Computer Science 9618/42 May/June 2025: question paper and mark scheme

Mark scheme

1(a) 1 mark each 2 • (Global) Stack as 1D array (of strings) with 20 elements initialised to string "–1" • (Global) TopOfStack initialised to –1 Example program code: Java public static String[] Stack = new String[20]; public static Integer TopOfStack; public static void main(String args[]){ for(Integer X = 0; X < 20; X++){ Stack[X] = "-1"; } TopOfStack = -1; } VB.NET Dim Stack(19) As String Dim TopOfStack As Integer For x = 0 To 19 Stack(x) = "-1" Next TopOfStack = -1 Python Stack = [] TopOfStack = -1 #main for x in range(20): Stack.append("-1") 1(b) 1 mark each 4 • Push function header (and end where appropriate) taking one (string) parameter • Checking if stack is full and returning integer –1 • (Otherwise) Incrementing TopOfStack • Storing parameter in the incremented the stack at TopOfStack and returning integer 1 Example program code: Java public static Integer Push(String Data){ if (TopOfStack == 19){ return -1; }else{ TopOfStack++; Stack[TopOfStack] = Data; return 1; } } VB.NET Function Push(ByVal Data) If TopOfStack = 19 Then Return -1 Else TopOfStack = TopOfStack + 1 Stack(TopOfStack) = Data Return 1 End If End Function Python def Push(Data): global Stack global TopOfStack if TopOfStack == 19: return -1 else: TopOfStack += 1 Stack[TopOfStack] = Data return 1 1(c) 1 mark each 4 • Function header (and end where appropriate) and returning a value in all cases. • Checking if stack is empty and returning string "–1" • (Otherwise) Decrementing TopOfStack • Returning element in stack at TopOfStack before TopOfStack is decremented Example program code: Java public static String Pop(){ if (TopOfStack == -1){ return "-1"; }else{ String ReturnValue = Stack[TopOfStack]; TopOfStack--; return ReturnValue; } } VB.NET Function Pop() If TopOfStack = -1 Then Pop = "-1" Else Pop = Stack(TopOfStack) TopOfStack = TopOfStack - 1 End If End Function Python def Pop(): global Stack global TopOfStack if TopOfStack == -1: return "-1" else: ReturnValue = Stack[TopOfStack] TopOfStack -= 1 return ReturnValue 1(d) 1 mark each 6 • Procedure header (and end where appropriate) taking one (string) parameter • Opening the file with the filename parameter and closing the file in an appropriate place • Looping through to end of file and reading in each line ... • … calling Push() once with each read in value … • … if any return value from Push() is integer –1 outputting "Stack full" • Exception handling for opening and reading from file with appropriate catch and output Example program code: Java public static void ReadData(String FileName){ Integer ReturnValue; try{ FileReader f = new FileReader(FileName); try{ BufferedReader Reader = new BufferedReader(f); String Line= Reader.readLine(); Line = Line.replace("\n",""); while (Line != null){ Line = Line.replace("\n",""); ReturnValue = Push(Line); if (ReturnValue == -1){ System.out.println("Stack full"); } Line = Reader.readLine(); } Reader.close(); }catch(IOException ex){} }catch(FileNotFoundException e){ System.out.println("File not found"); } } VB.NET Sub ReadData(ByVal FileName As String) Dim ReturnValue As String Try Dim FileReader As New System.IO.StreamReader(FileName) While Not FileReader.EndOfStream ReturnValue = Push(FileReader.ReadLine()) If ReturnValue = "-1" Then Console.WriteLine("Stack full") End If End While FileReader.Close() Catch ex As Exception Console.WriteLine("Cannot open file") End Try End Sub Python def ReadData(FileName): global Stack global TopOfStack try: File = open(FileName) for Line in File: ReturnValue = Push(Line.strip()) if ReturnValue == -1: print("Stack full") File.close() except: print("Cannot open file") 1(e) 1 mark for: 7 • Calculate() function header (and end where appropriate) • Looping until the stack is empty • Calling Pop() repeatedly within loop and storing/using return value • … working out if return value from Pop() call is an operator or a number / alternating between operator and number • Select to determine if the operator is +, -, /, * or ^ and attempt the matching calculation • … performing correct calculation using operator, number • … updating total from previous loops and returning this final value Example program code: Java public static Double Calculate(){ Double Total = Double.parseDouble(Pop()); String ReturnValue = ""; String LastOperator = ""; Boolean OperatorFlag = true; Integer