Data types, operations and programming constructs – A-level Computer Science exam questions with mark scheme

1Standard4 marks

The algorithm below is written in pseudo-code.

n ← 2026
total ← 0
WHILE n > 0
   total ← total + (n MOD 10)
   n ← n DIV 10
ENDWHILE
OUTPUT total

(a) Complete a trace table, with columns n and total, for the algorithm. Show the initial values in the first row. [3]

(b) State the purpose of the algorithm. [1]

(a) n column correct: 2026, 202, 20, 2, 0(1)
(a) total column correct up to and including 8: 0, 6, 8, 8(1)
(a) Final value of total is 10 (and no extra rows); ignore an OUTPUT column if 10 is shown(1)
(b) It calculates/outputs the sum of the digits of n (the starting number); do not accept 'adds up n'(1)
Answer: (a) n: 2026, 202, 20, 2, 0; total: 0, 6, 8, 8, 10. (b) Outputs the sum of the digits of the number.
Examiner tip: MOD 10 gives the last digit and DIV 10 removes it – students who use ordinary division get 202.6 and lose every later row.

2Standard6 marks

Write a program that:

repeatedly asks the user to enter a password until the password entered is valid
a valid password has at least 8 characters and contains at least one digit (0–9)
displays a suitable message each time an invalid password is entered
once a valid password has been entered, displays the number of attempts the user took.

You may write your answer in pseudo-code or a programming language you have studied. [6]

Gets a password from the user inside an indefinite loop (WHILE / REPEAT…UNTIL / while True with break)(1)
Checks the length of the password is ≥ 8 (accept > 7)(1)
Checks whether at least one character is a digit, by iterating through the characters or with a suitable built-in (e.g. isdigit, any(), character code between 48 and 57)(1)
Loop ends only when both conditions are true (correct use of AND / correct loop condition); R. if a password meeting only one condition is accepted(1)
Displays an error message for each invalid password, and not for the valid one(1)
Attempts counter initialised before the loop, incremented once per attempt and output after the loop ends (gives 1 if the first password is valid)(1)

Answer: e.g. (Python)

attempts = 0
valid = False
while not valid:
    pw = input("Password: ")
    attempts += 1
    hasDigit = False
    for c in pw:
        if c.isdigit():
            hasDigit = True
    if len(pw) >= 8 and hasDigit:
        valid = True
    else:
        print("Invalid password")
print("Attempts:", attempts)

Examiner tip: Check the counter: initialising it inside the loop or incrementing it only on invalid attempts gives a wrong count and loses the final mark.

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