2024 IB DP Computer Science 模擬試題連答案詳解

Direct changeover involves completely shutting down the legacy system and immediately launching the new system. Because there is no period of overlap or redundant operation, any unexpected failure in the new system can lead to severe data loss or total disruption of business activities since there is no backup system active.

Award [1 mark] for identifying the high risk or lack of a fallback system. Award [1 mark] for explaining the consequence, such as immediate data loss, business disruption, or operational downtime.

The Program Counter (PC) is a dedicated register within the CPU. Its primary role is to store the memory address of the next instruction that needs to be executed. During the fetch stage, the CPU retrieves the instruction from the memory address specified by the PC, and the PC is subsequently incremented (or updated in the case of a jump/branch) to point to the next instruction in sequence.

Award [1 mark] for stating that it stores the memory address of the next instruction to be executed. Award [1 mark] for explaining that it increments or updates to point to the subsequent instruction after a fetch.

Unlike circuit switching, packet switching does not require a dedicated physical channel to be established. This allows multiple data streams to share the same transmission lines, leading to higher bandwidth efficiency. Additionally, if a router or node along the path fails, individual packets can dynamically reroute around the failure, making the transmission highly fault-tolerant.

Award [1 mark] for each valid advantage stated up to a maximum of [2 marks]. Acceptable answers include: more efficient bandwidth/channel usage, higher resilience/fault tolerance (alternative paths), cheaper implementation, or packets can be buffered if a bottleneck occurs.

In computational thinking, abstraction is a key problem-solving technique. It involves simplifying complex real-world situations by ignoring or hiding details that are irrelevant to the current task, allowing the developer to focus purely on the core conceptual elements and high-level logic.

Award [1 mark] for mentioning the removal, hiding, or filtering of unnecessary/irrelevant details. Award [1 mark] for mentioning the focus on core, essential, or high-level characteristics of the problem.

A class acts as a template or blueprint written in code that defines the attributes (data) and behaviors (methods) that all instances will possess. It does not contain live data in itself. An object, however, is a specific, instantiated entity that exists at runtime, allocating actual memory space to hold state-specific values for those attributes defined in the class.

Award [1 mark] for describing a class (e.g., static blueprint, template, definition). Award [1 mark] for describing an object (e.g., dynamic instance, concrete entity with state, memory-allocated instantiation of a class).

A queue is a linear data structure that operates under the First-In-First-Out (FIFO) principle. For a shared office printer, this ensures a fair and predictable scheduling policy: the print job that was sent first is processed and printed first, preventing newer print jobs from overtaking older ones in the system queue.

Award [1 mark] for identifying the First-In-First-Out (FIFO) characteristic of a queue. Award [1 mark] for explaining why this ensures fairness or sequential processing order for print requests.

Virtual memory is divided into blocks called pages. When a program references an address on a page that is not currently loaded in the physical RAM (main memory), the Memory Management Unit (MMU) cannot translate the virtual address. This triggers a hardware interrupt known as a page fault, signaling the operating system to retrieve the missing page from secondary storage (swap space/hard drive) and load it into RAM.

Award [1 mark] for stating that the requested virtual page/data is not present in the physical RAM/main memory. Award [1 mark] for explaining that this triggers an interrupt/request for the operating system to retrieve the page from secondary storage (disk/swap space).

In a control system, sensors and actuators perform opposing but complementary functions. A sensor (e.g., a moisture or temperature sensor) measures physical state variables from the environment, converts them into signals, and sends them to the microprocessor. An actuator (e.g., a motorized window opener or heater) receives electrical signals/commands from the microprocessor and performs a physical action to change the state of the system.

Award [1 mark] for describing the sensor's role of gathering input / measuring environmental variables. Award [1 mark] for describing the actuator's role of taking physical action / output response to alter the environment based on system instructions.

When migrating from a legacy system, the older software often uses proprietary, obsolete, or structured data formats (e.g., flat-file databases or legacy text encodings) that do not align with the modern, relational, or object-oriented database schema of the new system. Consequently, developers must write complex conversion scripts, which introduces the risk of data corruption, mismatching data types, or complete loss of metadata.

