The End-of-Course Multiple-Choice Exam: Exam Skills Guide (Computer Science Principles)

How to use this guide

This guide is for any student preparing for the end-of-course exam in Computer Science Principles. It is about technique: how to read the questions, what to do with the code, and how to spend your time. It does not replace the topic study guides. Read it once, then work through the practice items before opening the answers. Every question here is original Marlbridge practice. None of it is past exam material, and the code is written in Marlbridge’s rendering of the exam reference notation. Exam details change, so check your teacher’s current instructions and the official course pages for the format in your exam year.

The section at a glance

Feature	Detail
Questions	70
Time	120 minutes
Weight	70% of the exam score (the Create performance task is the other 30%)
Single-select	57 questions, 4 options, one correct answer
Passage questions	5 single-select questions on a reading passage about a computing innovation
Select-two	8 questions, 4 options, two correct answers
Tools	No calculator. The Exam Reference Sheet is provided
Format	Fully digital

Roughly, the questions come from the course’s five big ideas: creative development, data, algorithms and programming, computer systems and networks, and the impact of computing. Algorithms and programming is the biggest single share, so reading and tracing code matters most. Do not skip the others: the impact of computing questions are often quick marks if you have read the passage carefully.

Reading the reference notation

Most mistakes in code questions come from reading the notation wrongly, not from misunderstanding the idea. Fix these in your mind.

Notation	Meaning	Watch for
x ← 5	Assign 5 to x	The arrow is not “equals”
a = b	Test whether a equals b	Gives true or false
a ≠ b	Test whether a differs from b
a MOD b	The remainder when a is divided by b	17 MOD 5 is 2; 10 MOD 5 is 0
aList[i]	Element at index i	First index is 1
LENGTH(aList)	Number of elements now	Changes after INSERT, APPEND, REMOVE
REPEAT n TIMES	Run the block n times	n is evaluated once
REPEAT UNTIL(cond)	Check cond first, run block until it becomes true	The block may run zero times
FOR EACH item IN aList	Visit each element in turn	item is a copy
RANDOM(a, b)	Random integer from a to b, both included	RANDOM(1, 6) can give 1 and 6
RETURN(value)	Leave the procedure and send back value	Nothing after it runs
DISPLAY(x)	Show x; separate items are shown in order

Two habits repay themselves in every code question.

1. Write the table. One column for each variable the code changes, one row for each pass or step. Update only the variable that the line changes, and use current values.
2. Read the whole block before you answer. A line that sets a variable back to 0 inside a loop, or a RETURN inside an IF, changes the whole meaning.

Worked example 1: a trace with MOD

count ← 0
FOR EACH n IN [4, 9, 12, 7, 6]
{
  IF(n MOD 3 = 0)
  {
    count ← count + 1
  }
}
DISPLAY(count)

What is displayed?

• (A) 2
• (B) 3
• (C) 4
• (D) 5

Answer

Answer: (B)

n	n MOD 3	n MOD 3 = 0?	count
(start)			0
4	1	no	0
9	0	yes	1
12	0	yes	2
7	1	no	2
6	0	yes	3

The code counts multiples of 3 in the list: 9, 12 and 6. Option (A) forgets 6; option (C) counts one extra element; option (D) counts every element and ignores the condition.

Single-select questions

A single-select question has one best answer among four. A useful routine:

1. Read the question stem first, then the code or scenario. Know what you are looking for before you read.
2. Decide what kind of question it is. Does it ask for an output, a value after code runs, a missing line, a reason, a definition, or which statement is true?
3. Predict the answer before looking at the options when you can. Predicting stops a tempting wrong option from steering you.
4. Eliminate. Cross out options that are plainly wrong. Common wrong options are the answer you get from a typical slip, such as counting from 0, using the old value of a variable, or off by one in a loop.
5. Choose, mark and move. If two options remain and the clock is running, pick one, flag the question and return later.

Wrong answers are designed

Each wrong option usually matches a specific mistake. Spotting the mistake it represents is a check on your own answer. If your answer equals the result of the classic slip, look again.

Worked example 2: choosing the missing line

A procedure should return the number of elements of aList that are greater than limit.

PROCEDURE countAbove(aList, limit)
{
  total ← 0
  FOR EACH item IN aList
  {
    <MISSING LINE>
  }
  RETURN(total)
}

Which replacement for <MISSING LINE> works?

