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Worked solutions

IB Computer Science SL · Paper 1

Computer Science, Standard Level

Session: November 2017
Marks: 70
Duration: 1 hour 30 min
Calculator: Not allowed
Questions: 13

Q1 Q2 Q3 Q4 Q5 Q6 Q7 Q8 Q9 Q10 Q11 Q12 Q13

Keep your copy of the paper open alongside. We don't reproduce the questions: each part below is labelled to match the November 2017 global paper, which your school or the IB can provide.

Practice mode

Hide our working until you've had a go.

Questions 1–10 · 25 marks

Question 1

2 marks

Essential features of a computer language

Computational thinking, problem-solving and programming

(a)

2 marks

Identify two essential features common to high-level computer programming languages:
1) Fixed vocabulary with defined reserved keywords and unambiguous semantic meanings.
2) Consistent formal syntax and grammar rules governing code structure, statements, loops, and data type definitions.

Mentor tip

Identify core linguistic elements: fixed vocabulary, strict grammar/syntax, or standard data types and control structures.

Question 2

2 marks

Fundamental computer operations

Computer organisation

(a)

2 marks

State two fundamental operations of a computer beyond add and retrieve:
1) Compare (performing logical evaluation or relational testing between data values).
2) Store (writing or saving data to a memory location or secondary storage register).

Mentor tip

State fundamental CPU operations such as Compare (or Check) and Store (or Write/Save).

Question 3

2 marks

Roles of client and server in networked environments

Networks

(a)

1 mark

State the role of a client in network architecture:
A hardware host or software application that requests services, resources, or data from a central server across a network connection.

Mentor tip

State clearly that a client sends service or resource requests to a server.

(b)

1 mark

State the role of a server in network architecture:
A host computer or program that listens for, processes, and fulfills incoming requests made by client devices.

Mentor tip

Clarify that a server fulfills and returns responses to requests from client systems.

Question 4

1 mark

Input methods for accessibility

System fundamentals

(a)

1 mark

Identify an input method that improves accessibility for users with impairments:
Voice recognition / speech-to-text input, allowing individuals with motor disabilities or visual impairments to control software and input data without a physical keyboard.

Mentor tip

Mention voice recognition, Braille keyboards, eye-tracking systems, or sip-and-puff switches.

Question 5

3 marks

Truth table construction for NOR and OR combination

Computer organisation

(a)

3 marks

Evaluate the Boolean expression column by column for all input combinations:
Inputs A and B:
Row 1: A = 0 (FALSE), B = 0 (FALSE). A NOR B = NOT(0 OR 0) = 1 (TRUE). (A NOR B) OR A = 1 OR 0 = 1 (TRUE).
Row 2: A = 0 (FALSE), B = 1 (TRUE). A NOR B = NOT(0 OR 1) = 0 (FALSE). (A NOR B) OR A = 0 OR 0 = 0 (FALSE).
Row 3: A = 1 (TRUE), B = 0 (FALSE). A NOR B = NOT(1 OR 0) = 0 (FALSE). (A NOR B) OR A = 0 OR 1 = 1 (TRUE).
Row 4: A = 1 (TRUE), B = 1 (TRUE). A NOR B = NOT(1 OR 1) = 0 (FALSE). (A NOR B) OR A = 0 OR 1 = 1 (TRUE).

(A NOR B) \lor A = \overline{A \lor B} \lor A = (\overline{A} \land \overline{B}) \lor A = A \lor \overline{B}

Mentor tip

Remember that A NOR B is TRUE only when both inputs are FALSE. In the final column, ORing with A forces TRUE whenever A is TRUE.

Question 6

3 marks

Logic diagram for compound Boolean expression

Computer organisation

(a)

3 marks

Construct the logic diagram for expression: NOT A OR B AND C:
1) Place a NOT gate connected to input A to produce NOT A.
2) Place an AND gate connected to inputs B and C to produce B AND C (respecting operator precedence where AND takes precedence over OR).
3) Connect the output of the NOT gate and the output of the AND gate into a two-input OR gate to produce the final output.

Output = \overline{A} \lor (B \land C)

Mentor tip

Observe standard operator precedence: B AND C is evaluated first, then ORed with the inverted input A.

