Faults & Errors (Cambridge (CIE) A Level Computer Science): Revision Note

Exam code: 9618

Written by: Robert Hampton

Reviewed by: James Woodhouse

Updated on 3 June 2025

Error types

What are the common types of error?

Designing algorithms is a skill that must be developed and when designing algorithms, mistakes and issues will occur
Trace tables can also help to find any kind of error in a program or algorithm
There are three main categories of errors that when designing algorithms a programmer must be able to identify & fix, they are:
- Syntax errors
- Logic errors
- Runtime errors

Syntax errors

What is a syntax error?

A syntax error is an error that breaks the grammatical rules of a programming language and stops it from running
Examples of syntax errors are:
- Typos and spelling errors
- Missing or extra brackets or quotes
- Misplaced or missing semicolons
- Invalid variable or function names
- Incorrect use of operators
- Incorrectly nested loops & blocks of code

Pseudocode - with syntax errors
FUNCTION generate_username(first_name STRING, last_name STRING) RETURNS STRING DECLARE username : STRING username = SUBSTRING(first_name, 1, 1) + SUBSTRING(last_name, 1, 1) + SUBSTRING(last_name, 1, 3) RETURN username ENDFUNCTION PROCEDURE main() DECLARE first_name : STRING DECLARE last_name : STRING DECLARE username : STRING OUTPUT "Enter your first name: " INPUT first_name OUTPUT "Enter your last name: " INPUT last_name username ← generate_username[first_name, last_name] OUTPUT "Here's a suggested username:" + username ENDPROCEDURE // Main program CALL Main
Pseudocode - without syntax errors
FUNCTION generate_username(first_name : STRING, last_name : STRING) RETURNS STRING DECLARE username : STRING username ← SUBSTRING(first_name, 1, 1) + SUBSTRING(last_name, 1, 1) + SUBSTRING(last_name, 1, 3) RETURN username ENDFUNCTION PROCEDURE main() DECLARE first_name : STRING DECLARE last_name : STRING DECLARE username : STRING OUTPUT "Enter your first name: " INPUT first_name OUTPUT "Enter your last name: " INPUT last_name username ← generate_username(first_name, last_name) OUTPUT "Here's a suggested username: ", username ENDPROCEDURE // Main program CALL main()
Syntax errors
`FUNCTION generate_username(first_name STRING...)` Missing colon : between parameter name and type `first_name : STRING and last_name : STRING` `username = SUBSTRING(...)` Uses = instead of correct assignment operator ← Should be `username ← ...` `username ← generate_username[first_name, last_name]` Incorrect use of square brackets [] instead of parentheses () for function call Should be `generate_username(first_name, last_name)` `CALL Main` Incorrect capitalisation and missing parentheses — procedure name is case-sensitive Should be `CALL main()`

Logic errors

What is a logic error?

A logic error is where incorrect code is used that causes the program to run, but produces an incorrect output or result
Logic errors can be difficult to identify by the person who wrote the program, so one method of finding them is to use 'Trace Tables'
Examples of logic errors are:
- Incorrect use of operators (< and >)
- Logical operator confusion (AND for OR)
- Looping one extra time
- Indexing arrays incorrectly (arrays indexing starts from 0)
- Using variables before they are assigned
- Infinite loops

Pseudocode
FUNCTION calculate_area(length : REAL, width : REAL) RETURNS REAL // Checks for valid dimensions and calculates area IF length > 0 OR width > 0 THEN DECLARE area : REAL area ← width - length RETURN area ELSE OUTPUT "Length and width must be positive values." ENDIF ENDFUNCTION PROCEDURE main() DECLARE length : REAL DECLARE width : REAL DECLARE area : REAL OUTPUT "Enter the length of the rectangle: " INPUT length OUTPUT "Enter the width of the rectangle: " INPUT width area ← calculate_area(length, width) OUTPUT "The area of the rectangle is approximately ", area, " square units." ENDPROCEDURE // Main program CALL main()

Pseudocode

FUNCTION calculate_area(length : REAL, width : REAL) RETURNS REAL
    // Checks for valid dimensions and calculates area
    IF length > 0 OR width > 0 THEN
        DECLARE area : REAL
        area ← width - length
        RETURN area
    ELSE
        OUTPUT "Length and width must be positive values."
    ENDIF
ENDFUNCTION

PROCEDURE main()
    DECLARE length : REAL
    DECLARE width : REAL
    DECLARE area : REAL

    OUTPUT "Enter the length of the rectangle: "
    INPUT length

    OUTPUT "Enter the width of the rectangle: "
    INPUT width

    area ← calculate_area(length, width)

    OUTPUT "The area of the rectangle is approximately ", area, " square units."
ENDPROCEDURE

// Main program
CALL main()

Logic errors

Line	Error	Correction
`IF length > 0 OR width > 0 THEN`	The check allows one value to be negative, which is not valid	Should be `IF length > 0 AND width > 0 THEN`
`area ← width - length`	Incorrect calculation – subtracts instead of multiplying to find the area	Should be `area ← length * width`

Runtime errors

What is a runtime error?

