📚 Case Study Practical Drill | 案例分析实战演练
Welcome to this Year 8 Cambridge Computer Science case study practical drill. In this session, we will apply computational thinking and programming basics to a realistic problem: designing a simple snack vending machine. You will learn how to break down a problem, create algorithms, use flowcharts and pseudocode, handle data, and evaluate a solution. Each section builds your skills in analysis, design, and testing, just like a real computer scientist.
欢迎参加本次 Year 8 剑桥计算机科学案例分析实战演练。我们将把计算思维和编程基础运用到一个真实场景中：设计一台简单的零食自动贩卖机。你将学会如何分解问题、创建算法、使用流程图和伪代码、处理数据以及评估解决方案。每个环节都在增强你的分析、设计和测试能力，就像真正的计算机科学家一样。
1. Understanding the Problem | 理解问题
Before coding, we must clearly define the problem. Our vending machine sells three items: a pack of crisps (50p), a bottle of juice (80p), and a chocolate bar (65p). A customer selects one item, inserts coins, and expects to receive the correct item and change if they overpay.
在编写代码之前，我们必须清晰地定义问题。我们的自动贩卖机出售三种商品：一包薯片（50便士）、一瓶果汁（80便士）和一块巧克力（65便士）。顾客选择一种商品，投入硬币，如果多付了钱就期待收到正确的商品和找零。
We ask questions: Which coins can the machine accept? What happens if the customer does not insert enough money? What output messages should the machine display? Clarifying these points prevents confusion later.
我们会提出问题：机器能接受哪些硬币？如果顾客投入的金额不够会怎样？机器应该显示什么输出信息？理清这些要点可以避免后续出现混乱。
2. Defining Requirements | 明确需求
Based on the problem, we list the functional requirements. The machine must: let the user press a button for item 1, 2, or 3; accept 10p, 20p, and 50p coins; calculate the total inserted; compare it to the item price; deliver the item and return change if the total is sufficient; and display ‘Insufficient funds’ if not enough money is inserted.
根据问题，我们列出功能性需求。机器必须：让用户按下商品1、2或3的按钮；接受10p、20p和50p硬币；计算投入的总金额；将其与商品价格比较；如果总金额足够，交付商品并退还找零；如果金额不足，显示“余额不足”。
We also note the non-functional requirements: the system should be easy to use, display clear messages, and handle coin inputs correctly without crashing.
我们还会注意到非功能性需求：系统应易于使用，显示清晰的信息，并能正确处理硬币输入而不崩溃。
3. Decomposing the Task | 任务分解
Computational thinking starts with decomposition. We break the vending machine system into smaller, manageable parts: input (user selection and coin insertion), processing (price lookup, total calculation, comparison), output (dispense item, return change, display messages), and storage (item prices and inventory counts).
计算思维从分解开始。我们将自动贩卖机系统拆分成更小、更易处理的部分：输入（用户选择和硬币投入）、处理（价格查找、总额计算、比较）、输出（交付商品、退还找零、显示信息）和存储（商品价格和库存数量）。
This modular view helps us focus on one piece at a time. For instance, we can design the coin counting logic separately from the change calculation algorithm.
这种模块化的视角帮我们一次专注于一个部分。例如，我们可以把硬币计数逻辑与找零计算算法分开设计。
4. Designing the Algorithm | 算法设计
An algorithm is a step-by-step solution. We design a high-level algorithm for the machine: 1. Display item menu and prices. 2. Wait for item selection (1, 2, or 3). 3. Prompt user to insert coins. 4. Keep accepting valid coins until user presses ‘done’. 5. Sum the inserted coins. 6. Retrieve the price of the chosen item. 7. If total >= price, dispense item and give change (total – price). Else, display ‘Insufficient funds’ and return coins.
算法是分步的解决方案。我们为机器设计一个高层算法：1. 显示商品菜单和价格。2. 等待商品选择（1、2或3）。3. 提示用户投入硬币。4. 持续接受有效硬币直到用户按“完成”。5. 累加投入的硬币。6. 获取所选商品的价格。7. 如果总额≥价格，交付商品并找零（总额-价格）；否则显示“余额不足”并退还硬币。
Notice how the algorithm uses sequence, selection (if condition) and iteration (repeat coin input). These are the three building blocks of programming.
注意算法如何使用顺序、选择（if条件）和迭代（重复硬币输入）。这是编程的三大基本结构。
5. Flowchart Representation | 流程图表示
Flowcharts give a visual map of the algorithm. We draw standard symbols: ovals for Start/End, rectangles for processes, diamonds for decisions, and parallelograms for input/output. The flowchart starts with ‘Start’, then ‘Display menu’, ‘Input item choice’, ‘Input coin’ in a loop, ‘Sum total’, ‘Is total >= price?’, and paths for Yes (give item and change) and No (error message, return coins).
流程图提供了算法的可视化地图。我们使用标准符号：椭圆形表示开始/结束，矩形表示处理，菱形表示决策，平行四边形表示输入/输出。流程图从“开始”开始，然后“显示菜单”，“输入商品选择”，在一个循环中“输入硬币”，“累加总额”，“总额≥价格？”，并为是（交付商品和找零）和否（错误信息，退还硬币）设计路径。
Imagine the decision diamond: if the condition is true, follow the left branch; if false, follow the right. Drawing a clear flowchart makes it easier to spot logical errors before writing code.