Question 7

6 marks

Nested loop execution analysis and while loop conversion

Computational thinking, problem-solving and programming

(a)

1 mark

Determine the number of times comparison K = J executes:
The outer loop iterates N times (K from 1 to N), and for each outer pass, the inner loop iterates N times (J from 1 to N).
Total comparison executions = N * N = N^2.

N \times N = N^{2}

Mentor tip

Multiply the iterations of the outer loop by the iterations of the inner loop to get N^2.

(b)

1 mark

Determine the number of times output K is executed:
The equality K = J occurs exactly once per outer loop pass (when J equals K).
Since the outer loop runs N times, output K executes exactly N times.

Mentor tip

Since K equals J exactly once for each of the N outer loop passes, it executes N times.

(c)

4 marks

Construct an algorithm performing the same task using a single while loop:
K = 1
loop while K <= N
output K
K = K + 1
end loop

Mentor tip

Observe that the nested loop simply outputs numbers 1 through N, so a single while loop from 1 to N is equivalent.

Question 8

3 marks

Machine instruction cycle and system buses

Computer organisation

(a)

1 mark

State where instructions and active data are held during execution:
Primary memory / Random Access Memory (RAM).

Mentor tip

Name primary memory / RAM.

(b)

2 marks

Outline the respective roles of the data bus and address bus:
1) Address bus: A unidirectional bus that transports the physical memory address from the CPU (MAR) to specify which memory cell is to be read from or written to.
2) Data bus: A bidirectional bus that transports the actual instruction or data word between the CPU (MDR) and the addressed memory location.

Mentor tip

State that the address bus carries memory addresses from the CPU, while the data bus carries the actual content to or from the CPU.

Question 9

1 mark

Definition of a bit

Computer organisation

(a)

1 mark

Define the term bit in computer science:
Short for binary digit; the fundamental and smallest unit of data in computing, representing a single binary state of either 0 or 1.

Mentor tip

Define bit as a binary digit representing the fundamental unit of storage taking a value of 0 or 1.

Question 10

2 marks

Beta testing methodology

System fundamentals

(a)

2 marks

Outline what is meant by beta testing:
Beta testing involves deploying a near-final version of a software product to real end users or an external testing cohort in practical operating environments.
Feedback and defect reports generated during beta testing allow development teams to resolve remaining software defects prior to final commercial deployment.

Mentor tip

Emphasize real user testing in production environments before formal public release.

Questions 11–13 · 45 marks

Question 11

15 marks

Office software features, disaster recovery, and system changeover

Computational thinking, problem-solving and programming
System fundamentals

(a)

2 marks

Describe how a word processor spellchecker functions:
The spellchecker compares each word in the document against an internal dictionary database.
If the word matches an entry, it is validated; if not found, it is flagged as misspelled.

Mentor tip

Describe dictionary comparison and highlighting words not present in the dictionary.

(b)

1 mark

State an advantage of automated periodic document saving:
In the event of an abrupt power failure or software crash, only modifications made since the most recent automatic save are lost rather than the entire document.

Mentor tip

Note that data loss is limited only to changes made since the last periodic save.

(c)

2 marks

Identify two additional word processor features useful in an office:
1) Mail merge (generating customized correspondence by merging a mailing list with a template document).
2) Track changes / collaborative commenting (enabling editorial review and revision tracking among colleagues).

Mentor tip

Mention features such as mail merge, macros, tables, styles, or track changes.

(d)

2 marks

Outline the purpose of another office application software package:
Spreadsheet software (e.g., Microsoft Excel): Used to organize numerical financial records, perform automated calculations using mathematical formulas, and render graphical charts for financial reporting.

Mentor tip

Name spreadsheet or database software and outline its primary business application.

(e)

2 marks

Outline measures taken to prevent data loss:
1) Implementing automated regular backups stored in secure, off-site cloud storage.
2) Installing uninterruptible power supply (UPS) units to protect equipment from abrupt power surges and sudden blackouts.

Mentor tip

Focus on off-site data backups and hardware protection against power loss.

(f)

2 marks

Outline a problem arising from installing new office hardware and software:
Employee disruption and temporary efficiency loss: Workers require time to learn new interfaces and operational procedures, temporarily slowing down administrative output.

Mentor tip

State user retraining hurdles and temporary productivity drops.