Award [1] for identifying a valid compatibility issue (e.g., mismatching database schemas, obsolete file formats, different character encoding standards).
Award [1] for outlining the consequence or implication of this issue during the migration process (e.g., requires data transformation scripts, risks data loss/corruption, or increases implementation time/cost).

During the fetch stage, the CPU copies the memory address stored in the Program Counter (PC) to the Memory Address Register (MAR) so the instruction can be retrieved from RAM. Immediately after this address is sent (or during the fetch cycle), the PC is incremented by one (or by the instruction length) to point to the address of the next sequential instruction to be executed.

Award [1] for stating that the PC holds the memory address of the next instruction to be fetched/executed.
Award [1] for stating that the PC is incremented (by 1 or instruction size) to point to the next instruction.

Dividing data into discrete packets allows for more efficient bandwidth utilization and error recovery. In the event of network congestion or packet loss, only the missing or damaged packet needs to be retransmitted by the sender, saving significant network overhead compared to resending the entire original file. Additionally, individual packets can take different physical routes to the destination, optimizing network load balancing.

Award [1] for identifying a valid benefit (e.g., selective retransmission, routing flexibility, avoiding channel monopolization).
Award [1] for explaining how this benefit occurs (e.g., only corrupted parts of the file are resent, or packets bypass congested routes to speed up delivery).

Abstraction simplifies complex real-world situations by ignoring non-essential details while keeping key characteristics. In a public transport simulation, physical details such as the model or color of the buses, individual driver profiles, and passenger names are omitted. Instead, the simulation only models critical parameters such as bus capacities, route distances, station locations, and scheduled arrival/departure times, which reduces computational complexity.

Award [1] for showing understanding of abstraction (e.g., filtering out unnecessary details / focusing only on important elements).
Award [1] for applying it directly to the public transport simulation context (e.g., ignoring bus aesthetic details or passenger identities, and modeling only transport schedules, capacities, or routes).

(a)
- LAN: Operates within a limited geographical area, such as a single office or distribution hub of FastTrack Cargo. It is typically owned and managed directly by the company, offering high bandwidth and low latency.
- WAN: Extends over a large geographical distance to connect separate offices and distribution hubs. It relies on external telecommunications providers and public internet infrastructure, usually suffering from lower speeds and higher latency compared to a LAN.

(b)
- Benefit 1 (Data Encryption): A VPN encrypts all data sent between the remote drivers and the central office database. This ensures that even if packet data is intercepted on a public or insecure Wi-Fi network, it cannot be read or understood by unauthorized third parties.
- Benefit 2 (Secure Tunneling and Access Control): A VPN establishes a secure 'tunnel' that authenticates the user's credentials before granting entry. This acts as a gateway protection mechanism, preventing unauthorized entities from gaining direct access to the private network resources.

(c)
- Packetization: The message or query from the driver is split into smaller, manageable pieces called packets. Each packet contains a header with control info (such as source IP, destination IP, and packet sequence number) and the payload (data).
- Independent Routing: Each packet is sent out onto the internet where routers dynamically determine the optimal path. Packets may travel via different routes depending on current network congestion.
- Reassembly: Once the packets arrive at the central database server, they are reordered and reassembled into the original message using the sequence numbers in the headers.
- Error Checking: If any packet is detected as corrupted or is missing, the receiving server requests a retransmission of that specific packet.

(d)
- Router: Connects the LAN to the WAN. Its primary role is to inspect the destination IP address of incoming packets and use a routing table to forward those packets along the most efficient route toward their target.
- Gateway: Serves as the entry and exit point of a network and performs protocol translation. Because the internal network of FastTrack Cargo and the public internet might run on different communication protocol suites, the gateway translates these protocols to ensure seamless communication between disparate systems.

Total: 15 marks

(a) [3 marks]
- 1 mark: Defining LAN's limited geographical scope (e.g., single office, warehouse).
- 1 mark: Defining WAN's large geographical scope (e.g., connecting multiple cities/hubs).
- 1 mark: Highlighting a key structural/ownership difference (e.g., LAN uses local private wiring, WAN uses third-party telecommunication lines or public internet).

