Cambridge A Level Computer Science 9608 — 2021 Oct/Nov Paper 2 · Variant 3

This document has 24 pages. Any blank pages are indicated. Cambridge International AS & A Level * 3 2 6 3 2 2 9 2 3 5 * DC (PQ/CGW) 208684/3 © UCLES 2021 [Turn over COMPUTER SCIENCE 9608/23 Paper 2 Fundamental Problem-solving and Programming Skills October/November 2021 2 hours You must answer on the question paper. No additional materials are needed. INSTRUCTIONS ● Answer all questions. ● Use a black or dark blue pen. ● Write your name, centre number and candidate number in the boxes at the top of the page. ● Write your answer to each question in the space provided. ● Do not use an erasable pen or correction fluid. ● Do not write on any bar codes. ● You may use an HB pencil for any diagrams, graphs or rough working. ● Calculators must not be used in this paper. INFORMATION ● The total mark for this paper is 75. ● The number of marks for each question or part question is shown in brackets [ ]. ● No marks will be awarded for using brand names of software packages or hardware.

2 9608/23/O/N/21 © UCLES 2021 1 (a) An algorithm contains the following steps: 1. prompt and input a product number 2. validate the product number 3. save the validated product number to FILE_A.txt (i) Write the pseudocode statement for step 1. … …[2] (ii) State the purpose of FILE_A.txt. … …[1] (b) State two benefits of creating program flowcharts when documenting a program. 1 … … 2 … … [2] (c) Draw one line from each technical term to its appropriate description. Technical term Description Corrective maintenance Array Adaptive maintenance Structure chart Stores data of the same data type in memory Amends an algorithm following identification of errors Amends an algorithm in response to specification changes Shows parameters passed between program modules [3]

3 9608/23/O/N/21 © UCLES 2021 [Turn over (d) The following pseudocode procedure, SetupVars(), initialises the variables used to check the status of a product. PROCEDURE SetupVars() Description "CONCRETE-SLAB" Destination "ELY" Mileage 200 EndOfYear 1 Limit 20000 Overdue FALSE ENDPROCEDURE Complete the table by evaluating each pseudocode expression. If the expression is invalid, write “ERROR” in the Evaluates to column. Refer to the Appendix on pages 22–23 for a list of built-in pseudocode functions and operators. Pseudocode expression Evaluates to EndOfYear * Limit / Mileage LENGTH(Description) / NUM_TO_STRING(Limit) MOD(20, LENGTH(Destination)) (EndOfYear < 2) AND (Limit = 20000) AND NOT(Overdue) MID(Description, 1, 5) & LEFT(Destination, 2) & NUM_TO_STRING(Limit / 1000) [5]

4 9608/23/O/N/21 © UCLES 2021 2 A company owns 50 cars that are available for hire. The cars are numbered from 1 to 50. The size of a car can be small, medium, or large. The following pseudocode procedure, SetOut(), takes as input the size of the car the customer has requested for hire and outputs details of the cars that are available for hire for that size of car. Line numbers are given for reference only. Refer to the Appendix on pages 22–23 for a list of built-in pseudocode functions and operators. 01 DECLARE Available : ARRAY[1:50] OF BOOLEAN 02 DECLARE CarSize : ARRAY[1:50] OF STRING 03 04 PROCEDURE SetOut(Size: STRING) 05 DECLARE Index : REAL 06 07 FOR Index 0 TO 50 08 IF Size = CarSize[Index] AND Available[Index] = "TRUE" 09 THEN 10 OUTPUT NUM_TO_STRING(Index) & ": " & Size & " Available" 11 ENDIF 12 ENDFOR 13 14 ENDPROCEDURE (a) Complete the table by identifying four line numbers that contain errors and give the correct pseudocode statement. Line number Correct pseudocode statement [4]

5 9608/23/O/N/21 © UCLES 2021 [Turn over (b) Study the pseudocode algorithm on page 4. Describe the algorithm using structured English. Do not use pseudocode or program code in your answer. … … … … … … … … [4] (c) The function PayUsingAccount() is required to process a payment for hiring a car as follows: 1. The function takes two parameters with REAL data type: • Balance is the amount of money in the account • CostOfHire is the cost of hiring the car. 2. If CostOfHire exceeds Balance, the rental is not authorised and FALSE is returned, otherwise TRUE is returned. Write pseudocode for the function PayUsingAccount(). … … … … … … … … … … … … … [3]