(g)

4 marks

Compare direct changeover and phased conversion:
1) Direct changeover: Complete switch from old to new system in a single step. Advantage: lower running costs with no duplicate data entry. Disadvantage: high catastrophic failure risk if errors emerge.
2) Phased conversion: Incremental implementation by department or module. Advantage: lower risk as defects are confined to a single department. Disadvantage: prolonged deployment period and temporary interface mismatches.

Mentor tip

Detail direct changeover vs phased rollout, identifying advantages and trade-offs for each.

Question 12

15 marks

Wireless local area network configuration and packet switching

Networks

(a)

1 mark

Identify one hardware component in a WLAN other than computers:
Wireless Access Point (WAP) / wireless router / network switch.

Mentor tip

Name a wireless access point, wireless router, or switch.

(b)

4 marks

Outline two disadvantages of implementing a WLAN compared to a wired LAN:
1) Reduced throughput and bandwidth contention: Wireless spectrum is shared among all connected devices, causing data transfer rates to degrade rapidly as client density increases.
2) Elevated security vulnerability: Radio frequency signals propagate beyond physical building perimeters, making transmissions susceptible to external interception or unauthorized eavesdropping.

Mentor tip

Discuss shared bandwidth/signal attenuation and higher interception risks over wireless frequencies.

(c)

3 marks

Identify three measures to reduce unauthorized access to confidential school data:
1) Enforce robust user authentication with role-based access control (differentiating student, faculty, and administrator permissions).
2) Implement end-to-end data encryption across storage drives and network traffic using modern standards (e.g., WPA2/WPA3 enterprise).
3) Enforce MAC address filtering to restrict network access exclusively to registered school-owned devices.

Mentor tip

Highlight strong authentication with role-based access, WPA2/3 encryption, and MAC address whitelisting.

(d)

1 mark

Define protocol:
A formal set of agreed-upon rules and standards governing the formatting, transmission, and reception of data between networked devices.

Mentor tip

Define protocol as a set of rules and conventions for transmitting data.

(e)

6 marks

Explain packet switching using the packet structure (Header 12B, Data 112B, Trailer 4B):
1) Message segmentation: The sender decomposes the complete data message into standardized payload segments (e.g., 112 bytes each).
2) Header encapsulation: Each segment is wrapped with a 12-byte header containing the sender's IP, recipient's IP, transmission protocol identifier, and packet sequence number.
3) Trailer attachment: A 4-byte trailer containing cyclic redundancy check (CRC) checksums and control codes is added for integrity verification.
4) Independent dynamic routing: Routers inspect the destination address within each header and forward packets independently across intermediate network hops along the most efficient available paths.
5) Error verification and reassembly: At the destination, the trailer checksums verify data integrity, re-requesting corrupted packets; the receiving host uses sequence numbers to reorder packets into the original message.

Mentor tip

Structure the response around segmentation, header addressing, dynamic routing, checksum error checks, and sequence reassembly.

Question 13

15 marks

Character arrays, trace tables, and substitution ciphers

Computational thinking, problem-solving and programming

(a)

1 mark

State the zero-based index of character 'U' in array S = ['P','S','E','U','D','O','C','O','D','E']:
S[0]='P', S[1]='S', S[2]='E', S[3]='U'.
Index = 3.

Mentor tip

Count using zero-based indexing to find index 3.

(b)

4 marks

Complete the trace table counting occurrences of 'E' in array S:
K = 0: CL = 0, K < 10 is TRUE, S[0]='P'='E' is FALSE.
K = 1: CL = 0, K < 10 is TRUE, S[1]='S'='E' is FALSE.
K = 2: CL = 1, K < 10 is TRUE, S[2]='E'='E' is TRUE.
K = 3 to 8: S[K]='E' is FALSE, CL remains 1.
K = 9: CL = 2, K < 10 is TRUE, S[9]='E'='E' is TRUE.
K = 10: K < 10 is FALSE -> Loop terminates; output CL = 2.

Mentor tip

Track CL incrementing only at K=2 and K=9 when the character 'E' is encountered, finally outputting 2.

(c)

5 marks

Construct an algorithm to generate array SUBSTITUTE by circular left shift of N positions:
IND = 0
loop for K from N to len(UPCASELETTERS) - 1
SUBSTITUTE[IND] = UPCASELETTERS[K]
IND = IND + 1
end loop
loop for K from 0 to N - 1
SUBSTITUTE[IND] = UPCASELETTERS[K]
IND = IND + 1
end loop

SUBSTITUTE [i] = UPCASELETTERS [(i + N) mod 26]

Mentor tip

Copy from index N to 25 first, then append from index 0 to N-1.