• (A) total ← total + item
• (B) IF(item > limit) { total ← total + 1 }
• (C) IF(item > limit) { RETURN(total + 1) }
• (D) IF(item > limit) { total ← 1 }

Answer

Answer: (B)

The procedure must count, so it adds 1 for each qualifying element and returns the total at the end. (A) adds the value of every element, which is a sum. (C) returns early, at the first element above the limit. (D) sets the total to 1 each time, so it can only ever give 0 or 1.

Test with aList ← [3, 8, 1, 9] and limit ← 2: elements above 2 are 3, 8 and 9, so the answer is 3, and only (B) gives that.

Select-two questions

In a select-two question you choose two options, and both must be correct. The question says “select two answers”. Treat it like four true/false statements.

1. Read the stem.
2. Judge each option separately as true or false, writing T or F beside it.
3. You should finish with exactly two T’s. If you have three, or one, one of your judgements is wrong, so recheck the closest call.
4. Do not mark a statement as correct because it sounds technical. Check it against what you know.

Worked example 3: select two

Which two statements about the Internet are true?

• (A) Data sent across the Internet is split into packets that may take different routes.
• (B) Every message must travel along the same single path from sender to receiver.
• (C) Having several possible routes between devices helps the Internet keep working when part of it fails.
• (D) The Internet and the World Wide Web are two names for exactly the same thing.

Answer

Answer: (A) and (C)

(A) is true: packets can be routed independently and put back in order at the destination. (B) is false, since single-path delivery would make failures far worse. (C) is true: redundancy gives fault tolerance. (D) is false: the Web is a service that runs on the Internet, which is the network of networks.

Reading a computing-innovation passage

Five questions refer to a passage about a computing innovation, such as a type of app, sensor system or online service. These questions test your reading as much as your knowledge.

• Read the passage once for the story, then read each question and go back to the exact lines it needs.
• Answer from the passage and from the course ideas. An option may be a reasonable thing in the real world but not supported by the text. Prefer the option the passage supports.
• Expect questions about data the innovation collects or uses, beneficial and harmful effects, privacy and security, bias, and what kind of data processing is going on.
• Match the vocabulary: if a question asks about “a harmful effect”, an option that describes only a benefit is wrong even if it is accurate.

Original practice passage

A fictional town library trials a “return-time predictor” app. It records the date each book is borrowed and returned, and the borrower’s age group. From the history, it estimates when each borrowed book will come back, so staff can tell waiting readers. The library says the data is stored on its own server and used only to improve lending.

Which two of the following are potential concerns about the app that follow from the passage?

• (A) The age-group data could reveal patterns about individuals if it is combined with other information.
• (B) The app records the date of each loan, so it cannot be used on a computer.
• (C) If the history mostly comes from one age group, the predictions may be less accurate for others.
• (D) The server is owned by the library, so no data can ever be lost.

Answer

Answer: (A) and (C)

(A) is a privacy concern: even without names, combined data can identify people. (C) is a bias concern: a data set that over-represents one group gives predictions that fit that group better. (B) does not follow from anything in the passage. (D) is false, because any system can fail or be attacked.

Pacing: 70 questions in 120 minutes

That is about 1 minute 43 seconds a question on average. Quick questions (definitions, short traces) will take 30 to 60 seconds, which pays for longer tracing items.

Checkpoint	Question number	Time used (guide)
Quarter	18	about 30 minutes
Half	35	about 60 minutes
Three quarters	53	about 90 minutes
End	70	105 minutes, leaving 15 to review

• First pass: answer everything you can in under two minutes. If you have not got a method after about 90 seconds, flag the question and go on.
• Second pass: return to the flagged items. Now you have no new questions to fear, and sometimes a later question reminds you of an idea.
• Final minutes: make sure every question has an answer selected. A blank cannot earn marks.
• Do not change an answer without a reason. Change it only when you find a specific error in your first reasoning.

Five technique habits for code

1. Boundary cases. Check an empty list, a list with one element, all-equal values, and the first and last positions. Many correct-looking procedures fail one of these.
2. Off by one. Ask: does the loop run n times or n + 1 times? Does an index go from 1 to LENGTH(aList), never 0 and never past the end?
3. Position of a statement. Moving total ← 0 into the loop, or a RETURN into the loop, changes the result. Notice which block each line sits in.
4. Order of values after swaps. To swap two variables you need a third temporary variable. Without one, the first assignment overwrites a value you still need.
5. Binary search needs sorted data. If the list is unsorted, binary search can miss a value that is present. Linear search works either way.

