Defensive design: validation, authentication, maintainability – GCSE Computer Science exam questions with mark scheme

1Grade 1–32 marks

Give two ways a programmer can make their code easier to maintain. [2]

One from: comments; meaningful / descriptive variable and subprogram names; indentation; using subprograms; using constants(1)
A second, different method from the list(1)
Answer: e.g. comments; meaningful variable names
Examiner tip: 'Make it neat' is too vague to score. Name the actual technique, such as indentation.

2Grade 4–54 marks

A school's online form collects pupil details. Name the most suitable validation check for each input. [4]

Input	Rule
Age	Must be between 11 and 18
Postcode	Must follow the pattern of a UK postcode
Surname	Must not be left blank
Password	Must be at least 8 characters

Age: range check(1)
Postcode: format check(1)
Surname: presence check(1)
Password: length check(1)
Answer: range; format; presence; length
Examiner tip: Name the check, e.g. 'range check'. Restating the rule ('between 11 and 18') does not score.

3Grade 6–74 marks

Write code that asks the user to enter their age. The program must keep asking until an age between 11 and 18 inclusive is entered. Use OCR Exam Reference Language or a high-level programming language. [4]

Inputs the age and converts it to an integer (e.g. int(input(...)))(1)
Uses a condition-controlled loop (while … endwhile or do … until)(1)
Correct condition covering both inclusive limits, e.g. while age < 11 OR age > 18 / until age >= 11 AND age <= 18; allow equivalents such as age <= 10 OR age >= 19(1)
The age is input inside the loop so the loop ends as soon as a valid age is entered (an error message is not required); allow a single input inside a do … until loop for marks 1 and 4(1)

Examiner tip: 'age < 11 AND age > 18' can never be true, so the loop never runs. Use OR for 'outside the range'.

4Grade 8–96 marks

A system stores the username "admin" and the password "Pa55word".

Write an algorithm that:

• asks the user to enter a username and password

• allows a maximum of 3 attempts

• outputs "Access granted" if both are correct

• outputs "Locked out" after 3 incorrect attempts.

Use OCR Exam Reference Language or a high-level programming language. [6]

Inputs both the username and the password inside the loop(1)
Uses a loop that allows at most 3 attempts; allow while / do … until / for loop with an exit (e.g. break) when the login is correct(1)
Counts the attempts (initialised before the loop, incremented inside it)(1)
Checks that both username AND password match the stored values(1)
Stops the loop when the login is correct and outputs "Access granted"(1)
Outputs "Locked out" only after 3 failed attempts (not after each failure); an 'Incorrect' message after each failure is not required(1)

Answer:

attempts = 0
loggedIn = false
while attempts < 3 AND loggedIn == false
    user = input("Username")
    pword = input("Password")
    attempts = attempts + 1
    if user == "admin" AND pword == "Pa55word" then
        loggedIn = true
    else
        print("Incorrect")
    endif
endwhile
if loggedIn == true then
    print("Access granted")
else
    print("Locked out")
endif

Examiner tip: Check that 'Locked out' is printed only after the third failure, and that a correct login stops the loop.

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