(d)

5 marks

Explain the algorithmic steps for method encode(CH, UPCASELETTERS, SUBSTITUTE):
1) Search array UPCASELETTERS for character CH using a linear search (or binary search).
2) Track the index POS where UPCASELETTERS[POS] equals CH.
3) If CH is located in UPCASELETTERS, return the corresponding replacement letter stored at SUBSTITUTE[POS].
4) If CH is not found in UPCASELETTERS (e.g., punctuation, space, or lowercase), return character CH unmodified.

Mentor tip

Search UPCASELETTERS for CH, return SUBSTITUTE[POS] if matched, or return CH unchanged if not an uppercase letter.

Our analysis

Where the marks are

Each part sorted by the topic it mostly tests. Computational thinking, problem-solving and programming carries the most marks on this paper.

Computational thinking, problem-solving and programming25 marks · 36%
Q1, Q7, Q11, Q13
Networks17 marks · 24%
Q3, Q12
System fundamentals16 marks · 23%
Q4, Q10, Q11
Computer organisation12 marks · 17%
Q2, Q5, Q6, Q8, Q9

Mentor insights

Traps to watch for

01
Compound Boolean evaluation precedence · Q5
In (A NOR B) OR A, remember that A NOR B is true exclusively when A = 0 and B = 0. Whenever A = 1, the overall expression evaluates to true.
02
Nested loop execution counts · Q7
The comparison K = J runs N * N = N^2 times because both loops run N iterations, but the inner condition K = J evaluates to true only once per outer pass, executing the output statement N times total.
03
Circular array shifting boundaries · Q13
When left-shifting UPCASELETTERS by N positions into array SUBSTITUTE, elements from index N through 25 are placed into indices 0 through (25 - N), and the wrapped elements from 0 to N - 1 fill the end of the array.

Solutions and tips are GeorgeTheTutor's own; the questions are not reproduced here. Not affiliated with or endorsed by the International Baccalaureate Organization.

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Worked solutions

IB Computer Science SL · Paper 1

Computer Science, Standard Level

Session: November 2017
Marks: 70
Duration: 1 hour 30 min
Calculator: Not allowed
Questions: 13

Q1 Q2 Q3 Q4 Q5 Q6 Q7 Q8 Q9 Q10 Q11 Q12 Q13

Keep your copy of the paper open alongside. We don't reproduce the questions: each part below is labelled to match the November 2017 global paper, which your school or the IB can provide.

Practice mode

Hide our working until you've had a go.

Questions 1–10 · 25 marks

Question 1

2 marks

Essential features of a computer language

Computational thinking, problem-solving and programming

(a)

2 marks

Identify two essential features common to high-level computer programming languages:
1) Fixed vocabulary with defined reserved keywords and unambiguous semantic meanings.
2) Consistent formal syntax and grammar rules governing code structure, statements, loops, and data type definitions.

Mentor tip

Identify core linguistic elements: fixed vocabulary, strict grammar/syntax, or standard data types and control structures.

Question 2

2 marks

Fundamental computer operations

Computer organisation

(a)

2 marks

State two fundamental operations of a computer beyond add and retrieve:
1) Compare (performing logical evaluation or relational testing between data values).
2) Store (writing or saving data to a memory location or secondary storage register).

Mentor tip

State fundamental CPU operations such as Compare (or Check) and Store (or Write/Save).

Question 3

2 marks

Roles of client and server in networked environments

Networks

(a)

1 mark

State the role of a client in network architecture:
A hardware host or software application that requests services, resources, or data from a central server across a network connection.

Mentor tip

State clearly that a client sends service or resource requests to a server.

(b)

1 mark

State the role of a server in network architecture:
A host computer or program that listens for, processes, and fulfills incoming requests made by client devices.

Mentor tip

Clarify that a server fulfills and returns responses to requests from client systems.

Question 4

1 mark

Input methods for accessibility

System fundamentals

(a)

1 mark

Identify an input method that improves accessibility for users with impairments:
Voice recognition / speech-to-text input, allowing individuals with motor disabilities or visual impairments to control software and input data without a physical keyboard.