(b) [4 marks]
- Award 2 marks for each well-explained benefit:
- 1 mark for identifying the technical mechanism (e.g., encryption, IP spoofing prevention, tunneling).
- 1 mark for context-based explanation (e.g., preventing interception on public Wi-Fi, securing driver credentials).

(c) [4 marks]
- 1 mark: Explanation of packetization (splitting data into packets with headers/sequence numbers).
- 1 mark: Explanation of dynamic/independent routing of individual packets across various paths.
- 1 mark: Explanation of reassembly at the receiving node using sequence numbers.
- 1 mark: Explanation of error-checking / retransmission protocols.

(d) [4 marks]
- Router (Max 2 marks):
- 1 mark: Identifies that it routes packets across different networks.
- 1 mark: Explains that it uses routing tables and IP addresses to determine the next hop.
- Gateway (Max 2 marks):
- 1 mark: Identifies that it functions as a network interface converter/entry-exit node.
- 1 mark: Explains that it converts between different network protocols/formats.

**(a) Truth Table**

| T | H | W | S | A |
|---|---|---|---|---|
| 0 | 0 | 0 | 0 | 0 |
| 0 | 0 | 1 | 0 | 0 |
| 0 | 1 | 0 | 0 | 0 |
| 0 | 1 | 1 | 0 | 1 |
| 1 | 0 | 0 | 1 | 0 |
| 1 | 0 | 1 | 0 | 1 |
| 1 | 1 | 0 | 1 | 0 |
| 1 | 1 | 1 | 0 | 1 |

**(b) (i) Boolean Expressions**
- \(S = T \cdot \overline{W}\) (or \(S = T \text{ AND NOT } W\))
- \(A = W \cdot (T + H)\) (or \(A = W \text{ AND } (T \text{ OR } H)\))

**(b) (ii) Logic Circuit Connection Description**
1. Connect the sensor inputs \(T\) and \(H\) to the two inputs of an OR gate.
2. Connect the output of this OR gate and the sensor input \(W\) to the two inputs of an AND gate.
3. The output of this AND gate produces the final signal for output \(A\).

**(c) Pseudocode Implementation**
```
loop while true
sensorData = readSensors()
T = sensorData[0]
H = sensorData[1]
W = sensorData[2]

// Control Sprinkler System (S)
if T == 1 and W == 0 then
turnSprinkler(true)
else
turnSprinkler(false)
end if

// Control Emergency Alarm (A)
if W == 1 and (T == 1 or H == 1) then
triggerAlarm(true)
else
triggerAlarm(false)
end if

sleep(10)
end loop
```

**(a) [4 marks]**
- **1 mark** for correctly and systematically listing all 8 possible input combinations of \(T\), \(H\), and \(W\).
- **1.5 marks** for the correct output column for \(S\) (1 when \(T=1, W=0\); 0 otherwise). Deduct 0.5 marks per error up to 1.5.
- **1.5 marks** for the correct output column for \(A\) (1 when \(W=1\) and either \(T=1\) or \(H=1\); 0 otherwise). Deduct 0.5 marks per error up to 1.5.

**(b) (i) [2 marks]**
- **1 mark** for correct Boolean expression for \(S\).
- **1 mark** for correct Boolean expression for \(A\).

**(b) (ii) [2 marks]**
- **1 mark** for identifying the correct first stage: Passing \(T\) and \(H\) through an OR gate.
- **1 mark** for identifying the correct second stage: Passing the OR gate output and \(W\) through an AND gate to get \(A\).

**(c) [7 marks]**
- **1 mark** for constructing an infinite loop structure (e.g., `loop while true` or `loop forever`).
- **1 mark** for invoking `readSensors()` correctly and storing the returned array.
- **1 mark** for extracting values using correct indices (`sensorData[0]` for \(T\), `sensorData[1]` for \(H\), `sensorData[2]` for \(W\)).
- **1 mark** for correct evaluation of sprinkler condition and executing `turnSprinkler(true)` and `turnSprinkler(false)` based on outcomes.
- **2 marks** for correct evaluation of alarm condition and executing `triggerAlarm(true)` and `triggerAlarm(false)` based on outcomes (1 mark for logical structure, 1 mark for correct boolean operators/nesting).
- **1 mark** for including `sleep(10)` at the end of the loop body to ensure correct execution timing.

