📘 O-Level Computer Science · Paper 2

Pseudocode Complete Notes

Everything you need — data types, loops, arrays, functions, files, SQL and full worked examples. Built for Cambridge O-Level CS.

Contents

01 Data Types & Variables 02 Operators & Expressions 03 IF / ELSE Statements 04 CASE OF 05 FOR Loop 06 WHILE Loop 07 REPEAT UNTIL Loop 08 1D Arrays 09 2D Arrays 10 Functions & Procedures 11 File Handling 12 SQL Basics 13 15-Mark Exam Questions

01 Data Types & Variables ▾

Every piece of data stored in a program has a type. Choosing the right type matters — it tells the computer how much memory to use and what operations are allowed.

INTEGER

Whole numbers only. No decimals.

55, 23, -8, 0, 105

STRING

Any text inside speech marks.

"Ali", "35", "TRUE", "Q"

REAL

Decimal / floating-point numbers.

23.4, 3.142, 55.0

BOOLEAN

Only two possible values.

TRUE, FALSE

CHAR

A single character in single quotes.

'a', 'M', '?', '1'

⚠️

Common trap: "35" is a STRING, not an integer — the speech marks make it text. You cannot do maths on it without converting it first.

Declaring Variables

Use DECLARE to tell the program a variable exists before using it.

Pseudocode

DECLARE colour : STRING
DECLARE age    : INTEGER
DECLARE price  : REAL
DECLARE flag   : BOOLEAN
DECLARE grade  : CHAR

Assigning Values

Use the arrow ← to assign a value to a variable (read it as "gets" or "is set to").

Pseudocode

colour      ← "red"
age         ← 18
price       ← 9.99
flag        ← FALSE
grade       ← 'A'

CONST pi   ← 3.1415   // constant — cannot be changed later
pi          ← 3.142    // ✗ ERROR — cannot reassign a constant

INPUT and OUTPUT

Pseudocode

OUTPUT "Enter your name"     // shows a message on screen
INPUT  name                  // waits for user to type something
OUTPUT "Hello, ", name       // outputs text + variable together

02 Operators & Expressions ▾

Operator	Meaning	Example	Result
+	Addition	7 + 3	10
-	Subtraction	7 - 3	4
*	Multiplication	7 * 3	21
/	Division (real result)	7 / 2	3.5
MOD	Remainder after division	7 MOD 2	1
DIV	Integer division (no decimal)	7 DIV 2	3
=	Equal to	x = 5	TRUE/FALSE
<>	Not equal to	x <> 5	TRUE/FALSE
> < >= <=	Comparison operators	age >= 18	TRUE/FALSE
AND	Both conditions must be TRUE	x > 0 AND x < 10	TRUE/FALSE
OR	At least one must be TRUE	x = 0 OR x = 1	TRUE/FALSE
NOT	Inverts TRUE/FALSE	NOT flag	TRUE/FALSE
&	String concatenation	"Hi " & name	"Hi Ali"

💡

MOD trick: To check if a number is even, use num MOD 2 = 0. To check divisibility by any number n, use num MOD n = 0.

03 IF / ELSE Statements ▾

Basic Structure

Pseudocode

IF age >= 18 THEN
    OUTPUT "Eligible to vote"
ELSE
    OUTPUT "Not eligible"
ENDIF

Nested IF (ELSE IF)

Pseudocode

IF hours > 50 THEN
    OUTPUT "Overtime Bonus"
ELSE IF hours > 30 AND hours <= 50 THEN
    OUTPUT "Normal Salary"
ELSE
    OUTPUT "Salary Deducted"
ENDIF

⚠️

Always end with ENDIF. A very common exam mistake is writing END IF as two words — it must be one word.

Practice Questions

Q1 Take temperature as input. Output "Too Hot" if > 40, else "Pleasant weather".

Solution

OUTPUT "Enter temperature"
INPUT  temperature
IF temperature > 40 THEN
    OUTPUT "Too Hot"
ELSE
    OUTPUT "Pleasant weather"
ENDIF

Q2 Take 2 numbers. If num1 is greater output num1 × 3. If num2 is greater output num2 × 5. If equal, output num1 + num2.

Solution

OUTPUT "Enter first number"
INPUT  num1
OUTPUT "Enter second number"
INPUT  num2

IF num1 > num2 THEN
    OUTPUT num1 * 3
ELSE IF num2 > num1 THEN
    OUTPUT num2 * 5
ELSE
    OUTPUT num1 + num2
ENDIF

04 CASE OF Statement ▾

Use CASE OF when you have one variable that can match many specific values. It is cleaner than writing lots of ELSE IF chains.