Mentor tip

Mention voice recognition, Braille keyboards, eye-tracking systems, or sip-and-puff switches.

Question 5

3 marks

Truth table construction for NOR and OR combination

Computer organisation

(a)

3 marks

Evaluate the Boolean expression column by column for all input combinations:
Inputs A and B:
Row 1: A = 0 (FALSE), B = 0 (FALSE). A NOR B = NOT(0 OR 0) = 1 (TRUE). (A NOR B) OR A = 1 OR 0 = 1 (TRUE).
Row 2: A = 0 (FALSE), B = 1 (TRUE). A NOR B = NOT(0 OR 1) = 0 (FALSE). (A NOR B) OR A = 0 OR 0 = 0 (FALSE).
Row 3: A = 1 (TRUE), B = 0 (FALSE). A NOR B = NOT(1 OR 0) = 0 (FALSE). (A NOR B) OR A = 0 OR 1 = 1 (TRUE).
Row 4: A = 1 (TRUE), B = 1 (TRUE). A NOR B = NOT(1 OR 1) = 0 (FALSE). (A NOR B) OR A = 0 OR 1 = 1 (TRUE).

(A NOR B) \lor A = \overline{A \lor B} \lor A = (\overline{A} \land \overline{B}) \lor A = A \lor \overline{B}

Mentor tip

Remember that A NOR B is TRUE only when both inputs are FALSE. In the final column, ORing with A forces TRUE whenever A is TRUE.

Question 6

3 marks

Logic diagram for compound Boolean expression

Computer organisation

(a)

3 marks

Construct the logic diagram for expression: NOT A OR B AND C:
1) Place a NOT gate connected to input A to produce NOT A.
2) Place an AND gate connected to inputs B and C to produce B AND C (respecting operator precedence where AND takes precedence over OR).
3) Connect the output of the NOT gate and the output of the AND gate into a two-input OR gate to produce the final output.

Output = \overline{A} \lor (B \land C)

Mentor tip

Observe standard operator precedence: B AND C is evaluated first, then ORed with the inverted input A.

Question 7

6 marks

Nested loop execution analysis and while loop conversion

Computational thinking, problem-solving and programming

(a)

1 mark

Determine the number of times comparison K = J executes:
The outer loop iterates N times (K from 1 to N), and for each outer pass, the inner loop iterates N times (J from 1 to N).
Total comparison executions = N * N = N^2.

N \times N = N^{2}

Mentor tip

Multiply the iterations of the outer loop by the iterations of the inner loop to get N^2.

(b)

1 mark

Determine the number of times output K is executed:
The equality K = J occurs exactly once per outer loop pass (when J equals K).
Since the outer loop runs N times, output K executes exactly N times.

Mentor tip

Since K equals J exactly once for each of the N outer loop passes, it executes N times.

(c)

4 marks

Construct an algorithm performing the same task using a single while loop:
K = 1
loop while K <= N
output K
K = K + 1
end loop

Mentor tip

Observe that the nested loop simply outputs numbers 1 through N, so a single while loop from 1 to N is equivalent.

Question 8

3 marks

Machine instruction cycle and system buses

Computer organisation

(a)

1 mark

State where instructions and active data are held during execution:
Primary memory / Random Access Memory (RAM).

Mentor tip

Name primary memory / RAM.

(b)

2 marks

Outline the respective roles of the data bus and address bus:
1) Address bus: A unidirectional bus that transports the physical memory address from the CPU (MAR) to specify which memory cell is to be read from or written to.
2) Data bus: A bidirectional bus that transports the actual instruction or data word between the CPU (MDR) and the addressed memory location.

Mentor tip

State that the address bus carries memory addresses from the CPU, while the data bus carries the actual content to or from the CPU.

Question 9

1 mark

Definition of a bit

Computer organisation

(a)

1 mark

Define the term bit in computer science:
Short for binary digit; the fundamental and smallest unit of data in computing, representing a single binary state of either 0 or 1.

Mentor tip

Define bit as a binary digit representing the fundamental unit of storage taking a value of 0 or 1.

Question 10

2 marks

Beta testing methodology

System fundamentals

(a)

2 marks

Outline what is meant by beta testing:
Beta testing involves deploying a near-final version of a software product to real end users or an external testing cohort in practical operating environments.
Feedback and defect reports generated during beta testing allow development teams to resolve remaining software defects prior to final commercial deployment.