6 9608/23/O/N/21 © UCLES 2021 3 The function AddressChecker() checks if an email address is in the format: <user>@<host>•<domain> The design requirements of the function are: • The <user> contains only lower-case characters. • The <domain> contains exactly three alphabetic characters. AddressChecker() will use three functions as described in the following table. Function Description Split() • Takes two parameters: • a STRING value containing the email address • a BOOLEAN value representing the required return value • Returns two different values: • a STRING containing <user> if the BOOLEAN parameter value is TRUE • a STRING containing <domain> if the BOOLEAN parameter value is FALSE IsLowerCase() • Takes an element of the email address as a STRING parameter • Returns a BOOLEAN value of TRUE if the string contains only lower-case characters, otherwise returns FALSE IsThreeLetter() • Takes the domain as a STRING parameter • Returns a BOOLEAN value of TRUE if the string consists of exactly three alphabetic characters, otherwise returns FALSE AddressChecker() will: • prompt and input an email address • check that the email address is valid • return the string "Invalid" if the email address does not meet the design requirements, otherwise return the email address.

7 9608/23/O/N/21 © UCLES 2021 [Turn over (a) Draw a program flowchart to represent the algorithm for the AddressChecker() function. Variable declarations are not required in program flowcharts. [6]

8 9608/23/O/N/21 © UCLES 2021 (b) The function AddressChecker() is changed to check a list of email addresses stored in a text file. The function’s header is changed as follows: FUNCTION AddressChecker(Filename : STRING, StartLine : INTEGER, LastLine : INTEGER) RETURNS BOOLEAN Assume that: • The function is called with a valid filename of the text file. • The text file exists and contains at least one email address. • Each email address is stored in a separate line in the text file. • The function does not contain logical errors. Give two reasons why the function may return an unexpected result. Reason 1 … … Reason 2 … … [2] (c) (i) Explain what is meant by the term syntax error. … … … … [2] (ii) Give an example of a syntax error in program code. Programming language … Program code … … … … [1]

9 9608/23/O/N/21 © UCLES 2021 [Turn over (d) The following incomplete structure chart shows part of the design of the function AddressChecker(). Complete the structure chart. AddressChecker() IsThreeLetter() [3] (e) State and describe two features of an Integrated Development Environment (IDE) that aid debugging. Feature 1 … … Description … … … Feature 2 … … Description … … … [4]

10 9608/23/O/N/21 © UCLES 2021 4 The following pseudocode contains a function that searches a text file for the string contained in the parameter Match. 01 DECLARE MatchItems : ARRAY[1:100] OF STRING 10 FUNCTION Extract(FileName : STRING, Match : STRING, Signal : INTEGER) RETURNS BOOLEAN 11 12 DECLARE FileLine : INTEGER 13 DECLARE Counter : INTEGER 14 DECLARE FileData, SubMatch : STRING 15 DECLARE Result : BOOLEAN 16 17 Counter 0 18 FileLine 1 19 Result FALSE 20 21 OPENFILE FileName FOR READ 22 23 WHILE NOT EOF(FileName) AND FileLine <= 100 24 READFILE FileName, FileData 25 SubMatch LEFT(FileData, LENGTH(Match)) 26 IF SubMatch = Match 27 THEN 28 MatchItems[Counter] FileData 29 Counter Counter + 1 30 ENDIF 31 FileLine FileLine + 1 32 ENDWHILE 33 34 CLOSEFILE FileName 35 IF Counter >= Signal 36 THEN 37 Result TRUE 38 ENDIF 39 RETURN Result 40 41 ENDFUNCTION (a) Complete the trace table when the function is called using this statement: Present Extract("DATA.txt", "TG12367", 1) You may assume that the text file DATA.txt contains the following lines: "XD43668#23/11/19#DSCP" "TG12367#24/01/19#MAHA" "HD44356#24/11/19#MAHA" "TG12367#24/11/19#GHFI"

11 9608/23/O/N/21 © UCLES 2021 [Turn over FileLine Counter LEFT(FileData, LENGTH(Match)) SubMatch = Match Result [5] (b) The function is changed as follows: • The statement on line 25 is changed to compare the last ten characters of FileData and the parameter Match. If the comparison is successful: • store the first seven characters of FileData in a global 1D array called Original • store the last four characters of FileData in a global 1D array called Backup • The count of the number of items stored in the array Backup is returned. (i) Rewrite the function header in pseudocode so that it meets the new requirements. … … … … [2]

