Iteration: Practice Questions (Computer Science Principles 3.8)

These are original Marlbridge practice questions, not past exam questions. All scenarios are fictional. The code is Marlbridge’s rendering of the exam reference notation (← assignment, = equality test, lists start at index 1, DISPLAY shows a value). Mark points are a suggested Marlbridge rubric, not an official scoring guideline. No calculator is needed or allowed. Try each question before opening its answer.

Question 1 (multiple choice · foundation)

What is displayed?

x ← 1
REPEAT 3 TIMES
{
  x ← x + 2
}
DISPLAY(x)

(A) 9
(B) 5
(C) 7
(D) 3

Answer and explanation

Answer: (C). The block runs three times and adds 2 each time: 1 → 3 → 5 → 7.

(A) is four passes.
(B) stops after two passes.
(D) is one pass.

Question 2 (multiple choice · core)

What is displayed?

n ← 10
REPEAT UNTIL(n > 5)
{
  n ← n + 1
}
DISPLAY(n)

(A) 16
(B) 10
(C) 6
(D) 11

Answer and explanation

Answer: (B). The condition n > 5 is already true at the start (10 > 5), so the loop body never runs and n stays 10.

(A) and (C) assume the loop counts toward or away from 5.
(D) assumes the block ran once.

Question 3 (multiple choice · core)

Which segment will never finish?

(A) n ← 9 then REPEAT UNTIL(n = 0) { n ← n - 3 }
(B) n ← 0 then REPEAT UNTIL(n = 0) { n ← n - 3 }
(C) n ← 10 then REPEAT UNTIL(n = 0) { n ← n - 3 }
(D) n ← 10 then REPEAT UNTIL(n ≤ 0) { n ← n - 3 }

Answer and explanation

Answer: (C). From 10 the values are 10, 7, 4, 1, −2, … and never equal 0.

(A) goes 9, 6, 3, 0 and stops.
(B) is already true, so the block never runs and the program finishes at once.
(D) stops as soon as n ≤ 0.

Question 4 (multiple choice · core)

What is displayed?

n ← 1
count ← 0
REPEAT UNTIL(n ≥ 100)
{
  n ← n * 2
  count ← count + 1
}
DISPLAY(count)
DISPLAY(n)

(A) 7 64
(B) 6 64
(C) 7 128
(D) 8 256

Answer and explanation

Answer: (C). n is doubled each pass: 2, 4, 8, 16, 32, 64, 128. The loop stops only when n ≥ 100, which first happens at 128 after 7 passes.

(A) pairs the pass count with the wrong value.
(B) stops at 64, which is still below 100.
(D) goes one pass too far.

Question 5 (multiple choice · core)

What is displayed?

count ← 0
i ← 1
REPEAT 10 TIMES
{
  IF(i MOD 3 = 0)
  {
    count ← count + 1
  }
  i ← i + 1
}
DISPLAY(count)

(A) 10
(B) 0
(C) 4
(D) 3

Answer and explanation

Answer: (D). i takes the values 1 to 10. The condition i MOD 3 = 0 is true for 3, 6 and 9, so count becomes 3.

(A) counts every pass.
(B) would be the result only if the condition were never true.
(C) needs a fourth multiple of 3 between 1 and 10, and there is none.

Question 6 (multiple choice · core)

A robot starts in the first square of a straight corridor facing the far wall. Which segment moves it forward until it is next to the wall, whatever the length of the corridor?

(A) IF(CAN_MOVE(forward)) { MOVE_FORWARD() }
(B) REPEAT UNTIL(NOT CAN_MOVE(forward)) { MOVE_FORWARD() }
(C) REPEAT UNTIL(CAN_MOVE(forward)) { MOVE_FORWARD() }
(D) REPEAT 5 TIMES { MOVE_FORWARD() }

Answer and explanation

Answer: (B). The loop tests the square ahead before each move and stops when it cannot move, so it works for any length.

(A) moves at most one square.
(C) stops immediately because the robot can move at the start.
(D) works only when the corridor needs exactly 5 moves.

Question 7 (multiple select, choose two · core)

Which two statements about REPEAT UNTIL(condition) are true?

(A) The condition is checked before each pass through the block.
(B) The block always runs at least once.
(C) The condition is checked after every statement in the block.
(D) The loop runs while the condition is true.
(E) The loop stops when the condition becomes true.

Answer and explanation

Answers: (A) and (E). REPEAT UNTIL tests first and stops on true.

(B) The block may run zero times.
(C) The condition is only checked before each pass.
(D) It is the opposite: it runs while the condition is false.

Question 8 (constructed response · core)

Write a code segment that uses a loop to find the sum of the first five even numbers (2, 4, 6, 8, 10) and displays it. Show a trace table of the variable that holds the total after each pass.

Answer and explanation

One correct answer:

total ← 0
even ← 2
REPEAT 5 TIMES
{
  total ← total + even
  even ← even + 2
}
DISPLAY(total)

Pass	`even` used	`total` after
1	2	2
2	4	6
3	6	12
4	8	20
5	10	30

30 is displayed. Check: 2 + 4 + 6 + 8 + 10 = 30 ✓.

Suggested Marlbridge rubric (5 marks)

Mark point	Marks
Initialises the total to 0 and the first even number to 2	1
A loop that runs five times (or until the 5th number is used)	1
Adds the current even number to the total and increases it by 2 inside the loop	1
Displays the total after the loop	1
Trace table values 2, 6, 12, 20, 30	1

Question 9 (constructed response · core)

A programmer writes the loop below to count how many weeks it takes for a plant that is 12 cm tall and grows 5 cm a week to reach at least 40 cm. It runs forever. (a) Explain why. (b) Rewrite it so that it works, and state how many weeks and what final height your code reports.

height ← 12
weeks ← 0
REPEAT UNTIL(height ≥ 40)
{
  weeks ← weeks + 1
}

Answer and explanation

(a) The loop body changes weeks but never changes height. The condition height ≥ 40 depends on height, which stays at 12, so the condition never becomes true.

(b) Corrected code:

height ← 12
weeks ← 0
REPEAT UNTIL(height ≥ 40)
{
  height ← height + 5
  weeks ← weeks + 1
}
DISPLAY(weeks)
DISPLAY(height)

Heights after each week: 17, 22, 27, 32, 37, 42. The loop stops when the height is 42 after 6 weeks. It displays 6 42.

Suggested Marlbridge rubric (5 marks)

Mark point	Marks
(a) The stopping condition depends on `height`, which never changes	1
(b) Adds the growth to `height` inside the loop	1
(b) Keeps the counter `weeks` increasing by 1 each pass	1
(b) States 6 weeks	1
(b) States the final height 42	1

How did you do?

Count the questions you got right first time, then read the explanation for every one you missed, including the wrong options you rejected correctly, because the distractors show you the typical slips. Anything you missed twice goes onto your revision list for the revision notes; re-read the matching section of the study guide before trying again.