📚 The Program Development Life Cycle | 程序开发生命周期
The program development life cycle (PDLC) is a structured sequence of stages that software developers follow to plan, create, test, and maintain a computer program. Understanding each phase helps ensure the final software meets user needs, is free of serious errors, and can be adapted over time. In the Cambridge IGCSE Computer Science syllabus, students are expected to recognise these stages and explain what happens during each one.
程序开发生命周期是软件开发人员规划、创建、测试并维护计算机程序所遵循的一组有序阶段。理解每一个阶段有助于确保最终软件满足用户需求、没有严重错误，并能够长期适应变化。在剑桥 IGCSE 计算机科学课程中，学生需要认识这些阶段并解释每个阶段发生的事情。
1. Understanding the Problem | 理解问题
Before any code is written, the developer must fully understand what the program is supposed to do. This involves talking to the client or end users, reading existing documentation, and observing current workflows. A clear problem description is produced so that everyone agrees on the goals and boundaries of the project.
在编写任何代码之前，开发人员必须充分理解程序要做什么。这涉及与客户或最终用户交谈、阅读现有文档以及观察当前工作流程。需要给出清晰的问题描述，使所有人对项目的目标和范围达成一致。
In many exam scenarios, a problem statement is provided, and the first task is to identify the inputs, processes, and outputs (IPO) required. This helps break down a complex requirement into manageable parts.
在许多考试情境中，会给出一个问题陈述，第一项任务就是确定所需的输入、处理和输出 (IPO)。这有助于将复杂的需求分解为可管理的部分。
2. Feasibility Study | 可行性研究
A feasibility study checks whether the proposed solution is realistic. Developers ask questions about budget, time, available technology, and the skills of the team. If the project is not feasible, it may be cancelled or adjusted at this early stage to save resources.
可行性研究检查所提议的解决方案是否现实。开发人员会询问预算、时间、可用技术和团队技能等问题。如果项目不可行，可以在早期阶段取消或调整，以节省资源。
Technical feasibility is particularly important: can the current hardware and software support the new system? Operational feasibility considers whether the new system will actually be used by staff. Legal feasibility checks for data protection and copyright issues.
技术可行性尤其重要：当前的硬件和软件能否支持新系统？操作可行性考虑员工是否真会使用新系统。法律可行性检查数据保护和版权问题。
3. Requirements Analysis | 需求分析
Once the project is deemed feasible, detailed requirements are gathered. Analysts create a requirements specification document that lists exactly what the system must do, including functional requirements (what the system should perform) and non-functional requirements (constraints like speed, security, and usability).
一旦项目被认为可行，就会收集详细的需求。分析人员会创建一份需求规格说明文档，准确列出系统必须做什么，包括功能需求（系统应执行的操作）和非功能需求（如速度、安全性和可用性等约束）。
Methods for gathering requirements include interviews, questionnaires, observation, and examining existing paperwork. No design or coding happens until all stakeholders agree on the specification, because changing requirements later is expensive.
收集需求的方法包括访谈、问卷调查、观察和检查现有文件。在所有利益相关方就规格说明达成一致之前，不会进行任何设计或编码，因为后期更改需求的成本很高。
4. System Design | 系统设计
In the design phase, the system architecture is planned. The developer decides on the structure of the program, data structures, user interface layouts, and algorithms. Design documents often include structure diagrams, flowcharts, pseudocode, and screen mock-ups.
在设计阶段，会规划系统架构。开发人员决定程序结构、数据结构、用户界面布局和算法。设计文档通常包括结构图、流程图、伪代码和屏幕原型。
Design should be modular, meaning the system is broken down into smaller, independent modules or subroutines. Each module can be designed, coded, and tested separately, which makes the project easier to manage and allows parallel work by different team members.
设计应是模块化的，也就是说系统被分解成更小的、独立的模块或子程序。每个模块可以单独设计、编码和测试，这使项目更易于管理，并允许不同团队成员并行工作。
Input and output designs are also specified. For example, a data entry form must be clear, with validation rules decided at this stage to avoid Garbage In, Garbage Out (GIGO) problems.
输入和输出设计也会被指定。例如，数据输入表单必须清晰，并在此阶段决定验证规则，以避免“垃圾进，垃圾出” (GIGO) 问题。
5. Coding and Implementation | 编码与实现
During coding, the design is translated into a programming language. Developers write source code following the design specifications. It is vital to use meaningful variable names, add comments, and stick to a consistent indentation style so that the code is readable and maintainable.
在编码阶段，设计被翻译成编程语言。开发人员按照设计规范编写源代码。使用有意义的变量名、添加注释并保持一致的缩进风格至关重要，这样代码才可读、可维护。
Each module is written and unit tested individually. Development teams may use version control systems to track changes and collaborate without overwriting each other's work. At IGCSE level, simple integrated development environments (IDEs) are typically used, offering a code editor, compiler, and debugger in one tool.