General Structure

CASE OF variable
    value1 : OUTPUT "..."
    value2 : OUTPUT "..."
    value3 : OUTPUT "..."
OTHERWISE :
    OUTPUT "Invalid input"
ENDCASE

Example — Simple Calculator

Pseudocode

OUTPUT "Enter two numbers"
INPUT  num1, num2
OUTPUT "Enter operator (+, -, *, /)"
INPUT  operator

CASE OF operator
    '+' : OUTPUT num1 + num2
    '-' : OUTPUT num1 - num2
    '*' : OUTPUT num1 * num2
    '/' : OUTPUT num1 / num2
OTHERWISE :
    OUTPUT "Invalid operator"
ENDCASE

Example — Grade to Points

Pseudocode

OUTPUT "Enter grade (A/B/C/D/F)"
INPUT  grade

CASE OF grade
    'A' : OUTPUT "Grade A — 4.0 points"
    'B' : OUTPUT "Grade B — 3.0 points"
    'C' : OUTPUT "Grade C — 2.0 points"
    'D' : OUTPUT "Grade D — 1.0 points"
    'F' : OUTPUT "Grade F — 0.0 points"
OTHERWISE :
    OUTPUT "Invalid grade"
ENDCASE

05 FOR Loop (Count-Controlled) ▾

📌

When to use it: Use a FOR loop when you know exactly how many times you want to repeat something before the loop starts.

General Structure

FOR index ← 1 TO 10
    // code repeated 10 times
NEXT index

// Counting down using STEP
FOR index ← 10 TO 1 STEP -1
    OUTPUT index
NEXT index

// Even numbers only using STEP 2
FOR index ← 2 TO 20 STEP 2
    OUTPUT index
NEXT index

Practice — Sum & Average of 20 Numbers

Pseudocode

sum ← 0
FOR index ← 1 TO 20
    OUTPUT "Enter a number"
    INPUT  num
    sum ← sum + num
NEXT index
OUTPUT "Sum: ", sum
OUTPUT "Average: ", sum / 20

Practice — Temperature Monitor (7 Days)

Pseudocode

hotDays  ← 0
niceDays ← 0
coldDays ← 0

FOR index ← 1 TO 7
    OUTPUT "Enter temperature for day ", index
    INPUT  temp
    IF temp > 30 THEN
        OUTPUT "Hot day"
        hotDays ← hotDays + 1
    ELSE IF temp >= 20 AND temp <= 30 THEN
        OUTPUT "Nice day"
        niceDays ← niceDays + 1
    ELSE
        OUTPUT "Cold day"
        coldDays ← coldDays + 1
    ENDIF
NEXT index

OUTPUT "Hot days:  ", hotDays
OUTPUT "Nice days: ", niceDays
OUTPUT "Cold days: ", coldDays

06 WHILE Loop (Pre-Condition) ▾

📌

When to use it: Use WHILE when you don't know how many times to loop. The condition is checked before entering the loop — so if it starts FALSE, the loop body never runs.

General Structure

index ← 1
WHILE index <= 10
    OUTPUT index
    index ← index + 1
ENDWHILE

Example — Sum Until User Enters 0

Pseudocode

sum ← 0
OUTPUT "Enter a number (0 to stop)"
INPUT  num
WHILE num <> 0
    sum ← sum + num
    OUTPUT "Enter a number (0 to stop)"
    INPUT  num
ENDWHILE
OUTPUT "Total: ", sum

Example — Password Login System (3 Attempts)

Pseudocode

flag  ← FALSE
tries ← 3

WHILE flag = FALSE AND tries > 0
    OUTPUT "Attempts remaining: ", tries
    OUTPUT "Enter password"
    INPUT  password
    IF password = "hello123" THEN
        flag ← TRUE
    ENDIF
    tries ← tries - 1
ENDWHILE

IF flag = TRUE THEN
    OUTPUT "Access Granted"
ELSE
    OUTPUT "Access Denied"
ENDIF

Example — Factorial (5! = 120)

Pseudocode

OUTPUT "Enter a number"
INPUT  num
index     ← 1
factorial ← 1
WHILE index <= num
    factorial ← factorial * index
    index     ← index + 1
ENDWHILE
OUTPUT "Factorial: ", factorial

07 REPEAT UNTIL Loop (Post-Condition) ▾

📌

Key difference: REPEAT UNTIL always runs at least once because the condition is checked after the code runs. Perfect for input validation.

WHILE pre-condition

Checks condition BEFORE running. May never run if condition starts FALSE.

REPEAT UNTIL post-condition

Checks condition AFTER running. Always runs at least once. Loops UNTIL condition is TRUE.