Mentor tip

Emphasize real user testing in production environments before formal public release.

Questions 11–13 · 45 marks

Question 11

15 marks

Office software features, disaster recovery, and system changeover

Computational thinking, problem-solving and programming
System fundamentals

(a)

2 marks

Describe how a word processor spellchecker functions:
The spellchecker compares each word in the document against an internal dictionary database.
If the word matches an entry, it is validated; if not found, it is flagged as misspelled.

Mentor tip

Describe dictionary comparison and highlighting words not present in the dictionary.

(b)

1 mark

State an advantage of automated periodic document saving:
In the event of an abrupt power failure or software crash, only modifications made since the most recent automatic save are lost rather than the entire document.

Mentor tip

Note that data loss is limited only to changes made since the last periodic save.

(c)

2 marks

Identify two additional word processor features useful in an office:
1) Mail merge (generating customized correspondence by merging a mailing list with a template document).
2) Track changes / collaborative commenting (enabling editorial review and revision tracking among colleagues).

Mentor tip

Mention features such as mail merge, macros, tables, styles, or track changes.

(d)

2 marks

Outline the purpose of another office application software package:
Spreadsheet software (e.g., Microsoft Excel): Used to organize numerical financial records, perform automated calculations using mathematical formulas, and render graphical charts for financial reporting.

Mentor tip

Name spreadsheet or database software and outline its primary business application.

(e)

2 marks

Outline measures taken to prevent data loss:
1) Implementing automated regular backups stored in secure, off-site cloud storage.
2) Installing uninterruptible power supply (UPS) units to protect equipment from abrupt power surges and sudden blackouts.

Mentor tip

Focus on off-site data backups and hardware protection against power loss.

(f)

2 marks

Outline a problem arising from installing new office hardware and software:
Employee disruption and temporary efficiency loss: Workers require time to learn new interfaces and operational procedures, temporarily slowing down administrative output.

Mentor tip

State user retraining hurdles and temporary productivity drops.

(g)

4 marks

Compare direct changeover and phased conversion:
1) Direct changeover: Complete switch from old to new system in a single step. Advantage: lower running costs with no duplicate data entry. Disadvantage: high catastrophic failure risk if errors emerge.
2) Phased conversion: Incremental implementation by department or module. Advantage: lower risk as defects are confined to a single department. Disadvantage: prolonged deployment period and temporary interface mismatches.

Mentor tip

Detail direct changeover vs phased rollout, identifying advantages and trade-offs for each.

Question 12

15 marks

Wireless local area network configuration and packet switching

Networks

(a)

1 mark

Identify one hardware component in a WLAN other than computers:
Wireless Access Point (WAP) / wireless router / network switch.

Mentor tip

Name a wireless access point, wireless router, or switch.

(b)

4 marks

Outline two disadvantages of implementing a WLAN compared to a wired LAN:
1) Reduced throughput and bandwidth contention: Wireless spectrum is shared among all connected devices, causing data transfer rates to degrade rapidly as client density increases.
2) Elevated security vulnerability: Radio frequency signals propagate beyond physical building perimeters, making transmissions susceptible to external interception or unauthorized eavesdropping.

Mentor tip

Discuss shared bandwidth/signal attenuation and higher interception risks over wireless frequencies.

(c)

3 marks

Identify three measures to reduce unauthorized access to confidential school data:
1) Enforce robust user authentication with role-based access control (differentiating student, faculty, and administrator permissions).
2) Implement end-to-end data encryption across storage drives and network traffic using modern standards (e.g., WPA2/WPA3 enterprise).
3) Enforce MAC address filtering to restrict network access exclusively to registered school-owned devices.

Mentor tip

Highlight strong authentication with role-based access, WPA2/3 encryption, and MAC address whitelisting.

(d)

1 mark

Define protocol:
A formal set of agreed-upon rules and standards governing the formatting, transmission, and reception of data between networked devices.

Mentor tip

Define protocol as a set of rules and conventions for transmitting data.