(a)
- Size: An array has a fixed size defined at the time of creation, whereas a collection is dynamic and can grow or shrink as elements are added or removed.
- Access: Array elements can be accessed directly using index notation (e.g. bookArray[i]), whereas collection elements must be accessed sequentially using an iterator (e.g. getNext()).
- Memory: Arrays occupy a contiguous block of memory, whereas collection elements may be stored in non-contiguous memory locations connected via pointers.

(b)
libraryBooks.resetNext()
maxBook = libraryBooks.getNext()
maxCount = maxBook.getBorrowCount()
while libraryBooks.hasNext()
currentBook = libraryBooks.getNext()
if currentBook.getBorrowCount() > maxCount then
maxBook = currentBook
maxCount = currentBook.getBorrowCount()
end if
end while
output maxBook.getTitle()

(c)(i)
libraryBooks.resetNext()
index = 0
while libraryBooks.hasNext()
bookArray[index] = libraryBooks.getNext()
index = index + 1
end while

(c)(ii)
loop i from 0 to numBooks - 2
loop j from 0 to numBooks - i - 2
if bookArray[j].getBorrowCount() < bookArray[j+1].getBorrowCount() then
temp = bookArray[j]
bookArray[j] = bookArray[j+1]
bookArray[j+1] = temp
end if
end loop
end loop

(a) Award 1 mark for each valid difference up to 3 marks.
- Array is static / has fixed size, while Collection is dynamic / size can change.
- Array elements are accessed directly via index (random access), while Collection elements are accessed sequentially via pointer/iterator.
- Array stores elements contiguously in memory, while Collection uses linked/non-contiguous structures.

(b) Award 4 marks as follows:
- 1 mark: Correctly resetting the collection and initializing tracking variables using the first element.
- 1 mark: Correctly implementing a loop using .hasNext().
- 1 mark: Correctly retrieving the next element using .getNext() and comparing its borrow count with the maximum.
- 1 mark: Correctly updating the maximum values and outputting the title of the book after the loop.

(c)(i) Award 3 marks as follows:
- 1 mark: Correctly resetting the collection and initializing index counter to 0.
- 1 mark: Correctly implementing the loop with .hasNext().
- 1 mark: Correctly assigning the book to bookArray at the index and incrementing the index.

(c)(ii) Award 5 marks as follows:
- 1 mark: Correctly implementing nested loops to traverse the array.
- 1 mark: Correctly setting inner loop boundaries to avoid index out of bounds (e.g., up to numBooks - i - 2).
- 1 mark: Correctly accessing and comparing the borrow count of adjacent elements using '<' (for descending order).
- 1 mark: Correctly swapping the elements using a temporary variable.
- 1 mark: Correctly indexing the array elements throughout the algorithm.

(a)
Inheritance (or "is-a" relationship, where CardioWorkout is a subclass/child class of Workout).

(b)
Inheritance allows code reuse. Instead of re-declaring and re-implementing common attributes (such as workoutID, duration, and baseMetabolicRate) and their associated getters/setters in CardioWorkout, the subclass automatically inherits them from Workout. This reduces redundancy and simplifies future software maintenance.

(c)
UML Class Diagram for CardioWorkout:
-------------------------------------------------------
| CardioWorkout |
-------------------------------------------------------
| - avgHeartRate : int |
-------------------------------------------------------
| + CardioWorkout(workoutID: String, duration: int, |
| baseMetabolicRate: double, avgHeartRate: int) |
| + calculateCalories() : double |
-------------------------------------------------------

(d)
Java code for Workout constructor:
```java
public Workout(String workoutID, int duration, double baseMetabolicRate) {
this.workoutID = workoutID;
this.duration = duration;
this.baseMetabolicRate = baseMetabolicRate;
}
```

(e)
Java code for CardioWorkout constructor:
```java
public CardioWorkout(String workoutID, int duration, double baseMetabolicRate, int avgHeartRate) {
super(workoutID, duration, baseMetabolicRate);
this.avgHeartRate = avgHeartRate;
}
```

Part (a) [1 mark]
- Award 1 mark for stating "Inheritance" or "is-a relationship".