Input Validation

Pseudocode

// Keeps asking until user enters a number between 1 and 12
REPEAT
    OUTPUT "Enter month (1-12)"
    INPUT  month
UNTIL month >= 1 AND month <= 12

Example — Even and Odd Sum (1 to 10)

Pseudocode

oddSum  ← 0
evenSum ← 0
index   ← 1
REPEAT
    IF index MOD 2 = 0 THEN
        evenSum ← evenSum + index
    ELSE
        oddSum  ← oddSum  + index
    ENDIF
    index ← index + 1
UNTIL index > 10
OUTPUT "Even sum: ", evenSum
OUTPUT "Odd sum:  ", oddSum

08 1D Arrays ▾

An array is a collection of values of the same data type, stored under one name and accessed by an index number.

Declaring, Assigning & Accessing

Pseudocode

// Declare — 10 integers, indexed 1 to 10
DECLARE Scores : ARRAY[1:10] OF INTEGER

// Assign directly
Scores[1] ← 85
Scores[2] ← 92

// Initialise all elements to 0 using a loop
FOR index ← 1 TO 10
    Scores[index] ← 0
NEXT index

// Take input from user
FOR index ← 1 TO 10
    OUTPUT "Enter score"
    INPUT  Scores[index]
NEXT index

Linear Search (Efficient Version)

Pseudocode

FUNCTION LinearSearch(ValToFind : INTEGER) RETURNS BOOLEAN
    exit  ← FALSE
    index ← 1
    REPEAT
        IF ValToFind = Scores[index] THEN
            exit ← TRUE
        ELSE
            index ← index + 1
        ENDIF
    UNTIL index > 10 OR exit = TRUE
    RETURN exit
ENDFUNCTION

💡

The efficient version stops as soon as it finds the value using exit ← TRUE — it doesn't keep checking the rest of the array unnecessarily.

Example — Find Largest Number

Pseudocode

DECLARE Values : ARRAY[1:5] OF INTEGER

FOR index ← 1 TO 5
    OUTPUT "Enter a value"
    INPUT  Values[index]
NEXT index

largest ← Values[1]   // start with first element
FOR index ← 2 TO 5
    IF Values[index] > largest THEN
        largest ← Values[index]
    ENDIF
NEXT index
OUTPUT "Largest value: ", largest

Example — Bubble Sort (Efficient)

Pseudocode

// Sorts Scores[1:10] in ascending order
upperBound ← 10
REPEAT
    upperBound ← upperBound - 1
    swap       ← FALSE
    FOR index ← 1 TO upperBound
        IF Scores[index] > Scores[index+1] THEN
            temp               ← Scores[index]
            Scores[index]     ← Scores[index+1]
            Scores[index+1]   ← temp
            swap               ← TRUE
        ENDIF
    NEXT index
UNTIL upperBound = 0 OR swap = FALSE   // stop early if already sorted

09 2D Arrays ▾

A 2D array is like a table with rows and columns. Think of it as a grid. You need two index numbers to access any one cell: Array[row, col].

Declaring & Accessing

Pseudocode

// 5 students, 3 columns (e.g. score in 3 subjects)
DECLARE Marks : ARRAY[1:5, 1:3] OF INTEGER

Marks[1,1] ← 85   // student 1, subject 1
Marks[2,3] ← 70   // student 2, subject 3

Input & Output with Nested Loops

Pseudocode

// Taking input
FOR rows ← 1 TO 5
    FOR cols ← 1 TO 3
        OUTPUT "Enter mark for student ", rows, " subject ", cols
        INPUT  Marks[rows, cols]
    NEXT cols
NEXT rows

// Outputting values
FOR rows ← 1 TO 5
    FOR cols ← 1 TO 3
        OUTPUT Marks[rows, cols]
    NEXT cols
NEXT rows

Linear Search in 2D Array (Efficient)

Pseudocode

OUTPUT "Enter value to find"
INPUT  ValToFind
rows ← 1
exit ← FALSE
REPEAT
    cols ← 1
    REPEAT
        IF Marks[rows, cols] = ValToFind THEN
            exit ← TRUE
        ELSE
            cols ← cols + 1
        ENDIF
    UNTIL cols > 3 OR exit = TRUE
    rows ← rows + 1
UNTIL rows > 5 OR exit = TRUE

IF exit = TRUE THEN
    OUTPUT "Value found at row ", rows-1, " col ", cols
ELSE
    OUTPUT "Value not found"
ENDIF

StudentData — Search by ID, Output Name

Pseudocode — StudentData[1:1000, 1:2] col1=ID col2=Name

OUTPUT "Enter the student ID"
INPUT  Id
found ← FALSE
rows  ← 1
REPEAT
    IF StudentData[rows, 1] = Id THEN
        found ← TRUE
    ELSE
        rows ← rows + 1
    ENDIF
UNTIL rows > 1000 OR found = TRUE

IF found = TRUE THEN
    OUTPUT "Name: ", StudentData[rows, 2]
ELSE
    OUTPUT "ID not found"
ENDIF

10 Functions & Procedures ▾

FUNCTION returns a value

Takes parameters, does something, and returns a result you can store in a variable.

