Learning Objectives
- Explain why input data is validated.
- Describe and apply range, length, type, presence, format and check-digit checks.
- Select suitable validation checks for a scenario.
- Explain why validation does not prove that data is correct.
Key Terms
- Validation
- Automatic checking that input data is reasonable and follows defined rules.
- Range check
- Checks that a value lies within stated limits.
- Length check
- Checks the number of characters or items.
- Type check
- Checks that data is of the required type.
- Presence check
- Checks that required data has been entered.
- Format check
- Checks that data follows a required pattern.
- Check digit
- An extra digit calculated from other digits and used to detect likely entry errors.
Purpose And Limitation
Validation reduces the chance that unsuitable data enters a system. It compares input with rules chosen during analysis. Invalid data is rejected or causes an error message.
Validation does not prove that data is true. An age of 25 may pass a range check even if the persons real age is 24. It only shows that the entered value is plausible according to the rule.
Range Check
A range check tests upper and lower limits. A mark may be accepted when 0 <= Mark <= 100. The limits may be inclusive or exclusive, so the condition must follow the wording exactly.
A range check is suitable for numerical or date values with meaningful bounds. It is not the same as checking the number of characters.
Length, Type And Presence Checks
A length check can require exactly eight characters, no more than 20 characters or at least one character. A type check ensures data such as an integer or date is entered in a form the system can process.
A presence check ensures a required field is not empty. It does not check whether the entered content is meaningful or correctly formatted.
Format Check
A format check tests a pattern, such as two letters followed by four digits. A value can have the correct length but the wrong format, so both checks may be useful.
The rule should describe positions or separators clearly. For example, a date may require DD/MM/YYYY. A format check alone does not prove the date exists unless additional checks are applied.
Check Digit
A check digit is calculated from the other digits in a code. When the code is entered, the calculation is repeated and compared with the supplied check digit. A mismatch suggests that one or more digits were entered incorrectly.
Topic 2 explains the use of check digits in data entry. In Topic 7, students should be able to recognise and apply check-digit validation as part of an algorithm. A check digit detects likely errors; it does not authenticate the owner or guarantee that the code belongs to the correct item.
Selecting A Check
| Data Requirement | Suitable Check |
|---|---|
| Age must be 12 to 18 inclusive | Range |
| Password must contain at least 10 characters | Length |
| Quantity must be an integer | Type |
| Email field cannot be empty | Presence |
| Code must be two letters followed by four digits | Format |
| Product barcode includes a calculated final digit | Check digit |
Worked Examples
Student Registration
Question: Suggest validation for age, student code and surname.
- Age 11 to 19: range check.
- Student code exactly eight characters: length check.
- Student code pattern two letters and six digits: format check.
- Surname required: presence check.
Answer: Several checks can be applied to one field because each tests a different rule.
Plausible But Wrong
Question: A user enters age 17 instead of 16. A range check accepts 17. Has validation failed?
- The rule checks whether the value is in the permitted range.
- 17 is in that range.
- The system cannot know the persons real age from the range rule.
Answer: The validation worked as defined, but it cannot prove factual correctness.
Examination Guidance
- Name the check and explain the exact rule in context.
- Use inclusive or exclusive limits accurately.
- Do not claim validation ensures correctness.
- Recognise that several checks may be needed for one field.
- For format checks, state the required pattern rather than only “correct format.”
Common Mistakes
- Calling a range check a length check.
- Using a presence check to claim that data is accurate.
- Assuming a check digit encrypts data.
- Forgetting to repeat input or produce an error after rejection.
- Applying a numerical range rule to a text pattern.
Knowledge Check
1. What is validation?
2. What does a range check test?
3. What does a presence check test?
4. Can validation prove data is true?
5. What is a check digit used for?
