be tested 5(b) Perfective 1

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6 A procedure Select() will: 1. take two integer values as parameters representing start and end values where both values are greater than 9 and the end value is greater than the start value 2. output each integer value between the start and the end value (not including the start and end values), where the sum of the last two digits is 6, for example, 142. (a) Write pseudocode for procedure Select(). Parameter validation is not required. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [7] (b) The check performed by procedure Select() on the last two digits is needed at several places in the program and will be implemented using a new function. The new function CheckNum() will: • allow the required sum to be specified (not just 6) • check one number • return an appropriate value. Describe the function interface and two advantages of this modular approach. Interface .................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... Advantage 1 ............................................................................................................................. ................................................................................................................................................... Advantage 2 ............................................................................................................................. ................................................................................................................................................... [4]

Mark scheme: 6(a) Max 7 marks 7 PROCEDURE Select(Start, End : INTEGER) DECLARE ThisNum, Total: INTEGER DECLARE ThisString : STRING DECLARE Char1, Char2 : CHAR FOR ThisNum  Start+1 TO End-1 ThisString  NUM_TO_STR(ThisNum) Char1  RIGHT(ThisString, 1) Char2  LEFT(RIGHT(ThisString, 2), 1) Total  STR_TO_NUM(Char1) + STR_TO_NUM(Char2) IF Total = 6 THEN OUTPUT ThisString ENDIF NEXT ThisNum ENDPROCEDURE MP1 Procedure heading and ending MP2 (Count-controlled) Loop MP3 …. with correct range from Start+1 to End-1 MP4 Convert ThisNum (loop counter) to a string MP5 Extract the last two/first/second ’character digit(s)’ required in a loop MP6 Extract the second individual ‘character digit’ required in a loop MP7 Calculate the sum of the last two digits MP8 If sum = 6 then OUTPUT the number (either string or integer) in a loop 6(b) Max 4 marks 4 MP1 The function will take two integer parameters - the number and the (required) total MP2 … and return a Boolean OR: CheckNum(Number,Total : INTEGER) RETURNS BOOLEAN MP1 MP2 Two marks for the advantages: MP3 CheckNum()can be called repeatedly as and when required MP4 CheckNum()is designed and tested once (then used repeatedly) MP5 Any subsequent change to CheckNum() needs to be made once only // is easier to maintain/modify

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7 A school has a library system which allows students to borrow books for a length of time. Information relating to students and books is stored in text files. Student information includes name, home address, email address, date of birth, tutor and subject choices. Book information includes author, title, subject category, library location and the date that the book was borrowed. A program helps the staff to manage the borrowing of books. (a) A new module needs to be written to generate emails to send to students who have an overdue book. Students who are sent an email are prevented from borrowing any more books until the overdue book is returned. The process of abstraction has been used when designing the new module. (i) State the purpose of applying abstraction to this problem. ........................................................................................................................................... ..................................................................................................................................... [1] (ii) Identify one item of information that is required and one item that is not required in the new module. Justify your choices. Item required ..................................................................................................................... Justification ....................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... Item not required ............................................................................................................... Justification ....................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... [2] (iii) Identify two operations that would be required to process data when an overdue book is returned. Operation 1 ....................................................................................................................... ........................................................................................................................................... Operation 2 ....................................................................................................................... ........................................................................................................................................... [2] (b) Part of the library program contains program modules with headers as follows: Pseudocode module header PROCEDURE Module-X() PROCEDURE Module-Y(BYREF RA : INTEGER, SA : REAL) PROCEDURE Overlay() FUNCTION Replace(RA : INTEGER, RB : BOOLEAN) RETURNS BOOLEAN FUNCTION Reset(TA : STRING) RETURNS INTEGER Module-X() and Module-Y() are both called from module Overlay(). Complete the structure chart. [3]

Mark scheme: 7(a)(i)  To filter out information (that is not necessary to solve the problem) // 1 to include only essential information 7(a)(ii) Required: 2 Student : Student name / email (address) Loan: Return/Issue date Book: Book title Not Required: Student: Home address / DoB / tutor / subject choices Book: Library location / category / author / book title 7(a)(iii) Max 2 marks 2 Examples could include:  Clear the loan // indicate that the book has been returned // update loan history  Take the student off 'block' // allow the student to borrow further books  Send acknowledgement to the student when book is returned 7(b) Max 3 marks 3 MP1 All modules correctly labelled and interconnected. MP2 Correct parameters and return type to Module-X and Reset MP3 Correct parameters and return type to Module-Y and Replace MP4 Correct parameters and return type to Overlay and Module-Y

