Identify Syntax & Logic Errors in Testing (OCR GCSE Computer Science)

What are syntax errors and logic errors?

• Syntax errors and logic errors are thoroughly covered earlier in the course, you can find that revision note here

• A summary of the two error types:

  • Syntax error: An error that breaks the grammatical rules of a programming language and stops it from running

  • Logic error: Incorrect code is used that allows the program to run, but produces an incorrect or undesired output

Identify Syntax & Logic Errors in Testing

How can you identify 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

Identifying logic errors

• Logic errors can be slightly more 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)

  • Division by 0

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

• In this example, the incorrect Boolean 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

• By changing the OR to AND, this corrects the logic error