Part (b) [2 marks]
- Award 1 mark for identifying a valid advantage (e.g., code reuse, reduced redundancy, easier maintenance).
- Award 1 mark for linking the advantage specifically to the scenario (e.g., inheriting workoutID/duration/baseMetabolicRate from Workout).

Part (c) [4 marks]
- Award 1 mark for the correct class name CardioWorkout and the attribute "- avgHeartRate : int" (must use '-' for private and correct colon-type notation).
- Award 1 mark for the constructor shown with '+' public visibility, all 4 parameters, and correct types.
- Award 1 mark for the overridden method "+ calculateCalories() : double" with '+' visibility and correct return type.
- Award 1 mark for applying correct UML syntax throughout (no Java syntax for declarations, correct use of ':' for types).

Part (d) [3 marks]
- Award 1 mark for the correct constructor signature: public Workout(String workoutID, int duration, double baseMetabolicRate).
- Award 1 mark for using 'this' keyword (or alternative distinct naming parameters) to resolve shadowing/ambiguity.
- Award 1 mark for assigning all three local parameters to their corresponding instance fields.

Part (e) [4 marks]
- Award 1 mark for the correct constructor signature with all 4 arguments: public CardioWorkout(String workoutID, int duration, double baseMetabolicRate, int avgHeartRate).
- Award 1 mark for using the correct "super(...)" keyword calling the superclass constructor.
- Award 1 mark for passing the correct parameters (workoutID, duration, baseMetabolicRate) to "super()" as the very first line of the constructor.
- Award 1 mark for correctly assigning "this.avgHeartRate = avgHeartRate;".

(a) Benefits of Modularity in development:
- **Division of Labor / Parallel Development**: Different programmers or development teams can work on different modules simultaneously. For example, one team can develop the complex path-finding engine while another designs the user interface module. This significantly speeds up the development process.
- **Reusability**: Once a module is developed and tested, it can be reused in other parts of the application or future projects. For instance, the external weather API connector module can be reused in a different meteorological app without rewriting the communication code.
- **Easier Debugging and Maintenance**: Since the system is divided into self-contained units, errors can be isolated quickly. If the system fails to display weather updates, developers can focus troubleshooting exclusively on the weather API module without searching through the entire codebase.

(b) OOP Relationships and Differences:
- **Route and GPSCoordinate**: This is an **Aggregation** (or Composition) relationship. It represents a "has-a" association where a Route acts as a container/whole and GPSCoordinates act as the components/parts. This relationship is structural and persistent; the Route maintains long-term references to its coordinates.
- **Vehicle and TrafficSensor**: This is a **Dependency** relationship. It represents a "uses-a" association where the Vehicle temporarily relies on the TrafficSensor to execute a specific task (e.g., retrieving an update passing through a method parameter), but does not own or maintain a long-term reference to it.
- **Difference**: Aggregation is a stronger, structural relationship where one object structurally contains others as part of its state. Dependency is a weaker, transient association where one class merely utilizes the services of another temporarily, resulting in lower coupling between the two classes.

Part (a) [4 marks]
- Award [1 mark] for identifying a valid benefit of modularity (up to 2 benefits).
- Award [1 mark] for explaining each benefit in the context of the EcoRoute application (up to 2 benefits).
- *Example benefits*: Division of labor, reusability of modules, ease of debugging/maintenance, encapsulation of changes.

Part (b) [4 marks]
- Award [1 mark] for correctly identifying the Route-GPSCoordinate relationship as Aggregation (or Composition/"has-a").
- Award [1 mark] for describing how Route contains GPSCoordinate structurally.
- Award [1 mark] for correctly identifying the Vehicle-TrafficSensor relationship as Dependency (or "uses-a").
- Award [1 mark] for explaining the difference (e.g., aggregation is persistent/structural containment, while dependency is transient/use-only, leading to looser coupling).

(a)
The purpose of a constructor is to initialize the instance variables (state) of an object when it is created/instantiated. It is automatically called when the new operator is used.

