A Level Computer Science Programming Guide

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Introduction

This Wikibook is not designed to teach you how to program, but rather to help you to remember and easily translate your code to the different languages.

All Pseudocode in this guide is based on various Cambridge Mark Schemes and Help Guides, so it is as accurate as possible.

When complete, the High-Level languages covered in this guide will be those that are set in the Cambridge syllabus; VB.Net, Python and Pascal/Delphi.

Variables, Constants and Data Types

Data types and their usage

Pseudocode
Data Type	Description	Usage	Example
INTEGER	A whole number	Uses the normal denary system	123, -123, 0
REAL	Any number that has fractional parts/decimals	Written with at least one digit either side of a decimal point	12.4, 3.0, -17.34
CHAR	A single character	A single character enclosed between single quotation marks	'a', '@', '5'
STRING	A series of zero or more characters	No, or a series of characters enclosed between double quotation marks	"banana", "Q173hf", "John Doe"
BOOLEAN	A logical Expression	True or False	TRUE, FALSE
DATE	Any valid date	Use the format: dd/mm/yyyy If a different format must be used, use a comment to explain your decision.	21/10/2015, 03/07/1985, 12/11/1955

Variable Declarations

Python has no variable declarations.

Language	General Usage	Example Usage
Pseudocode	DECLARE <Identifier> : <Data Type>	DECLARE Weight : INTEGER DECLARE Mass : REAL DECLARE Material : STRING
VB.NET	Dim <Identifier> As <Data Type>	Dim Weight As Integer Dim Mass As Double Dim Material As String

Constants

Python has no constants.

Language

General Usage

Example Usage

Pseudocode

CONSTANT <Identifier> = <Value>

CONSTANT Pi = 3.14
CONSTANT e = "2.718"
CONSTANT Multiplier = 10

VB.NET

Const <Identifier> As <Data Type> = <Value>

Const Pi As Double = 3.14
Const e As String = "2.17"
Const Multiplier as Integer = 10

Assignments

Language	General Usage	Example Usage
Pseudocode	<Identifier> ← <Value>	Name ← "Stephen" Age ← 47 Weight ← 19.32
VB.NET	<Identifier> = <Value>	Name = "Stephen" Age = 47 Weight = 19.32
Python	<Identifier> = <Value>	Name = "Stephen" Age = 47 Weight = 19.32

Arrays

Declaring Arrays

In Pseudocode, Arrays all have declarable Upper and Lower bounds.

In VB.NET, Arrays all have declarable Upper Bounds, but Lower Bounds are always 0.

One Dimensional Arrays

Language

General Usage

Example Usage

Pseudocode

DECLARE <Identifier> : ARRAY[<Lower Bound>:<Upper Bound>] OF <Data Type>

DECLARE NameList : ARRAY[1:5] OF STRING
DECLARE YearlyRainfall : ARRAY[1900:2100] OF REAL
DECLARE AgeList : ARRAY[0:40] OF INTEGER

VB.NET

Dim <Identifier>(<Upper Bound>) As <Data Type>

Dim NameList(4) As String
Dim YearlyRainfall(200) As Double
Dim AgeList(40) As Integer

Python

<Identifier> = []

NameList = []
YearlyRainfall = []
AgeList = []

Two Dimensional Arrays

Language

General Usage

Example Usage

Pseudocode

DECLARE <Identifier> : ARRAY[<Lower Bound>:<Upper Bound>, <Lower Bound>:<Upper Bound>] OF <Data Type>

DECLARE GameArea : ARRAY[1:32, 0:16] OF STRING
DECLARE SudokuGrid : ARRAY[1:9, 1:9] OF INTEGER
DECLARE PopulationDensity : ARRAY[1:50, 20:50] OF REAL

VB.NET

Dim <Identifier>(<Upper Bound>, <Upper Bound>) As <Data Type>

Dim GameArea(31, 16) As String
Dim SudokuGrid(8) As Integer
Dim PopulationDensity(49,30) As Double

Python

<Identifier1> = []
<Identifier2> = []
<Identifier1>.append(<Identifier2>)

Game = []
GameArea = []
GameArea.append(Game)

SudokuX = []
SudokuGrid = []
SudokuGrid.append(SudokuX)

Longitude = []
PopulationDensity = []
PopulationDensity.append(Longitude)

Using Arrays

One Dimensional Arrays

Language

General Usage

Example Usage

Pseudocode

<Identifier>[<Index>] ← <Value>

NameList[1] ← "Stephen"
YearlyRainfall[1985] ← 13.73
AgeList[39] ← 17

VB.NET

<Identifier>(<Index>) = <Value>

NameList(0) = "Stephen"
YearlyRainfall(85) = 13.73
AgeList(38) = 17

Python

<Identifier>.append(<Value>)

NameList.append("Stephen")
YearlyRainfall.append(13.73)
AgeList.append(17)