TheData = 0; while(ReturnValue != "-1"){ ReturnValue = Pop(); if(OperatorFlag == false){ TheData = Integer.parseInt(ReturnValue); if(LastOperator.compareTo("+")==0){ Total = Total + TheData; }else if(LastOperator.compareTo("-")==0){ Total = Total - TheData; }else if(LastOperator.compareTo("*")==0){ Total = Total * TheData; }else if(LastOperator.compareTo("/")==0){ Total = Total / TheData; }else if(LastOperator.compareTo("^")==0){ Total = Math.pow(Total, TheData); } OperatorFlag = true; }else{ LastOperator = ReturnValue; OperatorFlag = false; } } return Total; } VB.NET Function Calculate() Dim Total As Integer = Pop() Dim ReturnValue As String = "" Dim LastOperator As String = "" Dim OperatorFlag As Boolean = True Dim TheData As Integer = 0 While (ReturnValue <> "-1") ReturnValue = Pop() Select Case OperatorFlag Case False TheData = ReturnValue Select Case LastOperator Case "+" Total = Total + TheData Case "-" Total = Total - TheData Case "*" Total = Total * TheData Case "/" Total = Total / TheData Case "^" Total = Total ^ TheData End Select OperatorFlag = True Case Else LastOperator = ReturnValue OperatorFlag = False End Select End While Return Total End Function Python def Calculate(): global Stack global TopOfStack Total = Pop() Total = int(Total) Return = 0 LastOperator = "" Operator = True while(Return != "-1"): Return = Pop() if Operator == False: Data = int(Return) if LastOperator == "+": Total = Total + Data elif LastOperator == "-": Total = Total - Data elif LastOperator == "*": Total = Total * Data elif LastOperator == "/": Total = Total / Data elif LastOperator == "^": Total = Total ** Data Operator = True else: LastOperator = Return Operator = False return Total 1(f)(i) 1 mark each 2 • Taking a filename as input and calling ReadData() with input • Calling Calculate() and outputting the return value Example program code: Java TopOfStack = -1; System.out.println("Enter the filename"); Scanner scanner = new Scanner(System.in); String FileName = scanner.nextLine(); ReadData(FileName); Double ReturnValue = Calculate(); System.out.println(ReturnValue); VB.NET Console.WriteLine("Enter the filename") Dim FileName As String = Console.ReadLine() ReadData(FileName) Dim ReturnValue As Single ReturnValue = Calculate() Console.WriteLine(ReturnValue) Python FileName = input("Enter the filename: ") ReadData(FileName) ReturnValue = Calculate() print(ReturnValue) 1(f)(ii) 1 mark for screenshot showing input of StackData.txt and output of 131 2 1 mark for screenshot showing input of SecondStack.txt and output of 320 e.g.

Mark scheme

2(a) 1 mark each 2 • Record/class NewRecord declared • 3 variables within structure (all integer) Example program code: Java class NewRecord{ private Integer Key; private Integer Item1; private Integer Item2; public NewRecord(Integer pKey, Integer pItem1, Integer pItem2){ Key = pKey; Item1 = pItem1; Item2 = pItem2; } public Integer GetKey(){ return Key; } public Integer GetItem1(){ return Item1; } public Integer GetItem2(){ return Item2; }} VB.NET Structure NewRecord Dim Key As Integer Dim Item1 As Integer Dim Item2 As Integer End Structure Python class Record: def __init__(self, pKey, pItem1, pItem2): self.__Key = pKey #integer self.__Item1 = pItem1 #integer self.__Item2 = pItem2 #integer def GetKey(self): return self.__Key def GetItem1(self): return self.__Item1 def GetItem2(self): return self.__Item2 2(b)(i) 1 mark for 1 • HashTable (200 records) and Spare (100 records) declared as (global) arrays Example program code: Java public static NewRecord[] HashTable = new NewRecord[200]; public static NewRecord[] Spare = new NewRecord[100]; VB.NET Dim HashTable(199) As NewRecord Dim Spare(99) As NewRecord Python HashTable = [] Spare = [] 2(b)(ii) 1 mark each 2 • Procedure Initialise() header (and close where appropriate) that initialises all elements in both arrays … • … to an empty record with –1 in each of the 3 fields/elements Example program code: Java public static void Initialise(){ NewRecord EmptyRecord = new NewRecord(-1,-1,-1); for(Integer X = 0; X < 200; X++){ HashTable[X] = EmptyRecord; } for(Integer X = 0; X < 100; X++){ Spare[X] = EmptyRecord; } } VB.NET Sub Initialise() Dim EmptyRecord As NewRecord EmptyRecord.Key = -1 EmptyRecord.Item1 = -1 EmptyRecord.Item2 = -1 For X = 0 To 199 HashTable(X) = EmptyRecord Next For X = 0 To 99 Spare(X) = EmptyRecord Next End Sub Python def Initialise(): global