PROCEDURE no return value

Takes parameters and does something (e.g. outputs to screen), but returns nothing.

FUNCTION — Grade Calculator

Pseudocode

FUNCTION GradeCalc(m1:INTEGER, m2:INTEGER, m3:INTEGER) RETURNS CHAR
    total      ← m1 + m2 + m3
    percentage ← total / 3
    OUTPUT "Total: ", total, "  Percentage: ", percentage

    CASE OF percentage
        >= 90 : grade ← 'A'
        >= 80 : grade ← 'B'
        >= 70 : grade ← 'C'
        >= 60 : grade ← 'D'
    OTHERWISE : grade ← 'F'
    ENDCASE
    RETURN grade
ENDFUNCTION

// Calling the function
OUTPUT "Enter marks for 3 subjects"
INPUT  s1, s2, s3
result ← GradeCalc(s1, s2, s3)
OUTPUT "Your grade is: ", result

PROCEDURE — Draw Triangle of Stars

Pseudocode

PROCEDURE DrawTriangle(size : INTEGER)
    FOR rows ← 1 TO size
        FOR col ← 1 TO rows
            OUTPUT "*"
        NEXT col
        OUTPUT ""   // new line
    NEXT rows
ENDPROCEDURE

// Calling the procedure
DrawTriangle(5)   // prints a 5-row triangle

11 File Handling ▾

Files let you store data permanently. Without files, all data is lost when the program ends. There are two modes: READ and WRITE.

⚠️

Always OPENFILE before reading/writing, and always CLOSEFILE when done. Forgetting CLOSEFILE is a common exam error.

Writing to a File

Pseudocode

OPENFILE "data.txt" FOR WRITE
WRITEFILE "data.txt", "This is line one"
WRITEFILE "data.txt", "This is line two"
CLOSEFILE "data.txt"

Reading from a File (Until End of File)

Pseudocode

OPENFILE "data.txt" FOR READ
WHILE NOT EOF("data.txt")
    READFILE "data.txt", oneLine
    OUTPUT   oneLine
ENDWHILE
CLOSEFILE "data.txt"

Example — Split Scores into High / Medium / Low Files

Pseudocode

OPENFILE "exam_scores.txt" FOR READ
OPENFILE "high.txt"        FOR WRITE
OPENFILE "medium.txt"      FOR WRITE
OPENFILE "low.txt"         FOR WRITE

WHILE NOT EOF("exam_scores.txt")
    READFILE "exam_scores.txt", score
    IF score >= 80 THEN
        WRITEFILE "high.txt", score
    ELSE IF score >= 50 THEN
        WRITEFILE "medium.txt", score
    ELSE
        WRITEFILE "low.txt", score
    ENDIF
ENDWHILE

CLOSEFILE "exam_scores.txt"
CLOSEFILE "high.txt"
CLOSEFILE "medium.txt"
CLOSEFILE "low.txt"

12 SQL — Database Queries ▾

SQL (Structured Query Language) is used to retrieve data from a database table. In your exam you need to know four keywords.

Keyword	What it does	Required?
SELECT	Choose which columns (fields) to display. Use * to select all.	✅ Always
FROM	Which table to query.	✅ Always
WHERE	Filter rows based on a condition.	Optional
ORDER BY	Sort results. Add ASC (low→high) or DESC (high→low).	Optional

Examples

SQL — Table: STUDENT (StudentID, FirstName, LastName, YearGroup, Score, Grade)

-- Show all data
SELECT * FROM STUDENT

-- Show names of all Year 11 students scoring above 70, highest score first
SELECT FirstName, LastName, Score
FROM   STUDENT
WHERE  YearGroup = 11 AND Score > 70
ORDER BY Score DESC

-- Show all students in House 'Blue', sorted by name A-Z
SELECT FirstName, LastName
FROM   STUDENT
WHERE  HouseColour = 'Blue'
ORDER BY LastName ASC

-- Show all Grade A students
SELECT *
FROM   STUDENT
WHERE  Grade = 'A'

📝

Exam tip: Text values like 'Blue' go in single quotes. Numbers like 70 do not. Getting this wrong is an easy mark to lose.

