📚 Program Development Life Cycle | 程序开发生命周期
The Program Development Life Cycle (PDLC) is a structured, step-by-step process used by programmers to create high-quality software that meets the needs of users. It is sometimes called the Software Development Life Cycle (SDLC). For the CIE IGCSE Computer Science syllabus, you need to know six main stages: analysis, design, coding, testing, documentation and maintenance.
程序开发生命周期（PDLC）是程序员用来创建满足用户需求的高质量软件的逐步结构化过程，有时也被称为软件开发生命周期（SDLC）。在CIE IGCSE计算机科学大纲中，你需要掌握六个主要阶段：分析、设计、编码、测试、文档和维护。
The PDLC is not a one-way process. After testing and maintenance, problems may be found that require the developer to go back to an earlier stage. This idea of revisiting earlier steps is very important in exams — it is known as iteration or the cyclic nature of the PDLC.
PDLC并非单向过程。在测试和维护之后，如果发现问题，开发者可能需要回到更早的阶段。这种回顾早期步骤的思想在考试中非常重要——它被称为迭代或PDLC的循环特性。
1. What is the Program Development Life Cycle? | 什么是程序开发生命周期
Analysis is the first stage of the life cycle. The developer meets the client and users to find out exactly what the program must do. The main activities in this stage include identifying the problem, gathering requirements, considering the feasibility of the solution and producing a requirements specification document.
分析是生命周期的第一个阶段。开发者与客户和用户会面，以确切了解程序必须做什么。这一阶段的主要活动包括：识别问题、收集需求、考虑解决方案的可行性，以及编写需求规格说明文档。
- Interviews and questionnaires are used to collect information from users about what they expect the program to do.
- 访谈和问卷用于从用户那里收集关于他们期望程序做什么的信息。
- Observation of the current system helps developers understand the existing workflow before they design a new system.
- 观察现有系统有助于开发者在设计新系统之前理解现有的工作流程。
- The requirements specification describes the inputs, outputs and processing that the final program must perform.
- 需求规格说明描述最终程序必须完成的输入、输出和处理。
The output of this stage is a clear statement of what the program should do, not how it should do it. In exam questions, if you are asked what happens during analysis, remember the key phrases: 'find out the user requirements' and 'produce a requirements specification'.
这一阶段的输出是明确说明程序应该做什么，而不是如何做。在考试题中，如果被问到分析阶段发生了什么，请记住关键短语：“查明用户需求”和“编写需求规格说明”。
2. Stage 1: Analysis | 阶段一：分析
During the design stage, the developer plans how the program will solve the problem. This includes designing the algorithm, the user interface, the data structures and the testing strategy. No code is written at this stage — everything is planned on paper or using design tools.
在设计阶段，开发者规划程序将如何解决问题。这包括设计算法、用户界面、数据结构以及测试策略。此阶段不编写任何代码——一切都在纸上或使用设计工具进行规划。
The main design tools in the CIE syllabus are:
CIE大纲中的主要设计工具有：
| Tool | Description | Example |
| Flowchart（流程图） | A diagram using standard symbols to show the flow of control. | Decision diamond, process rectangle, start/end oval. |
| Pseudocode（伪代码） | An English-like way of writing an algorithm without real syntax. | IF score ≥ 50 THEN print "Pass" |
| Structure diagram（结构图） | Shows the hierarchical breakdown of a system into modules. | Main menu → input, process, output. |
The design stage also produces a test plan. This is a table that lists test numbers, test types (normal, boundary, erroneous), test data, expected results and actual results. A good test plan is essential for the testing stage.
设计阶段还会产出一个测试计划。这是一个表格，列出测试编号、测试类型（正常、边界、错误）、测试数据、预期结果和实际结果。一个良好的测试计划对测试阶段至关重要。
ry data → values at the edge of the range; Erroneous data → values outside the valid range
正常数据 → 在有效范围内的预期值；边界数据 → 位于范围边缘的值；错误数据 → 超出有效范围的值
3. Stage 2: Design | 阶段二：设计
Coding, also called implementation, is the stage where the algorithm from the design stage is translated into an actual programming language such as Python, Java, VB.NET or pseudocode-based code. The programmer writes instructions that the computer can understand and execute.
编码，也称为实现，是将设计阶段产生的算法翻译成实际编程语言（如Python、Java、VB.NET或基于伪代码的代码）的阶段。程序员编写计算机能够理解和执行的指令。
The choice of programming language depends on factors such as the type of application, the hardware platform and the programmer's experience. In exams, you are often asked to write a program, trace through code, or identify errors in given code.
编程语言的选择取决于多个因素，例如应用类型、硬件平台以及程序员经验。在考试中，你经常被要求编写程序、跟踪代码或识别给定代码中的错误。
During coding, programmers must write clean, well-structured code using meaningful variable names, indentation and comments. Good practice makes programs easier to debug, maintain and understand later.
在编码过程中，程序员必须使用有意义的变量名、缩进和注释来编写清晰、结构良好的代码。良好的实践使程序更容易调试、维护和日后理解。
4. Stage 3: Coding | 阶段三：编码
Testing is the process of running a program to find errors. The program is tested against the test plan produced in the design stage. Testing can be done using strategies such as black-box testing and white-box testing, but at IGCSE level you mainly need to know the types of test data and the three types of program errors.
测试是运行程序以发现错误的过程。程序按照设计阶段产生的测试计划进行测试。测试可以采用黑盒测试和白盒测试等策略，但在IGCSE阶段，你主要需要掌握测试数据类型和三类程序错误。
The three error types you must be able to distinguish are:
你必须能够区分的三类错误是：
- Syntax errors occur when the rules of the programming language are broken, e.g. misspelling a keyword or missing a colon. They are usually detected by the compiler or interpreter.
- 语法错误发生在违反编程语言规则时，例如拼错关键字或缺少冒号。它们通常由编译器或解释器检测出来。
- Logic errors occur when the program runs but produces the wrong output, e.g. using a minus sign instead of a plus sign. The program will not stop, but the result is incorrect.
- 逻辑错误发生在程序能运行但产生错误输出时，例如用减号代替加号。程序不会停止，但结果是错误的。
- Runtime errors occur while the program is running, e.g. dividing by zero or trying to open a non-existent file. These often crash the program.
- 运行时错误发生在程序运行期间，例如除以零或试图打开不存在的文件。这些往往会导致程序崩溃。
Trace table: columns for each variable + output; rows record values after each line of code
跟踪表：每个变量和输出对应一列；每一行记录一行代码执行后的值
5. Stage 4: Testing | 阶段四：测试
Documentation is the stage in which written guides are produced for the program. There are two kinds of documentation: technical documentation for programmers who need to maintain the code, and user documentation for the people who will use the finished software.
文档是编写程序配套书面指南的阶段。有两种文档：供需要维护代码的程序
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