每个模块单独编写并进行单元测试。开发团队可能会使用版本控制系统来跟踪更改并进行协作，以免覆盖彼此的工作。在 IGCSE 级别，通常使用简单的集成开发环境 (IDE)，它在一个工具中提供代码编辑器、编译器和调试器。
6. Testing | 测试
Testing is not a single activity done at the end; it happens throughout the life cycle, but a dedicated testing phase uses a well-planned test strategy. The test plan includes test data chosen to cover normal, abnormal (erroneous), and boundary cases. Each test is matched to a specific requirement.
测试不是在最后进行的单一活动；它贯穿于整个生命周期，但专门的测试阶段使用事先精心规划的测试策略。测试计划包括用于覆盖正常、异常（错误）和边界情况的测试数据。每个测试都与特定需求相对应。
Types of testing include unit testing (individual modules), integration testing (combined modules), system testing (the whole program), and acceptance testing (checking if the system meets the client's needs). A traceability matrix can be used to map each test to a requirement, ensuring full coverage.
测试类型包括单元测试（单个模块）、集成测试（组合模块）、系统测试（整个程序）和验收测试（检查系统是否满足客户需求）。可追溯矩阵可用于将每项测试映射到需求，确保全面覆盖。
When a test fails, the defect is logged, fixed, and the test is rerun. This process is known as debugging and regression testing, ensuring that changes do not introduce new errors.
当测试失败时，缺陷会被记录、修正，并重新运行测试。这个过程称为调试和回归测试，确保更改不会引入新的错误。
7. Documentation | 文档编制
Documentation is produced for both users and technical staff. User documentation includes installation guides, user manuals, and help files. It should be written in plain language, with screenshots and step-by-step instructions so that non-experts can operate the software.
文档不仅为用户编制，也为技术人员编制。用户文档包括安装指南、用户手册和帮助文件。它应该使用通俗语言编写，并配有屏幕截图和分步说明，以便非专业人员也能操作软件。
Technical documentation is aimed at those who will maintain and upgrade the system. It includes the requirements specification, design diagrams, commented source code, test plans, and a description of known bugs. Good documentation reduces the cost of future changes significantly.
技术文档面向那些将维护和升级系统的人员。它包括需求规格说明、设计图、带注释的源代码、测试计划以及已知错误的说明。良好的文档能显著降低未来变更的成本。
8. Installation and Deployment | 安装与部署
Once the software passes acceptance testing, it is installed in the live environment. There are several conversion strategies: direct changeover (instant switch), parallel running (old and new systems operate together), pilot running (trial in one part of the business), and phased conversion (step-by-step rollout).
一旦软件通过验收测试，就会被安装到生产环境中。有多种转换策略：直接切换（即时切换）、并行运行（新旧系统同时运行）、试点运行（在业务的一部分试运行）和分阶段转换（逐步推出）。
Parallel running is the safest option because the old system serves as a backup, but it requires extra resources. Direct changeover is risky and is only used when the old system cannot be kept running. The choice depends on the criticality of the system and available budget.
并行运行是最安全的选择，因为旧系统充当备份，但需要额外资源。直接切换风险较大，仅在无法保持旧系统运行时使用。选择取决于系统的关键程度和可用预算。
9. Maintenance | 维护
Maintenance is the longest phase in the program development life cycle. It involves corrective maintenance (fixing bugs that escaped testing), adaptive maintenance (modifying the software to work with new hardware or operating systems), and perfective maintenance (adding new features or improving performance).
维护是程序开发生命周期中最长的阶段。它包括纠正性维护（修复未在测试中发现的错误）、适应性维护（修改软件以适应新硬件或操作系统）和完善性维护（添加新功能或改善性能）。
Well-structured, modular code with clear comments is much easier to maintain. Without proper documentation, even small changes can become very time-consuming and introduce unexpected side effects.
结构良好、模块化且注释清晰的代码更容易维护。若没有适当的文档，即使微小的变更也可能非常耗时，并带来意想不到的副作用。
10. Evaluation | 评估
After the system has been in use for some time, an evaluation is carried out to judge whether the original objectives have been met. Developers compare the final product against the requirements specification and collect feedback from users on usability, performance, and reliability.
系统使用一段时间后，会进行评估，判断是否达到了最初的目标。开发人员将最终产品与需求规格说明进行比对，并收集用户关于可用性、性能和可靠性的反馈。
Criteria for a successful system include accuracy of output, speed, ease of use, robustness under extreme conditions, and cost-effectiveness. Any shortcomings may lead to a new iteration of the life cycle, so evaluation can trigger another cycle of analysis, design, and improvement.
成功系统的评判标准包括输出的准确性、速度、易用性、在极端条件下的健壮性以及性价比。任何不足之处都可能导致新一轮生命周期的开始，因此评估可以触发又一个分析、设计和改进的循环。
The program development life cycle is therefore not strictly linear; it is often represented as an iterative or cyclical model, especially in agile development approaches that emphasise continuous feedback and gradual refinement.
因此，程序开发生命周期并非严格线性；它通常被表示为一个迭代或循环模型，尤其是在强调持续反馈和逐步完善的敏捷开发方法中。
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