Worked example 4: spotting an edge-case failure

PROCEDURE biggest(aList)
{
  best ← 0
  FOR EACH item IN aList
  {
    IF(item > best)
    {
      best ← item
    }
  }
  RETURN(best)
}

A programmer says this procedure always returns the largest value in aList. Which input shows that this claim is false?

• (A) [3, 8, 5]
• (B) [7]
• (C) [-4, -2, -9]
• (D) [2, 2, 2]

Answer

Answer: (C)

best starts at 0. With all-negative values no element is greater than 0, so the procedure returns 0, which is not in the list. The inputs in (A), (B) and (D) all contain a positive value, so the procedure gives the right answer for them. A safer start is best ← aList[1].

Common misconceptions and slips

Slip	What it looks like	Fix
Counting from 0	Treating aList[1] as the second element	Say “first element is index 1” before every list question
Using the old value	Calculating x ← x + 1 twice from the starting value	Update the table row by row
Reading = as assignment	Thinking IF(a = 3) changes a	= only tests; ← changes
Ignoring the stem	Solving for output when asked for the number of times a line runs	Underline the thing asked for
Choosing a true but irrelevant option	Picking a correct statement that does not answer the question	Match the option to the stem
Over-reading a passage	Picking an option supported by general knowledge but not the text	Point to the line that supports it
Rushing select-two	Finding one correct answer and guessing the second	Test every option separately

Practice set: original Marlbridge practice

Do these in 12 minutes with no notes. Then check the answers and write down which slip, if any, you made.

1. What is displayed?

x ← 2
y ← 5
x ← y
y ← x + 1
DISPLAY(x)
DISPLAY(y)

• (A) 2 and 3
• (B) 5 and 6
• (C) 5 and 3
• (D) 2 and 6

2. A list begins as [5, 10, 15]. After INSERT(aList, 2, 7) and then REMOVE(aList, 3), which list results?

• (A) [5, 7, 15]
• (B) [5, 7, 10, 15]
• (C) [5, 10, 15]
• (D) [7, 10, 15]

3. A student wants a procedure that displays the numbers 1 to 5. Which loop is correct?

• (A) n ← 1 REPEAT 5 TIMES { DISPLAY(n) n ← n + 1 }
• (B) n ← 0 REPEAT 5 TIMES { DISPLAY(n) n ← n + 1 }
• (C) n ← 1 REPEAT 6 TIMES { DISPLAY(n) n ← n + 1 }
• (D) n ← 1 REPEAT 5 TIMES { n ← n + 1 DISPLAY(n) }

4. Which two are true of a sorted list of 1000 elements searched with binary search?

• (A) Binary search can be used because the list is sorted.
• (B) In the worst case it must look at all 1000 elements.
• (C) Each check removes about half of the remaining elements.
• (D) It also works if the list is in random order.

Answers and notes

1. (B). After x ← y, x is 5. Then y ← x + 1 uses the new x, so y is 6. Option (D) uses the old x of 2. Option (C) is the mixture that comes from skipping a row.

2. (A). After the INSERT the list is [5, 7, 10, 15]. Index 3 is now 10, so REMOVE(aList, 3) deletes 10, leaving [5, 7, 15]. Option (B) forgets the REMOVE; option (C) undoes both changes; option (D) removes the first element instead.

3. (A). It starts at 1 and runs five times, so it shows 1 to 5. (B) shows 0 to 4. (C) shows 1 to 6. (D) shows 2 to 6 because it adds before displaying.

4. (A) and (C). Binary search needs sorted data, and it halves the remaining range each time, so about 10 checks suffice for 1000 elements. (B) is false for binary search, but would be right for a linear search. (D) is false, since an unsorted list can make binary search miss a present value.

A checklist for exam week

• ☐ I can trace a loop, a list operation and a procedure call with a table.
• ☐ I know the exam notation: first index 1, MOD, RANDOM(a, b) includes both ends, RETURN ends a procedure.
• ☐ I have done at least one full timed practice section of my teacher’s choosing.
• ☐ I have a first-pass and second-pass routine and I know my checkpoints.
• ☐ For select-two questions I mark each option true or false.
• ☐ I read passages once for the story, then return to the lines each question needs.
• ☐ I have checked the current official exam instructions and my testing device set-up.

Where to go next

Use the unit reviews for mixed practice, the topic checklists to find gaps, and the guide to the Create performance task for the other 30% of your score.