(e)

6 marks

Explain packet switching using the packet structure (Header 12B, Data 112B, Trailer 4B):
1) Message segmentation: The sender decomposes the complete data message into standardized payload segments (e.g., 112 bytes each).
2) Header encapsulation: Each segment is wrapped with a 12-byte header containing the sender's IP, recipient's IP, transmission protocol identifier, and packet sequence number.
3) Trailer attachment: A 4-byte trailer containing cyclic redundancy check (CRC) checksums and control codes is added for integrity verification.
4) Independent dynamic routing: Routers inspect the destination address within each header and forward packets independently across intermediate network hops along the most efficient available paths.
5) Error verification and reassembly: At the destination, the trailer checksums verify data integrity, re-requesting corrupted packets; the receiving host uses sequence numbers to reorder packets into the original message.

Mentor tip

Structure the response around segmentation, header addressing, dynamic routing, checksum error checks, and sequence reassembly.

Question 13

15 marks

Character arrays, trace tables, and substitution ciphers

Computational thinking, problem-solving and programming

(a)

1 mark

State the zero-based index of character 'U' in array S = ['P','S','E','U','D','O','C','O','D','E']:
S[0]='P', S[1]='S', S[2]='E', S[3]='U'.
Index = 3.

Mentor tip

Count using zero-based indexing to find index 3.

(b)

4 marks

Complete the trace table counting occurrences of 'E' in array S:
K = 0: CL = 0, K < 10 is TRUE, S[0]='P'='E' is FALSE.
K = 1: CL = 0, K < 10 is TRUE, S[1]='S'='E' is FALSE.
K = 2: CL = 1, K < 10 is TRUE, S[2]='E'='E' is TRUE.
K = 3 to 8: S[K]='E' is FALSE, CL remains 1.
K = 9: CL = 2, K < 10 is TRUE, S[9]='E'='E' is TRUE.
K = 10: K < 10 is FALSE -> Loop terminates; output CL = 2.

Mentor tip

Track CL incrementing only at K=2 and K=9 when the character 'E' is encountered, finally outputting 2.

(c)

5 marks

Construct an algorithm to generate array SUBSTITUTE by circular left shift of N positions:
IND = 0
loop for K from N to len(UPCASELETTERS) - 1
SUBSTITUTE[IND] = UPCASELETTERS[K]
IND = IND + 1
end loop
loop for K from 0 to N - 1
SUBSTITUTE[IND] = UPCASELETTERS[K]
IND = IND + 1
end loop

SUBSTITUTE [i] = UPCASELETTERS [(i + N) mod 26]

Mentor tip

Copy from index N to 25 first, then append from index 0 to N-1.

(d)

5 marks

Explain the algorithmic steps for method encode(CH, UPCASELETTERS, SUBSTITUTE):
1) Search array UPCASELETTERS for character CH using a linear search (or binary search).
2) Track the index POS where UPCASELETTERS[POS] equals CH.
3) If CH is located in UPCASELETTERS, return the corresponding replacement letter stored at SUBSTITUTE[POS].
4) If CH is not found in UPCASELETTERS (e.g., punctuation, space, or lowercase), return character CH unmodified.

Mentor tip

Search UPCASELETTERS for CH, return SUBSTITUTE[POS] if matched, or return CH unchanged if not an uppercase letter.

Our analysis

Where the marks are

Each part sorted by the topic it mostly tests. Computational thinking, problem-solving and programming carries the most marks on this paper.

Computational thinking, problem-solving and programming25 marks · 36%
Q1, Q7, Q11, Q13
Networks17 marks · 24%
Q3, Q12
System fundamentals16 marks · 23%
Q4, Q10, Q11
Computer organisation12 marks · 17%
Q2, Q5, Q6, Q8, Q9

Mentor insights

Traps to watch for

01
Compound Boolean evaluation precedence · Q5
In (A NOR B) OR A, remember that A NOR B is true exclusively when A = 0 and B = 0. Whenever A = 1, the overall expression evaluates to true.
02
Nested loop execution counts · Q7
The comparison K = J runs N * N = N^2 times because both loops run N iterations, but the inner condition K = J evaluates to true only once per outer pass, executing the output statement N times total.
03
Circular array shifting boundaries · Q13
When left-shifting UPCASELETTERS by N positions into array SUBSTITUTE, elements from index N through 25 are placed into indices 0 through (25 - N), and the wrapped elements from 0 to N - 1 fill the end of the array.

Solutions and tips are GeorgeTheTutor's own; the questions are not reproduced here. Not affiliated with or endorsed by the International Baccalaureate Organization.

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