SYLLABUS 7.5 · PAPER 2 · ALSO FOR 0984 AND 2210

Validation and verification

Validation and verification both try to stop bad data getting into a system, but they check different things. Validation asks whether data is sensible; verification asks whether it was copied correctly.

Validation checks

Validation is done automatically by the program. It checks that data follows set rules, not that it is true.

Range check: the value is between a lower and an upper limit, e.g. a percentage from 0 to 100.
Length check: the data has an exact number of characters, or a number within limits, e.g. a password of at least 8 characters.
Type check: the data is the right data type, e.g. a whole number of tickets.
Presence check: something has been entered and the field isn’t empty.
Format check: the data matches a pattern, e.g. two letters then four digits.
Check digit: the last digit of a code is recalculated from the others to catch entry mistakes.

Verification checks

Verification checks that data has been entered or copied exactly as it was meant to be. In a double entry check the data is typed in twice and the two versions are compared, as with a new password. In a visual check the person entering the data reads it back on screen and compares it with the original.

Writing a validation loop

A validation routine keeps asking until the input passes. The REPEAT loop below always runs once, so it asks at least once, and it only ends on a valid value. Giving a message after a wrong entry is good practice.

REPEAT
    OUTPUT "Enter the number of eggs (0 to 500)"
    INPUT Eggs
    IF Eggs < 0 OR Eggs > 500 THEN
        OUTPUT "That is not between 0 and 500"
    ENDIF
UNTIL Eggs >= 0 AND Eggs <= 500

Where marks go

Saying validation checks data is correct: it only checks data is reasonable. A valid age can still be the wrong age.
Naming a check without saying what it checks for this piece of data.
Mixing up a length check (number of characters) with a range check (size of a value).
Using IF instead of a loop, so the user only gets one more try.
Getting the boundary wrong: using < instead of <=, so a limit like 500 is rejected.

Try a question

4 MARKS · MARKED ON THIS PAGE

A farmer records the number of eggs collected each day. The number must be from 0 to 500 inclusive. Write an algorithm in pseudocode that: - declares an integer variable Eggs and inputs a value into it - uses a range check with a loop so that, while Eggs is less than 0 or greater than 500, the message Invalid, re-enter is output and Eggs is input again - outputs the message Accepted once a valid value has been entered. Do not output any other text.

solutionPseudocode

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