A runtime error is where an error causes a program to crash
Examples of runtime errors are:
- Dividing a number by 0
- Index out of the range of an array
- Unable to read or write a drive

Pseudocode
FUNCTION calculate_area(length : REAL, width : REAL) RETURNS REAL // Calculates area of a rectangle after input validation IF length < 0 OR width < 0 THEN OUTPUT "Length and width must be positive values." ENDIF DECLARE area : REAL area ← length * width RETURN area ENDFUNCTION PROCEDURE main() DECLARE length : REAL DECLARE width : REAL DECLARE area : REAL OUTPUT "Enter the length of the rectangle: " INPUT length OUTPUT "Enter the width of the rectangle: " INPUT width area ← calculate_area(length, width) OUTPUT "The area of the rectangle is approximately ", area, " square units." ENDPROCEDURE // Main program CALL main()

Pseudocode

FUNCTION calculate_area(length : REAL, width : REAL) RETURNS REAL
    // Calculates area of a rectangle after input validation
    IF length < 0 OR width < 0 THEN
        OUTPUT "Length and width must be positive values."
    ENDIF

    DECLARE area : REAL
    area ← length * width
    RETURN area
ENDFUNCTION

PROCEDURE main()
    DECLARE length : REAL
    DECLARE width : REAL
    DECLARE area : REAL

    OUTPUT "Enter the length of the rectangle: "
    INPUT length

    OUTPUT "Enter the width of the rectangle: "
    INPUT width

    area ← calculate_area(length, width)

    OUTPUT "The area of the rectangle is approximately ", area, " square units."
ENDPROCEDURE

// Main program
CALL main()

Runtime error

Line	Error	Correction
`area ← calculate_area(length)`	Only one parameter is passed instead of two – will crash at runtime	Should be `area ← calculate_area(length, width)`

Worked Example

Each pseudocode statement in the following table may contain an error due to the incorrect use of the function or operator.

Describe the error in each case, or write ‘NO ERROR’ if the statement contains no error.

You can assume that none of the variables referenced are of an incorrect type.[5]

Statement	Error
`Result ← 2 & 4`
`SubString ← MID("pseudocode", 4, 1)`
`IF x = 3 OR 4 THEN`
`Result ← Status AND INT(x/2)`
`Message ← "Done" + LENGTH(MyString)`

Answer

Statement	Error
`Result ← 2 & 4`	Should be arithmetic operator (not &) OR 2 and 4 should be CHAR / STRING
`SubString ← MID("pseudocode", 4, 1)`	NO ERROR
`IF x = 3 OR 4 THEN`	Not Boolean values / incorrect operator OR Condition incorrect
`Result ← Status AND INT(x/2)`	`INT(X/2)` doesn't evaluate to a Boolean value / incorrect operator
`Message ← "Done" + LENGTH(MyString)`	Can’t add string to number / "Done" is not a number

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Faults & Errors (Cambridge (CIE) A Level Computer Science): Revision Note

Error types

What are the common types of error?

Syntax errors

What is a syntax error?

Logic errors

What is a logic error?

Logic errors

Runtime errors

What is a runtime error?

Runtime error

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1. Information representation

Data Representation

Binary Prefixes

Number Systems

Binary Arithmetic

Uses of Number Systems

Character Encoding

Multimedia

Bitmap Images

Vector Graphics

Sound Representation

Compression

Compression

Compression Techniques

2. Communication

Networks & the Internet

Network Types

Network Models

Network Topologies

Cloud Computing

Transmission Media

Network Hardware

Ethernet

Bit Streaming

The Internet

IP Addressing

Locating Resources on the World Wide Web

3. Hardware

Computers & Components

System Components

Embedded Systems

Hardware Devices

Memory Types

Monitoring & Control

Logic Gates & Logic Circuits

Logic Gates

Logic Construction

4. Processor Fundamentals

Central Processing Unit (CPU) Architecture

System Architecture

Components of the CPU

System Performance

Ports & Interfaces

Fetch-Execute Cycle

Interrupts

Assembly Language

Assembly Language Basics

Assembly Process

Instruction Groups

Addressing Modes

Bit Manipulation

Bitwise Operations

5. System Software

Operating Systems

Operating System Fundamentals

Utility Software

Program Libraries

Language Translators

Translator Types

Development Tools

6. Security, privacy & data integrity

Data Security

Security Measures

Threats

Data Protection

Data Integrity