Two Dimensional Arrays

Language

General Usage

Example Usage

Pseudocode

<Identifier>[<Index>,<Index>] ← <Value>

GameArea[16, 5] ← "Fire"
SudokuGrid[9, 3] ← 7
PopulationDensity[14, 32] ← 242.023

VB.NET

<Identifier>(<Index>,<Index>) = <Value>

GameArea(15, 5) = "Fire"
SudokuGrid(8, 2) = 7
PopulationDensity(13, 12) = 242.023

Python

<Identifier1> = []
<Identifier2> = []
<Identifier1>.append(<Identifier2>)

Game.append("Fire")
GameArea.append(Game)

SudokoX.append(7)
SudokuGrid.append(SudokuX)

Longitude.append(242.023)
PopulationDensity.append(Longitude)

User-Defined Data Types

Warning: Display title "User-Defined Data Types" overrides earlier display title "Variables, Constants and Data Types".

Enumerated Data Types

Declaring Enumerated Data Types

Language

General Usage

Example Usage

Pseudocode

TYPE <Identifier> = (<Value 1>, <Value 2>, <Value 3>, ...)

TYPE Season = (Summer, Autumn, Winter, Spring)
TYPE Direction = (North, North_East, East, South_East, South, South_West, West, North_West)
TYPE SpiceIntensity = (No_Preference ,None, Mild, Medium, Hot, Extreme)

VB.NET

Enum <Identifier>
 <Value 1> = <Reference>
 <Value 2> = <Reference>
 <Value 3> = <Reference>
 ....
End Enum

Enum Season
 Summer = 1
 Autumn = 2
 Winter = 3
 Spring = 4
End Enum

Enum SpiceIntensity
 None = 0
 Mild = 1
 Medium = 2
 Hot = 3
 Extreme = 4
End Enum

Using Enumerated Data Types

Language

General Usage

Example Usage

Pseudocode

DECLARE <Identifier> : <Data Type>
<Identifier> ← <Value>

DECLARE Seasons : Season
Seasons ← Autumn

DECLARE CompassPointer : Direction
CompassPointer ← North

DECLARE SpicePreference : SpiceIntensity
SpicePreference ← Extreme

VB.NET

Dim <Identifier> As <Data Type>
<Integer Variable> = <Identifier>.<Value>

Dim Seasons As Season
SeasonCounter = Seasons.Autumn

Dim CompassPointer As Direction
CurrentDirection = CompassPointer.North

Dim SpicePreference As SpiceIntensity
SpiceCounter = SpicePreference.Extreme

Pointer Data Types

There is no VB.Net equivalent for a Pointer Data Type.

Language	General Usage	Example Usage
Pseudocode	TYPE <Pointer> = ^<Type Name>	Type AddPointer = ^INTEGER Type NextLocation = ^STRING Type Pass = ^REAL

Record Data Types

Language

General Usage

Example Usage

Pseudocode

TYPE <Identifier>
 DECLARE <Identifier> : <Data Type>
 DECLARE <Identifier> : <Data Type>
 ...
END TYPE

TYPE Person
 DECLARE Name : STRING
 DECLARE Birthday : DATE
 DECLARE Weight : REAL
 DECLARE Address : STRING
 DECLARE SpicePreference : SpiceIntensity
END TYPE

VB.NET

Structure <Identifier>
 Dim <Identifier> As <Datatype>
 Dim <Identifier> As <Datatype>
 ...
End Structure

Structure Person
 Dim Name As String
 Dim Birthday As Date
 Dim Weight As Double
 Dim Address As String
 Dim SpicePreference As SpiceIntensity
End Structure

Common Operations

Warning: Display title "Common Operations" overrides earlier display title "User-Defined Data Types".

Input And Output

Language

General Usage

Example Usage

Pseudocode

INPUT <Identifier>

Input Answer
Input Age
Input DistanceTravelled

VB.NET

<Identifier> = Console.Readline()

Dim <Identifier> As <Data Type> = Console.ReadLine()

Answer = Console.ReadLine()
Dim Age As Integer = Console.ReadLine()
DistanceTravelled = Console.ReadLine()

Arithmetic, Relational and Logic Operators

Language	Arithmetic Operators				Integer Division		Relational Operators						Logic Operators
Language	Addition	Subtraction	Multiplication	Division	Modulus	Division	Greater Than	Less Than	Greater Than or Equal to	Less Than or Equal to	Equal To	Not Equal To	AND	OR	NOT
Pseudocode	`+`	`-`	`*`	`/`	`MOD`	`DIV`	`>`	`<`	`>=`	`<=`	`=`	`<>`	`AND`	`OR`	`NOT`
VB.NET	`+`	`-`	`*`	`/`	`MOD`	`\`	`>`	`<`	`>=`	`<=`	`=`	`<>`	`AND`	`OR`	`NOT`

String Operators

Language	General Usage	Example Usage
Pseudocode
VB.NET

Random Number Generation

Language General Usage Example Usage

Pseudocode

RANDOMBETWEEN(<Minimum>, <Maximum>)

Generates a random INTEGER between the Minimum and Maximum.

