Programming concepts: sequence, selection, iteration & Boolean/relational operators – GCSE Computer Science exam questions with mark scheme

1Grade 4–54 marks

This algorithm calculates the price of a ticket.

age ← STRING_TO_INT(USERINPUT)
IF age < 5 THEN
   price ← 0
ELSE IF age < 16 THEN
   price ← 4
ELSE IF age ≥ 65 THEN
   price ← 5
ELSE
   price ← 9
ENDIF
OUTPUT price

State the output when the user enters:

(a) 3 [1]
(b) 16 [1]
(c) 70 [1]
(d) 15 [1]

(a) 0(1)
(b) 9(1)
(c) 5(1)
(d) 4(1)
Answer: (a) 0 (b) 9 (c) 5 (d) 4
Examiner tip: 16 is not less than 16, so it drops through to the ELSE. Check boundaries carefully.

2Grade 4–54 marks

Look at this algorithm.

scores ← [12, 5, 18, 7, 10]
total ← 0
count ← 0
FOR i ← 0 TO LEN(scores) - 1
   IF scores[i] ≥ 10 THEN
      total ← total + scores[i]
      count ← count + 1
   ENDIF
ENDFOR
OUTPUT total
OUTPUT count

(a) State the two values output by the algorithm. [2]

(b) Explain why a FOR loop has been used rather than a WHILE loop. [1]

(c) Write one line of AQA pseudocode, to be added at the end of the algorithm, that outputs the mean of the scores that were added to total. Your line must not use any fixed numbers, so that it would still give the correct mean if the values in scores were changed. You may assume at least one score is 10 or more. [1]

(a) 40 (total)(1)
(a) 3 (count)(1)
(b) The number of repetitions is known in advance / the loop always runs once for each item in the array (definite / count-controlled iteration); allow a FOR loop updates the counter automatically so there is no need to increment i (and no risk of an infinite loop)(1)
(c) OUTPUT total / count; accept brackets e.g. OUTPUT (total / count); allow mean ← total / count followed by OUTPUT mean; reject total DIV count (integer division does not give the mean); reject dividing by LEN(scores) or 5 (that is the mean of all scores); reject hard-coded values such as OUTPUT 40 / 3 or OUTPUT total / 3 (must use the variables total and count)(1)
Answer: (a) 40 and 3 (b) number of iterations known (definite iteration) (c) OUTPUT total / count
Examiner tip: ≥ 10 includes 10 itself, so 10 is counted – missing it gives the wrong answers 30 and 2.

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