13 Worked 15-Mark Exam Questions ▾

🎯

How to approach 15-mark questions: Read the requirements as a checklist. Write comments in your code for each requirement. Cover validation, loops, arrays, and output with meaningful messages.

Question 1 — Cricket League Points System

12 clubs. Track wins/draws/losses. Calculate points (Win=12, Draw=5, Loss=0). Find the club(s) with most points.

Full Solution

// Step 1: Input and validate number of matches (max 22)
REPEAT
    OUTPUT "Enter number of matches played (max 22)"
    INPUT  Matches
UNTIL Matches >= 1 AND Matches <= 22

// Step 2: Input club names and match results
FOR index ← 1 TO 12
    OUTPUT "Enter name of club ", index
    INPUT  Clubs[index]

    // Validate: wins + draws + losses must equal Matches
    REPEAT
        OUTPUT "Enter matches won"
        INPUT  won
        OUTPUT "Enter matches drawn"
        INPUT  drawn
        OUTPUT "Enter matches lost"
        INPUT  lost
        IF (won + drawn + lost) <> Matches THEN
            OUTPUT "Error: totals must add up to ", Matches
        ENDIF
    UNTIL (won + drawn + lost) = Matches

    Statistics[index, 1] ← won
    Statistics[index, 2] ← drawn
    Statistics[index, 3] ← lost

    // Step 3: Calculate points
    Points[index] ← (won * 12) + (drawn * 5)
NEXT index

// Step 4: Find the highest points total
highest ← -1
FOR index ← 1 TO 12
    IF Points[index] > highest THEN
        highest ← Points[index]
    ENDIF
NEXT index

// Step 5: Find and output all clubs with the highest points
DECLARE topTeams : ARRAY[1:12] OF INTEGER
count ← 0
FOR index ← 1 TO 12
    IF Points[index] = highest THEN
        count          ← count + 1
        topTeams[count] ← index
    ENDIF
NEXT index

FOR index ← 1 TO count
    OUTPUT "Winner: ", Clubs[topTeams[index]]
    OUTPUT "Wins:   ", Statistics[topTeams[index], 1]
    OUTPUT "Points: ", highest
NEXT index

Question 2 — Grid Treasure Hunt Game

5×5 grid. Place 'X' randomly (not at [1,1]). Player starts at [1,1]. 10 moves to find X using W/A/S/D.

Full Solution

// Step 1: Initialise grid with empty strings
FOR row ← 1 TO 5
    FOR col ← 1 TO 5
        Grid[row, col] ← ""
    NEXT col
NEXT row

// Step 2: Place 'X' in a random cell (not [1,1])
REPEAT
    Xrow ← ROUND(RANDOM() * 4, 0) + 1
    Xcol ← ROUND(RANDOM() * 4, 0) + 1
UNTIL Xrow <> 1 OR Xcol <> 1
Grid[Xrow, Xcol] ← 'X'

// Step 3: Initialise player position and move counter
PlayerRow ← 1
PlayerCol ← 1
moveCount ← 0
win       ← FALSE

// Step 4: Game loop — up to 10 moves
REPEAT
    OUTPUT "Position: [", PlayerRow, ",", PlayerCol, "]  Moves left: ", 10 - moveCount
    OUTPUT "W=Up  S=Down  A=Left  D=Right"
    INPUT  move
    move  ← UCASE(move)
    error ← FALSE
    tempR ← PlayerRow
    tempC ← PlayerCol

    CASE OF move
        'W' : tempR ← PlayerRow - 1
        'S' : tempR ← PlayerRow + 1
        'A' : tempC ← PlayerCol - 1
        'D' : tempC ← PlayerCol + 1
    OTHERWISE : error ← TRUE
    ENDCASE

    // Validate move is within grid bounds
    IF tempR < 1 OR tempR > 5 OR tempC < 1 OR tempC > 5 THEN
        error ← TRUE
        OUTPUT "Move out of bounds — try again"
    ENDIF

    IF error = FALSE THEN
        PlayerRow ← tempR
        PlayerCol ← tempC
        moveCount ← moveCount + 1
    ENDIF

    // Check win condition
    IF Grid[PlayerRow, PlayerCol] = 'X' THEN
        win ← TRUE
        OUTPUT "You Win! Found it in ", moveCount, " moves."
    ENDIF
UNTIL win = TRUE OR moveCount = 10

IF win = FALSE THEN
    OUTPUT "You Lose! The X was at [", Xrow, ",", Xcol, "]"
ENDIF

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