(b)
public Participant(String name, String category, int runningTime) {
this.name = name;
this.category = category;
this.runningTime = runningTime;
}

(c)
public Participant getGoldMedalist(String targetCategory) {
Participant goldMedalist = null;
int bestTime = Integer.MAX_VALUE;

for (int i = 0; i < count; i++) {
if (runners[i] != null) {
if (runners[i].getCategory().equals(targetCategory)) {
if (runners[i].getRunningTime() < bestTime) {
bestTime = runners[i].getRunningTime();
goldMedalist = runners[i];
}
}
}
}
return goldMedalist;
}

(a) [2 marks]
Award 1 mark for stating that it initializes instance variables/state.
Award 1 mark for stating that it runs automatically during instantiation / when using the new keyword.

(b) [3 marks]
Award 1 mark for correct method header (matches class name exactly, includes matching arguments: String, String, int).
Award 1 mark for correct assignments (e.g., this.name = name; or equivalent).
Award 1 mark for correct overall Java syntax (no return type specified).

(c) [10 marks]
Award 1 mark for correct method header: public Participant getGoldMedalist(String targetCategory).
Award 1 mark for declaring and initializing a tracker variable for the minimum/best time (e.g., to a very high value like Integer.MAX_VALUE).
Award 1 mark for declaring and initializing a Participant reference to null (to track the gold medalist).
Award 1 mark for writing a loop that correctly iterates up to count elements.
Award 1 mark for checking that the current array element runners[i] is not null to prevent NullPointerException.
Award 1 mark for calling getCategory() correctly on the runner object.
Award 1 mark for using .equals() to compare String category values (do not accept == for string comparison).
Award 1 mark for comparing the current runner's time (retrieved via getRunningTime()) to the current best time.
Award 1 mark for updating the tracker variables (both the minimum time and the Participant reference) inside the conditional block.
Award 1 mark for returning the tracked Participant reference at the end of the method.

**Part (a)**
- **Aggregation ('has-a')**: This is a specialized, association-based relationship where one class is a container of another. Here, a `Patient` object has a `MedicalRecord` object. They have a lifetime relationship, but the `MedicalRecord` can exist independently of the `Patient` (e.g., if the patient leaves the clinic, the record remains in the archive).
- **Dependency ('uses')**: This is a weaker, temporary relationship where a change in one class (`DatabaseConnector`) can affect another class (`Patient`). The `Patient` class does not maintain a persistent instance of `DatabaseConnector` as an attribute; rather, it temporarily 'uses' it (for instance, invoking a static method or instantiating it locally inside a `save()` method) to perform an operation.

**Part (b)**
- **Ethical/Security Concern**: Patient health data is highly sensitive. Transmitting unencrypted data directly creates a vulnerability to packet sniffing or unauthorized data interception, which violates the ethical principle of patient-doctor confidentiality and legal frameworks (e.g., HIPAA/GDPR).
- **Design Modification**: Implement the **Principle of Least Privilege** or **Encapsulation** by introducing an intermediary security wrapper or helper class (e.g., `SecureDataEncryptor`). The `Patient` class passes data to `SecureDataEncryptor` first, which outputs ciphertext. The `DatabaseConnector` is then only allowed to receive and transmit this encrypted data, preventing raw medical information from being exposed within the communication subsystem.

**Part (a) [4 marks]**
- **[1 mark]** For defining/describing **Aggregation** as a 'has-a' / whole-part relationship.
- **[1 mark]** For explaining that aggregated objects have independent lifetimes (e.g., `MedicalRecord` can exist without `Patient`).
- **[1 mark]** For defining/describing **Dependency** as a 'uses' relationship where one class depends on the definition of another.
- **[1 mark]** For explaining the temporary nature of dependency in code (e.g., `DatabaseConnector` is only used inside a method scope/parameter, not stored as an instance field).

**Part (b) [4 marks]**
- **[1 mark]** For identifying a valid ethical concern (e.g., violation of patient confidentiality, breach of data protection laws).
- **[1 mark]** For explaining *how* the direct transmission of unencrypted data by `Patient` leads to this concern (e.g., interception by third parties, exposure of raw medical diagnostics).
- **[1 mark]** For proposing a valid OO design change (e.g., adding an intermediary encryption class, wrapper class, or using private encapsulation access modifiers).
- **[1 mark]** For explaining how this change resolves the issue (e.g., ensures `DatabaseConnector` only handles encrypted ciphertext, isolating sensitive data exposure).