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8 A computer shop assembles desktop computers, using items bought from several suppliers. A text file Stock.txt contains information about each item. Information for each item is stored as a single line in the Stock.txt file in the format: <ItemNum><SupplierCode><Description> Item information is as follows: Format Comment unique number for each item in the range “0001” ItemNum 4 numeric characters to “5999” inclusive SupplierCode 3 alphabetic characters code to identify the supplier of the item Description a string a minimum of 12 characters The file is organised in ascending order of ItemNum and does not contain all possible values in the range. The programmer has defined the first program module as follows: Module Description ChangeSupp() • called with two parameters Code1 and Code2 of type string that represent valid supplier codes • creates a new file NewStock.txt from the contents of the file Stock.txt where any reference to Code1 is replaced by Code2 • returns a count of the number of items that have had their supplier code changed (a) Write pseudocode for module ChangeSupp(). ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [8] (b) A new module is required: Module Description Report_1() • takes a parameter of type string that represents a SupplierCode • searches the Stock.txt file for each line of item information that contains the given SupplierCode • produces a formatted report of items for the given SupplierCode, for example, for supplier DRG, the output could be: Report for Supplier: DRG Item Description 1234 USB Printer Cable 3 m 1273 32GB USB Flash Drive 1350 Mouse Mat 320 x 240 mm Number of items listed: 3 Write pseudocode for module Report_1(). ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ...................................................................................................................................................

Mark scheme: 8(a) FUNCTION ChangeSupp(Code1, Code2 : STRING) RETURNS 8 INTEGER DECLARE Count : INTEGER DECLARE ThisLine, ThisCode : STRING OPENFILE "Stock.txt" FOR READ OPENFILE "NewStock.txt" FOR WRITE Count  0 WHILE NOT EOF("Stock.txt") READFILE("Stock.txt ", ThisLine) // brackets optional ThisCode  MID(ThisLine, 5, 3) IF ThisCode = Code1 THEN ThisLine  LEFT(ThisLine, 4) & Code2 & RIGHT(ThisLine, LENGTH(ThisLine) - 7) Count  Count + 1 ENDIF WRITEFILE("NewStock.txt", ThisLine) // brackets optional ENDWHILE CLOSEFILE "NewStock.txt" CLOSEFILE "Stock.txt" RETURN Count ENDFUNCTION MP1 Open both files, in correct modes, and subsequently close MP2 Conditional loop until EOF(“Stock.txt”) MP3 Read a line from Stock.txt AND extract ThisCode in a loop MP4 Test ThisCode = Code1 AND if true, increment Count (must have been Initialised)in a loop MP5 Update ThisLine using substring functions and '&' in a loop MP6 completely correct update of ThisLine in a loop MP7 Write ThisLine to NewStock.txt in a loop MP8 Return count after loop 8(b) PROCEDURE Report_1(Supp : STRING) 6 DECLARE Count : INTEGER DECLARE ThisItemNum, ThisDesc, ThisLine, ThisCode : STRING Count  0 OPENFILE "Stock.txt" FOR READ OUTPUT "Report for Supplier:" & Supp OUTPUT "" //Blank line as per example OUTPUT "Item Description" OUTPUT "" //Blank line as per example WHILE NOT EOF("Stock.txt") READFILE("Stock.txt", ThisLine) ThisCode  Mid(ThisLine, 5, 3) IF ThisCode = Supp THEN ThisItemNum  LEFT(ThisLine, 4) ThisDesc  RIGHT(ThisLine, LENGTH(ThisLine) - 7) OUTPUT ThisItem & " " & ThisDesc Count  Count + 1 ENDIF ENDWHILE CLOSEFILE "Stock.txt" OUTPUT "" //Blank line as per example OUTPUT "Number of items listed: ", Count ENDPROCEDURE MP1 Output report header (blank lines optional) – Must contain the parameter code MP2 Conditional loop until EOF("Stock.txt") MP3 Read a line from Stock.txt AND extract SupplierCode in a loop MP4 Test if SupplierCode = Supp then increment count (must have been Initialised) MP5 Extract AND output item and description in a loop MP6 Output the final line with count 8(c)(i) Max 2 marks 2 MP1 Must ‘calculate’ the count before any item + description output / after the file is read once MP2 Lines to be output have to be stored … MP3 The file has to be read twice 8(c)(ii) One mark per point: 3 MP1 Loop through the file calculating the count MP2 Save ‘selected’ items in an array MP3 (After all lines have been read), output the header lines / count MP4 Loop through the array to output each array element

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