Pseudocode

Pseudocode is a way of writing algorithms using structured English that resembles programming code but is not written in any specific programming language. It is used to plan and communicate algorithms before they are implemented in code. Pseudocode is a core part of GCSE Computer Science — both AQA and OCR use pseudocode extensively in exam papers, and you are expected to read, write, and trace pseudocode.

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Why Use Pseudocode?

Pseudocode has several advantages:

• Language-independent — it does not depend on the syntax of any particular programming language
• Easier to read — it uses English words and is more accessible than raw code
• Focuses on logic — you concentrate on the algorithm's structure rather than worrying about syntax errors
• Quick to write — no need to worry about semicolons, brackets, or indentation rules
• Easy to modify — changes can be made without rewriting actual code

Exam Tip: In the exam, you may be asked to write pseudocode, complete partially written pseudocode, or trace through pseudocode using a trace table. Practice all three skills. The exam board's reference guide shows the specific pseudocode conventions they use — make sure you are familiar with them.

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Pseudocode Conventions

Although pseudocode is not standardised globally, GCSE exam boards use specific conventions. The following conventions are commonly used:

Variables and Assignment

name ← "Alice"
age ← 16
score ← 0

The arrow ← is used for assignment (storing a value in a variable). Some exam boards also accept = for assignment.

Input and Output

OUTPUT "Enter your name:"
INPUT name
OUTPUT "Hello, " + name

Arithmetic Operators

Operator	Meaning	Example
+	Addition	5 + 3 = 8
-	Subtraction	10 - 4 = 6
*	Multiplication	6 * 7 = 42
/	Division (real)	7 / 2 = 3.5
DIV	Integer division	7 DIV 2 = 3
MOD	Modulus (remainder)	7 MOD 2 = 1
^	Exponentiation	2 ^ 3 = 8

Comparison Operators

Operator	Meaning
=	Equal to
≠ or !=	Not equal to
<	Less than
>	Greater than
≤ or <=	Less than or equal to
≥ or >=	Greater than or equal to

Logical Operators

Operator	Meaning	Example
AND	Both conditions must be true	age >= 13 AND age <= 19
OR	At least one condition must be true	colour = "red" OR colour = "blue"
NOT	Reverses the truth value	NOT found

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Selection in Pseudocode

IF...THEN...ELSE

IF score >= 50 THEN
    OUTPUT "Pass"
ELSE
    OUTPUT "Fail"
END IF

Nested IF

IF score >= 70 THEN
    grade ← "A"
ELSE IF score >= 60 THEN
    grade ← "B"
ELSE IF score >= 50 THEN
    grade ← "C"
ELSE
    grade ← "U"
END IF

CASE / SWITCH

CASE day OF
    "Monday": OUTPUT "Start of the week"
    "Friday": OUTPUT "Almost the weekend"
    "Saturday", "Sunday": OUTPUT "Weekend!"
    DEFAULT: OUTPUT "Midweek"
END CASE

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Iteration in Pseudocode

FOR Loop (Count-Controlled)

FOR i ← 1 TO 10
    OUTPUT i
END FOR

This outputs the numbers 1 to 10. The loop variable i is automatically incremented by 1 each time.

WHILE Loop (Condition-Controlled, Pre-Check)

attempts ← 0
WHILE attempts < 3
    INPUT password
    IF password = "correct" THEN
        OUTPUT "Access granted"
        attempts ← 3
    ELSE
        attempts ← attempts + 1
    END IF
END WHILE

The condition is checked before each iteration. If it is false initially, the loop body never executes.

REPEAT...UNTIL Loop (Condition-Controlled, Post-Check)

REPEAT
    OUTPUT "Enter a positive number:"
    INPUT number
UNTIL number > 0

The condition is checked after each iteration. The loop body always executes at least once.

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Arrays and Strings in Pseudocode

Arrays

names ← ["Alice", "Bob", "Charlie", "Diana"]
OUTPUT names[0]          // Outputs "Alice" (0-indexed)
names[2] ← "Catherine"   // Changes "Charlie" to "Catherine"
OUTPUT LENGTH(names)     // Outputs 4

Strings

word ← "COMPUTER"
OUTPUT LENGTH(word)                  // Outputs 8
OUTPUT SUBSTRING(word, 0, 3)         // Outputs "COM"
OUTPUT word[0]                       // Outputs "C"

Common String Operations

Operation	Description	Example
LENGTH(s)	Returns the number of characters	LENGTH("hello") = 5
SUBSTRING(s, start, length)	Extracts part of a string	SUBSTRING("hello", 1, 3) = "ell"
UPPER(s)	Converts to uppercase	UPPER("hello") = "HELLO"
LOWER(s)	Converts to lowercase	LOWER("HELLO") = "hello"

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Functions and Procedures in Pseudocode

Procedure (Does Not Return a Value)

PROCEDURE greet(name)
    OUTPUT "Hello, " + name + "!"
END PROCEDURE

greet("Alice")    // Outputs "Hello, Alice!"

Function (Returns a Value)

FUNCTION calculateArea(length, width)
    area ← length * width
    RETURN area
END FUNCTION

result ← calculateArea(5, 3)
OUTPUT result    // Outputs 15

Exam Tip: Know the difference between a function and a procedure. A function returns a value that can be stored in a variable or used in an expression. A procedure performs an action but does not return a value.

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Summary

Pseudocode is a structured way to write algorithms without using a specific programming language. It uses clear conventions for assignment, input/output, selection, iteration, arrays, strings, and subroutines. In GCSE Computer Science, you must be able to read, write, complete, and trace pseudocode. Familiarise yourself with your exam board's specific pseudocode reference guide.

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Extended Study: Using Pseudocode as a Design Language

Pseudocode is the language of computational thinking. It is deliberately more relaxed than real code so you can focus on the shape of the algorithm rather than the quirks of a compiler. AQA's exam papers are peppered with pseudocode — reading it fluently, writing it cleanly and tracing it carefully are three distinct skills you must master. This section goes beyond the syntactic basics and treats pseudocode as a design tool for the four computational-thinking pillars.

Why pseudocode supports computational thinking