12 9608/23/O/N/21 © UCLES 2021 (ii) Rewrite the WHILE structure in pseudocode so that it meets the new requirements. The WHILE structure is given here for reference. 23 WHILE NOT EOF(FileName) AND FileLine <= 100 24 READFILE FileName, FileData 25 SubMatch LEFT(FileData, LENGTH(Match)) 26 IF SubMatch = Match 27 THEN 28 MatchItems[Counter] FileData 29 Counter Counter + 1 30 ENDIF 31 FileLine FileLine + 1 32 ENDWHILE … … … … … … … … … … … … … … … [3]

13 9608/23/O/N/21 © UCLES 2021 [Turn over Question 4 continues on the next page.

14 9608/23/O/N/21 © UCLES 2021 (c) A procedure, Encrypt(), is required to encrypt the data stored in each line of the text file DATA.txt. You may assume that the text file DATA.txt contains the following lines: "XD43668#23/11/19#DSCP" "TG12367#24/01/19#MAHA" "HD44356#24/11/19#MAHA" "TG12367#24/11/19#GHFI" The requirements of the procedure are: • the day of the month from the line of data is extracted as the encryption key • the encryption key is added to the ASCII value of each character of the line to form the encrypted data • the encrypted data are appended to the text file DATA-EN.txt until there are no more lines in the text file DATA.txt Write program code for the procedure Encrypt(). Visual Basic and Pascal: You should include the declaration statements for variables. Python: You should show a comment statement for each variable used with its data type. Programming language … Program code … … … … … … … … … … … … … … …

15 9608/23/O/N/21 © UCLES 2021 [Turn over … … … … … [6]

16 9608/23/O/N/21 © UCLES 2021 5 A geocode string contains four alphanumeric characters, followed by the character '+' and then two alphanumeric characters. For example, the geocode string for the Avocado Restaurant in Cambridge UK is: "1DFG+9N" A website allows a user to upload a review of a restaurant. A programmer has defined the following two arrays: Array Description GeoData • a global 1D array of data type STRING • stores the geocode for each restaurant Example: GeoData[1] "1DFG+9N" Review • a global 1D array of data type STRING • stores the following data items as a single element in the array: • the four-character user ID of the reviewer • the geocode of the restaurant • the text of the review • the hash symbol (#) separates the data items in each single element Example of a review for the Avocado restaurant submitted by user ID JH76: Review[1000] "JH76#1DFG+9N#Good restaurant" Assume that: • both arrays are initialised with an empty string • both arrays can store up to 20 000 elements • new reviews are added in the next unused element of the Review array. (a) The programmer writes a function CheckReview(). The design requirements of the function are shown in the following table: Description Parameter Returns • searches the GeoData array to determine if the user’s latest review contains a listed geocode • a STRING value containing the user ID of the reviewer • the comment from the review if the geocode exists in the GeoData array • a STRING value of "LOCATION NOT FOUND" if the geocode of the user’s review does not exist in the GeoData array • a STRING value of "NO REVIEW" if no review exists for the user ID

17 9608/23/O/N/21 © UCLES 2021 [Turn over Write pseudocode to complete the function CheckReview(). DECLARE GeoData : ARRAY[1:20000] OF STRING DECLARE Review : ARRAY[1:20000] OF STRING FUNCTION CheckReview(UserID : STRING) RETURNS STRING … … … … … … … … … … … … … … … … … … … … … … … … … … ENDFUNCTION [8]

18 9608/23/O/N/21 © UCLES 2021 (b) Before a review is published on the website, it is checked to find out whether the review contains a valid geocode. The function AddReview() has the following requirements: Description Parameters Returns • calls CheckReview() • if the text of the review is returned from CheckReview(), store the text in the global 1D array Accepted • a STRING value containing the user ID of the reviewer • a BOOLEAN value of FALSE if the text of the review is not added in the array Accepted, otherwise returns a BOOLEAN value of TRUE Assume that: • the 1D array Accepted is initialised with empty strings • the array contains up to 20 000 elements. Write program code for the function AddReview(). Visual Basic and Pascal: You should include the declaration statements for variables. Python: You should show a comment statement for each variable used with its data type. Programming language … Program code … … … … … … … … … … … … … … …