想象决策菱形：如果条件为真，沿着左分支；如果为假，沿着右分支。绘制清晰的流程图有助于在编写代码之前发现逻辑错误。
[Insert flowchart diagram in your notebook]
6. Pseudocode Writing | 伪代码编写
Pseudocode describes the algorithm in plain-language statements that resemble code. Here is the pseudocode for our vending machine:
伪代码用类似代码的简明语言描述算法。以下是自动贩卖机的伪代码：
ITEM1_PRICE = 50
ITEM2_PRICE = 80
ITEM3_PRICE = 65
OUTPUT “Select item (1,2,3)”
INPUT choice
total = 0
REPEAT
OUTPUT “Insert coin (10,20,50) or type 0 to finish”
INPUT coin
IF coin is valid THEN total = total + coin
UNTIL coin = 0
IF choice = 1 THEN price = ITEM1_PRICE
ELSE IF choice = 2 THEN price = ITEM2_PRICE
ELSE price = ITEM3_PRICE
IF total >= price THEN
change = total – price
OUTPUT “Dispensing item. Your change: “, change
ELSE
OUTPUT “Insufficient funds. Returning coins.”
This pseudocode uses constants, variables, loops, and conditional statements, just like a real program.
这个伪代码使用了常量、变量、循环和条件语句，就像真正的程序一样。
7. Variable and Data Types | 变量与数据类型
When we implement the solution, we need variables to store data. We identify: ‘choice’ (integer), ‘coin’ (integer), ‘total’ (integer), ‘price’ (integer), ‘change’ (integer). All hold whole numbers, so we use the integer data type.
当实现解决方案时，我们需要变量来存储数据。我们确定：“choice”（整数）、“coin”（整数）、“total”（整数）、“price”（整数）、“change”（整数）。它们都保存整数，因此使用整数数据类型。
We also might use a string variable for output messages like “Insufficient funds”. Choosing the right data type saves memory and prevents errors.
我们还可能使用字符串变量来存放诸如“余额不足”的输出信息。选择正确的数据类型可以节省内存并防止错误。
We can summarise the core variables in a table:
我们可以用一个表格总结核心变量：
| Variable | Data Type | Purpose |
|---|---|---|
| choice | Integer | Selected item number |
| coin | Integer | Value of inserted coin |
| total | Integer | Sum of coins inserted |
| price | Integer | Price of chosen item |
| change | Integer | Amount to return |
8. Testing with Sample Data | 示例数据测试
Before trusting the algorithm, we test it with sample data. We create a test table that lists inputs, expected outputs, and actual outputs. This is part of verification.
在信任算法之前，我们用示例数据来测试它。我们创建一个测试表格，列出输入、预期输出和实际输出。这是验证的一部分。
Example test cases:
示例测试用例：
| Test | Choice | Coins inserted | Total | Expected output |
|---|---|---|---|---|
| 1 | 1 (Crisps) | 20p, 20p, 10p | 50p | Item dispensed, change 0p |
| 2 | 2 (Juice) | 50p, 50p | 100p | Item dispensed, change 20p |
| 3 | 3 (Chocolate) | 20p, 20p | 40p | Insufficient funds, coins returned |
Running these tests mentally or in a simple Python simulation confirms that the logic works correctly.
在头脑中或用一个简单的Python模拟运行这些测试，可以确认逻辑工作正常。
9. Binary and Logic Connection | 二进制与逻辑连接
Year 8 Cambridge Computer Science often integrates binary numbers. Suppose we give each item a 4-bit binary code to represent its selection. This mimics how a real machine might use switches or a keypad encoder.
Year 8 剑桥计算机科学常常融入二进制数。假设我们给每种商品分配一个4位二进制代码来表示它的选择。这模拟了真实机器如何使用开关或键盘编码器。
| Item | Binary Code | Decimal |
|---|---|---|
| Crisps | 0001 | 1 |
| Juice | 0010 | 2 |
| Chocolate | 0011 | 3 |
We also encounter logic when checking coin validity. A coin is accepted only if its value is 10 OR 20 OR 50. This uses a logical OR combination. If we had a sensor for coin diameter and weight, we could link binary patterns to physical signals, but our simplified model uses numeric values.
在检查硬币有效性时，我们也会遇到逻辑。只有当硬币是10或20或50时才会被接受。这使用了逻辑或的组合。如果我们有硬币直径和重量的传感器，就可以把二进制模式与物理信号联系起来，但我们的简化模型使用数值。
Binary number conversion practice: if the machine stores change in binary registers, change of 20p is 10100₂ because 16+4=20. This reinforces the connection between theory and hardware.
二进制数转换练习：如果机器用二进制寄存器存储找零，20便士的找零就是10100₂，因为16+4=20。这强化了理论与硬件之间的联系。
10. Evaluation and Improvements | 评估与改进
Every design can be improved. We evaluate the vending machine solution: it works for three items but adding new items requires editing the code. A better approach would use a data structure like a list or array to store item names and prices, making it scalable.
任何设计都可以改进。我们评估贩卖机解决方案：它对三种商品有效，但添加新商品需要修改代码。更好的方法是使用列表或数组等数据结构来存储商品名称和价格，使其具有可扩展性。
We also notice that the machine does not check stock levels. Adding a stock variable for each item would prevent selling out-of-stock products. Furthermore, the user interface could be improved with a display screen and a card payment option.
我们还注意到机器没有检查库存水平。为每种商品增加一个库存变量可以防止出售缺货产品。此外，用户界面可以通过显示屏和刷卡支付选项来改进。
This reflection cycle — analyse, design, test, evaluate — is the essence of computational thinking. Practising with case studies builds your confidence for exams and real-world problem solving.
这种“分析、设计、测试、评估”的反思循环是计算思维的精髓。通过案例学习进行练习，可以增强你面对考试和真实世界问题解决的信心。
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