Be able to identify, locate and correct program errors (logic, syntax,…

The idea

Debugging means finding why a program does not behave as intended, working out where the fault is, and fixing it. Syntax errors break the rules of the language, so the program will not run; an example is a missing closing bracket or a misspelled keyword. Logic errors let the program run but produce wrong results, such as using addition where subtraction is needed or writing a condition that is never true. Runtime errors occur while the program is running, for example dividing by zero or accessing an index outside the bounds of a list. To debug, read the error message, trace the values of variables, add temporary output statements, and test with known inputs. Once the fault is located, correct the code and re-test to confirm the fix and check that nothing else has broken.

What earns the marks

• Syntax errors: the code breaks language rules, so it fails to run; examples include missing brackets or misspelled keywords.
• Logic errors: the program runs but gives incorrect output, such as a wrong operator or a faulty condition.
• Runtime errors: the fault appears while running, such as division by zero or an out-of-range list index.
• Locating errors: use error messages, trace tables, temporary output and test data to find the faulty line.
• Correcting errors: change the code so it follows the rules, produces the intended result, or handles the failing case.
• Verification: re-run the program with normal, boundary and erroneous test data to confirm the correction.

Common mistakes and how to put them right

• Treating every error as a syntax error.

  Put it right: Check whether the program runs at all; if it runs but gives wrong output, suspect a logic error, and if it crashes mid-run, suspect a runtime error.

• Assuming a runtime error is always on the line shown in the message.

  Put it right: The reported line may be where the fault surfaces, so trace back to the value or index that caused it.

• Fixing a symptom rather than the cause, such as changing an output value instead of the calculation.

  Put it right: Locate the faulty statement and correct the underlying logic, then re-test.

Exam tips

• Classify the error first: does the program fail to run, run with wrong output, or crash while running? The category guides the fix.
• Use a trace table or add temporary output to follow variable values and pinpoint the line where behaviour diverges from expectation.
• After correcting, re-test with the original failing input plus a normal case to show the fix works and has not introduced a new fault.