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Input validation methods

Validation checks whether input meets stated acceptance rules before a program uses it. If the input is invalid, a robust program should reject it, explain the problem clearly and allow a suitable correction.

Validation does not prove that entered data is true. An age of 15 may pass a range check even if the user is actually 16. Validation only shows that the value follows the rules the programmer has chosen.

OCR questions often ask you to identify a suitable validation method and explain how it applies to a scenario. The scenario matters: “a range check checks a range” is too vague to earn an explanation mark.

Common validation methods

Method	What it checks	Example use
Range check	A number is within permitted minimum and maximum values.	A ticket quantity is from 1 to 8 inclusive.
Type check	Data has the required data type.	A quantity is a whole number.
Presence check	A required value has not been left empty.	A contact name has been entered.
Length check	Text has an acceptable number of characters.	A club code contains exactly six characters.
Format check	Data follows a required pattern.	A date follows `DD/MM/YYYY`.
Lookup check	A value appears in an accepted list or table.	An event code is one of `ART`, `CODE` or `MUSIC`.

Different checks can be combined. For example, a six-character event code could need both a length check and a lookup check.

Fill in the blanks4 marks

Match each rule to its validation method. Checking that a value is between limits is a label 1 check. Rejecting an empty required field is a label 2 check. Comparing an entry with an accepted list is a label 3 check. Checking a required pattern is a label 4 check.

range
presence
lookup
format
length
type

Multiple choice1 mark

A school event code must be exactly six characters long. Which validation method directly checks this rule?

APresence check
BRange check
CLength check
DType check

Multiple choice, several answers2 marks

A booking must use one of the codes ART, CODE or MUSIC and must not be left empty. Choose two appropriate validation methods.

ALookup check
BPresence check
CRange check
DType check

Apply the rule to the actual input

Imagine a mental-maths game that displays two random whole numbers from 1 to 12 and asks the player to enter their total. The player's total is the input being validated. The random numbers are generated by the program, so validating those is not the answer to the question.

The smallest possible total is 2 and the largest is 24. A range check on the player's answer could therefore reject totals below 2 or above 24. A type check could ensure the answer is a whole number.

This is an important exam habit: identify exactly which value the user enters, then apply the validation rule to that value.

Written answer6 marks

A geography quiz displays two numbers from 5 to 20 and asks the player to enter their sum. Identify two validation methods and explain how each could be applied to the player's answer.

For each method, name it, state the exact rule and explain what unsuitable input it rejects. The possible answer is from 10 to 40 inclusive.

Students type their answer here.

What should happen to invalid data?

A check is useful only if the program responds appropriately. A clear pattern is:

receive the input;
test it against the acceptance rule;
if it is invalid, display a helpful message and request a replacement;
continue only when an acceptable value has been received.

Silently changing an answer or continuing with invalid data can create unexpected results. The message should tell the user what rule to follow, such as “Enter a whole number from 1 to 8”.

Multiple choice1 mark

Which response is most useful when a ticket quantity of 12 is rejected because only 1 to 8 tickets are allowed?

AContinue with 12 and hope enough tickets remain
BReplace 12 with 1 without telling the user
CStop the entire system without an explanation
DDisplay `Enter a whole number from 1 to 8` and ask again

Written answer2 marks

Explain why passing validation does not guarantee that input is correct.

Use one example of a value that follows a rule but may still be factually wrong.

Students type their answer here.

Written answer6 marks

A youth theatre registration form requires a stage name, an age from 11 to 18, and a selected workshop code from ACT, LIGHT or SOUND. Recommend three validation checks and justify each choice.

Apply one check to each field. Use field, rule, rejected input as the structure for each point.

Students type their answer here.

Review

Without looking back, list the six validation methods and give one new example of any three. Then answer this final distinction: validation checks input during use, while testing is a development activity used to discover whether a program behaves as expected. The testing worksheets will return to this difference.