RND()

Generates a random REAL number between 0 and 1.

DECLARE Random : INTEGER
Random = RANDOMBETWEEN(10, 17)

DECLARE Random : REAL
Random = RND()

VB.NET

<Variable> = <Minimum> + Rnd() * (<Maximum> - <Minimum>)

Generates a random INTEGER between the Minimum and Maximum.

Rnd()

Generates a random REAL number between 0 and 1.

Dim Random As Integer
Random = RANDOMBETWEEN(10, 17)
Random = 10 + Rnd() * (17 - 10)

Dim Random As Double
Random = Rnd()

Selection

Warning: Display title "Selection" overrides earlier display title "Common Operations".

IF Statements

Language

General Usage

Example Usage

Pseudocode

/ If...Then Statement

IF <Condition>
    THEN
        <Statements>
ENDIF

/ If...Then...Else Statement

IF <Condition>
    THEN
        <Statements>
    ELSE
        <Statements>
ENDIF

/ ElseIf Statement

IF <Condition>
    THEN
        <Statements>
ELSEIF <Condition>
    THEN
        <Statements>
    ELSE
        <Statements>
ENDIF

/ If...Then Statement

IF Age < 16
    THEN
        CanDrive ← FALSE
ENDIF

/ If...Then...Else Statement

IF IsGreen
    THEN
        OUTPUT "Green"
    ELSE
        OUTPUT "Blue"
ENDIF

/ ElseIf Statement

IF Score > 1000
    THEN
        Multiplier ← 10
ELSEIF Score < 500
    THEN
        Multiplier ← 0
    ELSE
        Multiplier ← 5
ENDIF

VB.NET

'If...Then Statement

If <Condition> Then
    <Statements>
End If

'If...Then...Else Statement

If <Condition> Then
    <Statements>
Else
    <Statements>
End If

'Else If Statement

If <Condition> Then
    <Statements>
Else If <Condition> Then
    <Statements>
Else
    <Statements>
End IF

'If...Then Statement

If Age < 16 Then
    CanDrive = False
End If

'If...Then...Else Statement

If IsGreen Then
    Console.WriteLine("Green")
Else
    Console.WriteLine("Blue")
End If

'Else If Statement

If Score > 1000 Then
    Multiplier = 10
Else If Score < 500 Then
    Multiplier = 0
Else
    Multiplier = 5
End If

Nested IF Statements

Language

General Usage

Example Usage

Pseudocode

IF <Condition>
    THEN
        IF <Condition>
            THEN
                <Statements>
        ENDIF
ENDIF

IF Found = False
    THEN
        IF Searching = false
            THEN
                CALL Search()
        ENDIF
ENDIF

IF Age >= 16
    THEN
        IF PassedLicenceTest = True
            THEN
                CALL GenerateLicence()
                
        ELSEIF TakenTest = False
            THEN
                OUTPUT "You need to take the test to get your licence"
            ELSE
                OUTPUT "You Failed Your Test"
                OUTPUT "You need to pass the test to get your licence"
        ENDIF
    ELSE
        OUTPUT "Ineligible for Licence"
ENDIF

VB.NET

If <Condition> Then
    If <Condition> Then
        <Statements>
    End If
End If

If Found = False Then
    If Searching = false Then
        Search()
    End If
End If

If Age >= 16 Then
    If PassedLicenceTest = True Then
        GenerateLicence()
    Else If TakenTest = False Then
        Console.WriteLine("You need to take the test to get your licence")
    Else
        Console.WriteLine("You Failed Your Test")
        Console.WriteLine("You need to pass the test to get your licence")
    End If
Else
    Console.WriteLine("Ineligible for Licence")
End If

CASE Statements

Language

General Usage

Example Usage

Pseudocode

CASE OF <identifier>
    <value 1> : <statement>
    <value 2> : <statement>
    ...
ENDCASE

CASE OF <identifier>
    <value 1> : <statement>
    <value 2> : <statement>
    ...
    OTHERWISE <statement>
ENDCASE

/ If...Then Statement

IF Age < 16
    THEN
        CanDrive ← FALSE
ENDIF

/ If...Then...Else Statement

IF IsGreen
    THEN
        OUTPUT "Green"
    ELSE
        OUTPUT "Blue"
ENDIF

/ ElseIf Statement

IF Score > 1000
    THEN
        Multiplier ← 10
ELSEIF Score < 500
    THEN
        Multiplier ← 0
    ELSE
        Multiplier ← 5
ENDIF