HashTable global Spare for X in range(200): HashTable.append(Record(-1,-1,-1)) for X in range(100): Spare.append(Record(-1,-1,-1)) 2(c) 1 mark each 2 • Function header (and close where appropriate), taking one (integer) parameter and returning the calculated value • Calculation of parameter MOD 200 Example program code: Java public static Integer CalculateHash(Integer TheKey){ return(TheKey % 200); } VB.NET Function CalculateHash(Key) Return Key Mod 200 End Function Python def CalculateHash(Key): return Key % 200 2(d) 1 mark each: 6 • Procedure header (and end where appropriate) taking one record as a parameter • Calling CalculateHash() with key from parameter record and storing/using return value • Checking if HashTable at return value from CalculateHash is empty record … • … if it is empty, store parameter in location • … otherwise, locating next free space in Spare … • … and storing in that index only (i.e. not in all other free spaces) Example program code: Java public static void InsertIntoHash(NewRecord TheRecord){ Integer HashValue = CalculateHash(TheRecord.GetKey()); if(HashTable[HashValue].GetKey().equals(-1)){ HashTable[HashValue] = TheRecord; }else{ for(Integer X = 0; X < 99; X++){ if(Spare[X].GetKey().equals(-1)){ Spare[X] = TheRecord; X = 99; } } } } VB.NET Sub InsertIntoHash(TheRecord) Dim HashValue As Integer = CalculateHash(TheRecord.Key) If HashTable(HashValue).Key = -1 Then HashTable(HashValue) = TheRecord Else For X = 0 To 99 If Spare(X).Key = -1 Then Spare(X) = TheRecord X = 100 End If Next End If End Sub Python def InsertIntoHash(TheRecord): global HashTable global Spare HashValue = CalculateHash(TheRecord.GetKey()) if HashTable[HashValue].GetKey() == -1: HashTable[HashValue] = TheRecord else: for x in range(0, 100): if Spare[x].GetKey() == -1: Spare[x] = TheRecord break 2(e) 1 mark each to max 5 5 • Procedure header (and end where appropriate), opening and closing file HashData.txt • Reading in all lines of data … • … splitting each line by commas • Creating record with correct values with each line read in from file • Calling InsertIntoHash() with each record they have created • Exception handling for opening and reading from file with appropriate catch and output. Example program code: Java public static void CreateHashTable(){ String[] Data = new String[3]; Integer NewKey; Integer NewItem1; Integer NewItem2; try{ FileReader File = new FileReader("HashData.txt"); try{ BufferedReader Reader = new BufferedReader(File); String Line= Reader.readLine(); while (Line != null){ Line = Line.replace("\n",""); Data = Line.split(","); NewKey = Integer.parseInt(Data[0]); NewItem1 = Integer.parseInt(Data[1]); NewItem2 = Integer.parseInt(Data[2]); NewRecord ReadData = new NewRecord(NewKey, NewItem1, NewItem2); InsertIntoHash(ReadData); Line= Reader.readLine(); } Reader.close(); }catch(IOException ex){} }catch(FileNotFoundException e){System.out.println("File not found");} } VB.NET Sub CreateHashTable() Dim Line As String Dim Data(3) As String Dim TheRecord As NewRecord Try Dim FileReader As New System.IO.StreamReader("HashData.txt") While Not FileReader.EndOfStream Line = FileReader.ReadLine() Data = Split(Line, ",") TheRecord.Key = Integer.Parse(Data(0)) TheRecord.Item1 = Integer.Parse(Data(1)) TheRecord.Item2 = Integer.Parse(Data(2)) InsertIntoHash(TheRecord) End While FileReader.Close() Catch ex As Exception Console.WriteLine("Cannot open file") End Try End Sub Python def CreateHashTable(): global HashTable global Spare try: File = open("HashData.txt") for Line in File: Data = Line.strip() Data = Line.split(",") InsertIntoHash(Record(int(Data[0]), int(Data[1]), int(Data[2]))) File.close() except: print("Cannot open file") 2(f)(i) 1 mark each 2 • Procedure header (and end where appropriate) and looping through each element in Spare … • … checking if record is empty and outputting key field if not empty Example program code: Java public static void PrintSpare(){ Integer X = 0; while(Spare[X].GetKey() != -1){ System.out.println(Spare[X].GetKey()); X++; } } VB.NET Sub PrintSpare() Dim X As Integer = 0 While Spare(X).Key <> -1 Console.WriteLine(Spare(X).Key) X = X + 1 End While End Sub Python def PrintSpare(): global Spare X = 0 while Spare[X].GetKey() != -1: print(Spare[X].GetKey()) X +=1 2(f)(ii) 1 mark for calling Initialise() then CreateHashTable() then PrintSpare() 1 Example program code: Java Initialise(); CreateHashTable(); PrintSpare(); VB.NET Initialise() CreateHashTable() PrintSpare() Python Initialise() CreateHashTable() PrintSpare() 2(f)(iii) 1 mark for output 1 For example:

