Robust and secure programming: erroneous data.

The idea

Erroneous data is test data that is incorrect or invalid for the problem, such as entering letters where a number is expected or a date like 31/02/2025. It is used to check that a program rejects or handles invalid input gracefully, for example by displaying an error message or prompting again, rather than crashing or producing a wrong result. In the context of robust programming, testing with erroneous data helps confirm that validation routines and error handling work as intended. For a program that asks for a test score between 0 and 100, erroneous data could be -5, 101, or 'abc'. The program should not accept these values; it should identify them as invalid and respond appropriately. This is distinct from normal data, which is valid and expected, and boundary data, which tests the limits of valid ranges.

What earns the marks

• Erroneous data is data that is invalid or incorrect for the given problem.
• Examples include values outside the valid range, wrong data types, or impossible dates.
• The program should reject erroneous data, often by showing an error message or asking for re-entry.
• Testing with erroneous data helps verify that validation and error handling routines work correctly.

Common mistakes and how to put them right

• Confusing erroneous data with boundary data.

  Put it right: Boundary data tests the edges of valid input, while erroneous data is outside the valid range or of the wrong type.

• Thinking that erroneous data should be accepted and processed.

  Put it right: The purpose is to ensure the program rejects it gracefully.

• Believing that any unexpected input is erroneous.

  Put it right: Some unexpected inputs may still be valid, so the definition depends on the problem's requirements.

Exam tips

• When asked to give examples of erroneous data, choose values that are clearly invalid for the stated problem, such as a negative age or a non-numeric input.
• Explain what the program should do when it receives erroneous data, such as displaying an error message or asking for input again.
• Remember that erroneous data is one of the three main types of test data: normal, boundary, and erroneous.