VB.NET

'If...Then Statement

If <Condition> Then
    <Statements>
End If

'If...Then...Else Statement

If <Condition> Then
    <Statements>
Else
    <Statements>
End If

'Else If Statement

If <Condition> Then
    <Statements>
Else If <Condition> Then
    <Statements>
Else
    <Statements>
End IF

'If...Then Statement

If Age < 16 Then
    CanDrive = False
End If

'If...Then...Else Statement

If IsGreen Then
    Console.WriteLine("Green")
Else
    Console.WriteLine("Blue")
End If

'Else If Statement

If Score > 1000 Then
    Multiplier = 10
Else If Score < 500 Then
    Multiplier = 0
Else
    Multiplier = 5
End If

File Handling

Warning: Display title "File Handling" overrides earlier display title "Selection".

Note for VB.NET users:

While the methods used here are still valid, they are generally not used. They have been included in this guide as they are more similar to the Pseudocode file management, hopefully making them easier to understand. Even though it is not practice to use them, they should still be acceptable in an exam.

Handling Text Files

Opening Text Files

Language

General Usage

Example Usage

Pseudocode

OPENFILE <File Identifier> FOR <File Mode>

OPENFILE Beans.txt FOR APPEND
OPENFILE Styles.css FOR READ
OPENFILE Names.tmp FOR WRITE

VB.NET

FileOpen(<File Number>], <File Identifier>, OpenMode.<File Mode>)

FileOpen(17, “Beans.txt”, OpenMode.Append)
FileOpen(FileNumber, “Styles.css”, OpenMode.Input)
FileOpen(NamesFile, “Names.tmp”, OpenMode.Output)

In VB.NET you can use the FreeFile() function to automatically obtain an unused file number, avoiding any problems that could occur if accidentally using the same number for two files.

Dim FileNumber As Integer = FreeFile()

It is good practice to always keep track of your file numbers, as it will help reduce bugs (aka forgetting or using the wrong file number), remove any Magic Numbers and make the code easier to read and understand.

File Mode Usage
File Mode Identifiers		Description
Pseudocode	VB.NET	Description
APPEND	OpenMode.Append	Used when Writing data to the file. If the file already exists, the new data will be added to the file after any existing data.
READ	OpenMode.Input	Used when Reading data from the file.
Write	OpenMode.Output	Used when Writing data to the file. If the file already exists, the file will be deleted and a new file will be created with the new data.

Reading From Text Files

Once a file is opened in 'Read' mode, you can use the following commands to read the data from the files.

Language General Usage Example Usage

Pseudocode

READFILE <File Identifier>, <Identifier>

/Datatypes declared as strings

READFILE Styles.css, NextLine
READFILE TransactionHistory.txt, Transaction
READFILE Address.db, CurrentAddress

WRITEFILE <File identifier>, <Variable>

VB.NET

<Identifier> = LineInput(<File Identifier>)

The File Identifer is the file number used identify the file when it was opened.

NextLine = LineInput(StylesFileNumber)
Transaction = LineInput(TransactionHistoryFileNumber)
CurrentAddress = LineInput(AddressFileNumber)

The Variable should be of data type STRING. This command reads the text file line by line.

The EOF() function can be used in Pseudocode and VB.NET to determine whether the file pointer is at the end of the file. It returns a Boolean Value, and can be useful when implementing loops.

EOF(<File Identifier>)


Pseudocode	VB.NET
EOF(file.txt)	EOF(FileNumber)

Writing To Text Files

Once a file is opened in 'Write' or 'Append' mode, you can use the following commands to write the data to the files.

Language General Usage Example Usage

Pseudocode

READFILE <File Identifier>, <Identifier>

WRITEFILE Beans.txt, BeanData
WRITEFILE Names.tmp, CurrentUser
WRITEFILE Recipes.db, NewRecipe

VB.NET

PrintLine(<File Identifier>, <Identifier>)

The File Identifer is the file number used identify the file when it was opened.

PrintLine("Beans.txt", BeanData)
PrintLine("Name.tmp", CurrentUser)
PrintLine("Recipes.db" NewRecipe)

The Variable should be of data type STRING.

Closing Text Files

Language	General Usage	Example Usage
Pseudocode	CLOSEFILE <File identifier>	CLOSEFILE Beans.txt CLOSEFILE Styles.css CLOSEFILE Names.tmp
VB.NET	FileClose(<File Identifier>) The `File Identifer` is the file number used identify the file when it was opened.	FileClose(17) FileClose(FileNumber) FileClose(NamesFile)

You should always remember to close your files. Failure to do so can result in major problems later on when any program tries to open the file, or the program tries to open or edit a different file with the same identifier.

Algorithms

Sorting

Bubble Sort
Insertion Sort

Searching

Binary Sort

Listing

Stacks
Queues
Linked Lists
Binary Trees
Hash Tables