Mark scheme

3(a)(i) 1 mark each 4 • Class header (and end when appropriate) • Four attributes with appropriate data types • Constructor header (and end where appropriate) within class taking (min) 4 parameters … • … assigning each parameter to its attribute Example program code: Java class Animal{ public String Name; public String Sound; public Integer Size; public Integer Intelligence; public Animal(String pName, String pSound, Integer pSize, Integer pIntelligence){ Name = pName; Sound = pSound; Size = pSize; Intelligence = pIntelligence; } } VB.NET Class Animal Public Name As String Public Sound As String Public Size As Integer Public Intelligence As Integer Sub New(pName, pSound, pSize, pIntelligence) Name = pName Sound = pSound Size = pSize Intelligence = pIntelligence End Sub End Class Python class Animal: def __init__(self, pName, pSound, pSize, pIntelligence): self.Name = pName #string self.Sound = pSound #string self.Size = pSize #integer self.Intelligence = pIntelligence #integer 3(a)(ii) 1 mark each 3 • Description() method header (and end where appropriate) with no parameter • Concatenating the attributes with the given message … • … and returning the created message Example program code: Java public String Description(){ String Message = "The animal's name is " + Name + ", it makes a " + Sound + ", its size is " + Size + " and its intelligence level is " + Intelligence; return Message; } VB.NET Function Description() Dim Message As String = "The animal's name is " & Name & ", it makes a " & Sound & ", its size is " & CStr(Size) & " and its intelligence level is " & CStr(Intelligence) Return Message End Function Python def Description(self): Message = "The animal's name is " + self.Name + ", it makes a " + self.Sound + ", its size is " + str(self.Size) + " and its intelligence level is " + str(self.Intelligence) return Message 3(b)(i) 1 mark each 4 • Class header (and end where appropriate) inherits from Animal • Constructor header (and end where appropriate) taking 6 parameters within class and calling parent constructor with the four parameters … • … WingSpan and NumberWords attributes defined with data types and parameters assigned within constructor • ChangeNumberWords() method header (and end where appropriate) takes one parameter and adds parameter to attribute NumberWords Example program code: Java class Parrot extends Animal{ public Integer WingSpan; public Integer NumberWords; public Parrot(String pName, String pSound, Integer pSize, Integer pIntelligence, Integer pWingSpan, Integer pNumberWords){ super(pName, pSound, pSize, pIntelligence); WingSpan = pWingSpan; NumberWords = pNumberWords; } public void ChangeNumberWords(Integer Change){ NumberWords = NumberWords + Change; }} VB.NET Class Parrot Inherits Animal Dim WingSpan As Integer Dim NumberWords As Integer Sub New(pName, pSound, pSize, pIntelligence, pWingSpan, pNumberWords) MyBase.New(pName, pSound, pSize, pIntelligence) WingSpan = pWingSpan NumberWords = pNumberWords End Sub Sub ChangeNumberWords(Change) NumberWords = NumberWords + Change End Sub End Class Python class Parrot(Animal): def __init__(self, pName, pSound, pSize, pIntelligence, pWingSpan, pNumberWords): super().__init__(pName, pSound, pSize, pIntelligence) self.WingSpan = pWingSpan #integer self.NumberWords = pNumberWords #integer def ChangeNumberWords(self, Change): self.NumberWords = self.NumberWords + Change 3(b)(ii) 1 mark each 2 • Description() method header (and end where appropriate) taking no parameters and overriding/overloads/extending/using parent method • Concatenating and returning the correct string Example program code: Java public String Description(){ String Message = "The animal's name is " + Name + ", it makes a " + Sound + ", its size is " + Size + " and its intelligence level is " + Intelligence + ". It has a wingspan of " + WingSpan + "cm and can say " + NumberWords + " words."; return Message; } VB.NET Overloads Function Description() Dim Message As String = "The animal's name is " & Name & ", it makes a " & Sound & ", its size is " & CStr(Size) & " and its intelligence level is " & CStr(Intelligence) & ". It has a wingspan of " & CStr(WingSpan) & "cm and can say " & CStr(NumberWords) & " words." Return Message End Function