Types Of Errors Aqa

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

• There are two main types of errors that when designing algorithms a programmer must be able to identify & fix, they are:

  • Syntax errors

  • Logic errors

Syntax & Logic 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

Examples

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

Example

• An algorithm is written to take as input the number of miles travelled. The algorithm works out how much this will cost, with each mile costing £0.30 in petrol. If this is greater than £10.00 then it is reduced by 10%.

Identify & Categorise Errors

How can you identify and categorise errors?

• As a result of a syntax error breaking the rules of the programming language, the program will not execute

• These are easily identifiable as the IDE will provide information about what the error is

  • This is done to help the programmer be able to fix the issue

• To help practise this skill, a selection of program screenshots will be used

The errors

• In the code below, there is a program which allows the user to enter how many items they wish to add to a shopping list

• The code then allows the user to

  • Enter their chosen number of items into a list

  • Output the list to the screen

• The code contains 3 syntax errors, highlighted with blue boxes

Error 1

• Line 3: Missing a second bracket at the end of integer input

  • The error message below is what the IDE provided to help the programmer identify the error

  • A top tip when programming is to look at the line of code above the one given in the error message

    • For example, this error message claims line 6 is the issue, however, the code above line 6 (line 3) is the line that contains the error

Error 2

• Line 6: Missing a colon at the end of a for loop, while loop or if statement

  • As mentioned earlier, the error message identifies the line after the actual error

  • The error is on line 6, however, the syntax error message identifies the line below it

    • Always check the line above for any potential errors

Error 3

• Line 7: Using == which is used for a comparison instead of a single = to declare a variable

  • This would return the same error as above, this time the user would more easily be able to identify the issue as it is on line 7

The fix

• Once all fixes are in place, the code should appear as follows and the program should execute as intended