Solutions

Question 1

Define the following terms for structure charts and give examples: module, sub-module, and flag.

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• External Entity In DFDs, an external entity represents sources or destinations of data outside the system, such as users or external systems. Example: In a banking system, a Customer is an external entity.

• Data Store A data store in DFDs symbolizes where data is stored within the system, like databases or files. Example: A Customer Database storing user information.

• Process Processes in DFDs depict activities that transform inputs into outputs. Example: Process Payment converts payment details into transaction records.

• Module In structure charts, a module is a distinct unit of code that performs a specific function. Example: A User Authentication module handles login procedures.

• Sub-Module A sub-module is a component of a module that carries out a subset of the module’s functionality. Example: Within the User Authentication module, a Password Verification sub-module checks user passwords.

• Flag A flag is a variable indicating a particular condition or status within the system. Example: An isLoggedIn flag that is set to true after successful user authentication.

Question 2

Use a table to compare DFDs and structure charts.

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Aspect	Data Flow Diagrams (DFDs)	Structure Charts
Purpose	Illustrate how data moves through a system	Show the hierarchical organisation of system modules
Focus	Data processes, flows, and storage	Functional decomposition and module relationships
Representation	Processes, data flows, data stores, external entities	Modules, sub-modules, control structures
Hierarchy Levels	Context diagram (Level 0), Level 1, Level 2, etc.	Top-down decomposition into modules and sub-modules
Use Case	Understanding data processing and storage	Designing program structure and module interactions
Notations Used	Circles/ovals (processes), arrows (data flow), open-ended rectangles (data stores), squares (external entities)	Rectangles (modules), arrows (control flow), diamonds (decision points)

Question 3

Discuss the nested hierarchy of structure charts.

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Both DFDs and structure charts utilize nested hierarchies to represent system details at varying levvels of abstraction.

• DFDs Begin with a high-level Context Diagram (Level 0) showing the system’s interaction with external entities. Subsequent levels (Level 1, Level 2, etc.) break down processes into more detailed subprocesses, providing a deeper understanding of data flows within the system.

• Structure Charts Start with a top-level module representing the overall system function. This module is decomposed into smaller sub-modules, each handling specific tasks. This hierarchical breakdown continues until reaching modules that perform atomic operations, ensuring clarity in system functionality and module interactions.

Questions 4-9 are based on the structure chart below for an email application.

Question 4

List the main modules and submodules shown in the structure chart of the email system.

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Main modules

• Authenticate user

• Manage mailbox

Sub-modules

• enter login details

• verify login details

• view email

• compose email

• send email

Question 5

Describe the process and sequence when users log into their email accounts and view their messages. Include any decision points involved.

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1. Authenticate User The user enters their login credentials, which are processed by the Authenticate User module.

2. Verify Login Details The system verifies the credentials through the Verify Login Details submodule.

3. Decision Point If the credentials are valid, the system proceeds; otherwise, it prompts the user to re-enter their credentials.

4. Retrieve User Data Upon successful authentication, the Retrieve User Data submodule fetches the user’s email data.

5. Display Inbox The Display Inbox submodule presents the list of received emails to the user.

6. Open Message The user can select an email to read, which is handled by the Open Message submodule.

Question 6

How does the verify log in details module interact with the other system components?

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The Verify Login Details submodule interacts with the following components:

• Authenticate User Module Receives user credentials from the Authenticate User module and verifies their accuracy.

• Retrieve User Data Submodule Upon successful verification, it triggers the Retrieve User Data submodule to access the user’s email information.

• Decision Logic If verification fails, it prompts the system to request re-entry of credentials or deny access.Answer

Question 7

Explain the decision-making process represented by the diamond symbol. What choices are available, and how does the system proceed based on the decision made?

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The diamond symbol represents a decision point where the system evaluates the validity of user login credentials:

• Valid Credentials If the credentials are correct, the system proceeds to retrieve and display the user’s inbox.

• Invalid Credentials If the credentials are incorrect, the system would prompt the user to re-enter their login information or denies access after multiple failed attempts.

Question 8

Outline the sequence of events that occurs when a user sends a message. How do the 'compose email' and 'send email' modules interact, and what happens once the message is sent?

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1. Compose Email The user creates a new email using the Compose Email module, which may involve writing the message, adding recipients, and attaching files.

2. Send Email Upon initiating a send, the Send Email module processes the outgoing message, and asks for confirmation.

3. Confirm Send After initiating sending, the system asks the user to confirm they are ready to send the email.

Question 9

How does the system handle login verification? Describe the interaction between the enter login details, verify log in details, and authentication modules, including what happens if the login details are incorrect.

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1. Enter Login Details The user inputs their username and password.

2. Authenticate User The Authenticate User module receives these credentials and passes them to the Verify Login Details submodule.

3. Verify Login Details This submodule checks the credentials against stored user data. If Correct: The system proceeds to retrieve and display the user’s inbox. If Incorrect: The system prompts the user to re-enter their credentials. After multiple failed attempts, access may be temporarily blocked or additional security measures initiated.