Validate length, presence and format
Use character count, required-field checks and pattern rules to reject incomplete or malformed input.
This sequence follows Cambridge IGCSE Computer Science 0478, 2026–2028 syllabus Version 6. It covers subject content Topic 7 only. The separately published 2029–2031 syllabus is not mapped here. Practice is original, self-checked and does not award an official Cambridge grade. Full timed papers and human subject-teacher review remain outstanding.
Before you begin
- No programming language is required. Be comfortable with place value and multiplication; each lesson introduces the computer-science notation it uses.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which check would require a code to have 8 characters?
What you will learn
- Explain a length check
- Explain a presence check
- Explain a format check
Different checks test different properties
A length check ensures that text contains the required number of characters or falls within a permitted length. A presence check ensures a required field has not been left blank.
Apply and check the rule
A format check tests whether data follows a pattern, such as a date written in the required order or a code with letters followed by digits. A format check does not necessarily prove that the represented real-world fact is correct.
Pause and explain
A date must use DD/MM/YYYY. Which check tests that pattern?
Worked example
A membership code must contain exactly eight characters and cannot be blank. Which checks apply?
Show the worked solution
- Check that the field is present.
- Count the characters.
- Require length eight.
Answer Use a presence check and a length check requiring exactly eight characters.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A username is required and must be 5–12 characters. Name the checks.
Give me a hint
One check handles blank values; another checks character count.
Compare my reasoning
- Presence check rejects an empty field.
- Length check counts characters.
- Accept lengths from 5 through 12.
Use a presence check and a length check from 5 to 12 characters inclusive.
Look for these in your work
- Presence
- Length
- Bounds
A postcode field expects two letters followed by four digits. Which validation check can test its shape?
Give me a hint
Describe the expected character pattern.
Compare my reasoning
- State the required sequence of character classes.
- Compare the entered text with that pattern.
- Reject entries that do not follow it.
Use a format check for two letters followed by four digits.
Look for these in your work
- Pattern stated
- Format check
- Rejection
An input matches the format 31/02/2026 but that date does not exist. Explain what additional rule or check is needed.
Give me a hint
A pattern check may accept impossible values.
Compare my reasoning
- The string has the expected DD/MM/YYYY shape.
- The calendar combination is invalid.
- Validate the actual day against the selected month and leap-year rules.
The format check is not enough. Check that the day exists in that month and year, including leap-year rules for February.
Look for these in your work
- Format limitation
- Invalid date
- Semantic check
Common mistakes
- Treating a syntactically valid date as a verified event
- Confusing a length check with a range check
What can you explain now?
Which check ensures a mandatory field is not empty?
Compare with the explanation
Presence check
The field must contain an entered value.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
What does a length, presence or format check test?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.