Python def Description(self): Message = "The animal's name is " + self.Name + ", it makes a " + self.Sound + ", its size is " + str(self.Size) + " and its intelligence level is " + str(self.Intelligence) + ". It has a wingspan of " + str(self.WingSpan) + "cm and can say " + str(self.NumberWords) + " words." return Message 3(c)(i) 1 mark each 4 • Class header (and end where appropriate) inherits from Animal • Constructor header (and end where appropriate) taking 5 parameters within class and calling parent constructor with parameters • Attribute Territory defined as int and parameter assigned within constructor • SetTerritory() method header (and end) takes 1 parameter and adds parameter to attribute TerritorySize Example program code: Java class Wolf extends Animal{ public Integer TerritorySize; public Wolf(String pName, String pSound, Integer pSize, Integer pIntelligence, Integer pTerritorySize){ super(pName, pSound, pSize, pIntelligence); TerritorySize = pTerritorySize; } public void SetTerritorySize(Integer Change){ TerritorySize = TerritorySize + Change; }} VB.NET Class Wolf Inherits Animal Dim TerritorySize As Integer Sub New(pName, pSound, pSize, pIntelligence, pTerritory) MyBase.New(pName, pSound, pSize, pIntelligence) TerritorySize = pTerritory End Sub Sub SetTerritorySize(Change) TerritorySize = TerritorySize + Change End Sub End Class Python class Wolf(Animal): def __init__(self, pName, pSound, pSize, pIntelligence, pTerritorySize): super().__init__(pName, pSound, pSize, pIntelligence) self.TerritorySize = pTerritorySize #integer def SetTerritorySize(self, Change): self.TerritorySize = self.TerritorySize + Change 3(c)(ii) 1 mark each 2 • Description() method header (and end where appropriate) taking no parameters and overriding/overloads/extending/using parent method • Concatenating and return correct message Example program code: Java public String Description(){ String Message = "The animal's name is " + Name + ", it makes a " + Sound + ", its size is " + Size + " and its intelligence level is " + Intelligence + ". Its territory is " + TerritorySize + " square miles."; return Message; } VB.NET Overloads Function Description() Dim Message As String = "The animal's name is " & Name & ", it makes a " & Sound & ", its size is " & CStr(Size) & " and its intelligence level is " & CStr(Intelligence) + ". Its territory is " & CStr(TerritorySize) & " square miles." Return Message End Function Python def Description(self): Message = "The animal's name is " + self.Name + ", it makes a " + self.Sound + ", its size is " + str(self.Size) + " and its intelligence level is " + str(self.Intelligence) + " it's territory is " + str(self.TerritorySize) +" square miles." return Message 3(d)(i) 1 mark each 2 • 1 correct instance created and stored in a suitable variable/structure • 2nd and 3rd correct instances created and stored in a suitable variable/structure Example program code: Java Parrot Animal1 = new Parrot("Chewie", "Squawk", 1, 10, 30, 29); Wolf Animal2 = new Wolf("Nighteyes", "Howl", 8, 7, 100); Animal Animal3 = new Animal("Copper", "Neigh", 10, 6); VB.NET Dim Animal1 As Parrot Animal1 = New Parrot("Chewie", "Squawk", 1, 10, 30, 29) Dim Animal2 As Wolf Animal2 = New Wolf("Nighteyes", "Howl", 8, 7, 100) Dim Animal3 As Animal Animal3 = New Animal("Copper", "Neigh", 10, 6) Python Animal1 = Parrot("Chewie","Squawk",1,10,30,29) Animal2 = Wolf("Nighteyes","Howl",8,7,100) Animal3 = Animal("Copper", "Neigh", 10, 6) 3(d)(ii) 1 mark each 3 • Calling SetTerritorySize(-20) for instance of Nighteyes • Calling ChangeNumberWords(2) for instance of Chewie • Calling Description() for all 3 animals (after any updates) and outputting return values Example program code: Java Animal2.SetTerritorySize(-20); Animal1.ChangeNumberWords(2); System.out.println(Animal1.Description()); System.out.println(Animal2.Description()); System.out.println(Animal3.Description()); VB.NET Animal2.SetTerritorySize(-20) Animal1.ChangeNumberWords(2) Console.WriteLine(Animal1.Description()) Console.WriteLine(Animal2.Description()) Console.WriteLine(Animal3.Description()) Python Animal2.SetTerritorySize(-20) Animal1.ChangeNumberWords(2) print(Animal1.Description()) print(Animal2.Description()) print(Animal3.Description()) 3(d)(iii) 1 mark each: 2 • All three messages accurate with all relevant values • … showing correctly updated territory for Nighteyes and words for Chewie e.g.