Types of Errors (Edexcel GCSE Computer Science): Revision Note

Syntax, Logic & Runtime Errors

• Designing algorithms is a skill that must be developed and when designing algorithms, mistakes and issues will occur

• 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

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

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

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

Identifying Logic Errors

• Logic errors can be slightly challenging to locate compared to syntax errors

• This is because the program will still run but will not produce the expected output that the programmer intended

• The most obvious areas to check for logic errors are:

  • Logical operators (<, >, ==, !=)

  • Boolean operators (AND, OR, NOT)

• To help demonstrate this skill, a snippet of program code is used

• In this example, the incorrect logical operator has been used, OR instead of AND

• The result means the else clause in the if statement would never be caught

• At first glance, entering normal test data such as 14, the program works as intended

• Entering erroneous data or boundary test data which is outside of the range would result in the error

  • When entering the age of 21, it still outputs that the user is in secondary school

• Changing the OR to AND corrects the logic error