19 9608/23/O/N/21 © UCLES 2021 [Turn over … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … [4]

20 9608/23/O/N/21 © UCLES 2021 (c) When reviewers write a review for a restaurant, they give the restaurant a score out of 10. The 2D global array ReviewScores stores the review data as follows: • The first column element stores the seven-character alphanumeric geocode string of the restaurant with its average score, separated by the hash symbol (#). • The second column element stores the comment from the review that has the highest score. The array has dimensions of 20 000 rows and 2 columns. For example, the contents of the 2D array ReviewScores for review 20 are: ReviewScores[20, 1] "1DFG+9N#8.3" ReviewScores[20, 2] "Food was good but I waited too long" The procedure BestRestaurants()is required as follows: Description Parameter Output • searches the array ReviewScores for restaurants with an average score that exceeds 8.0 • a STRING value consisting of the geocode of the restaurant • if the parameter matches a geocode in the array ReviewScores and the average score exceeds 8.0, output the geocode concatenated with the review comment Write pseudocode for the procedure BestRestaurants(). … … … … … … … … … … … … … … …

21 9608/23/O/N/21 © UCLES 2021 [Turn over … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … [5]

22 9608/23/O/N/21 © UCLES 2021 Appendix Built-in functions (pseudocode) Each function returns an error if the function call is not properly formed. ASC(ThisChar : CHAR) RETURNS INTEGER returns the ASCII value of ThisChar Example: ASC('A') returns 65 CHR(x : INTEGER) RETURNS CHAR returns the character whose ASCII value is x Example: CHR(87) returns 'W' LENGTH(ThisString : STRING) RETURNS INTEGER returns the integer value representing the length of string ThisString Example: LENGTH("Happy Days") returns 10 LEFT(ThisString : STRING, x : INTEGER) RETURNS STRING returns leftmost x characters from ThisString Example: LEFT("ABCDEFGH", 3) returns "ABC" MID(ThisString : STRING, x : INTEGER, y : INTEGER) RETURNS STRING returns a string of length y starting at position x from ThisString Example: MID("ABCDEFGH", 2, 3) returns "BCD" RIGHT(ThisString: STRING, x : INTEGER) RETURNS STRING returns rightmost x characters from ThisString Example: RIGHT("ABCDEFGH", 3) returns "FGH" MOD(ThisNum : INTEGER, ThisDiv : INTEGER) RETURNS INTEGER returns the integer value representing the remainder when ThisNum is divided by ThisDiv Example: MOD(10,3) returns 1 NUM_TO_STRING(x : REAL) RETURNS STRING returns a string representation of a numeric value Note: This function will also work if x is of type INTEGER Example: NUM_TO_STRING(87.5) returns "87.5" STRING_TO_NUM(x : STRING) RETURNS REAL returns a numeric representation of a string Note: This function will also work if x is of type CHAR Example: STR_TO_NUM("23.45") returns 23.45

23 9608/23/O/N/21 © UCLES 2021 Operators (pseudocode) Operator Description & Concatenates (joins) two strings Example: "Summer" & " " & "Pudding" produces "Summer Pudding" AND Performs a logical AND on two Boolean values Example: TRUE AND FALSE produces FALSE OR Performs a logical OR on two Boolean values Example: TRUE OR FALSE produces TRUE

24 9608/23/O/N/21 © UCLES 2021 BLANK PAGE Permission to reproduce items where third-party owned material protected by copyright is included has been sought and cleared where possible. Every reasonable effort has been made by the publisher (UCLES) to trace copyright holders, but if any items requiring clearance have unwittingly been included, the publisher will be pleased to make amends at the earliest possible opportunity. To avoid the issue of disclosure of answer-related information to candidates, all copyright acknowledgements are reproduced online in the Cambridge Assessment International Education Copyright Acknowledgements Booklet. This is produced for each series of examinations and is freely available to download at www.cambridgeinternational.org after the live examination series. Cambridge Assessment International Education is part of the Cambridge Assessment Group. Cambridge Assessment is the brand name of the University of Cambridge Local Examinations Syndicate (UCLES), which itself is a department of the University of Cambridge.