📚 Edexcel A-Level Programming: Core Coding Techniques and Exam Skills | 爱德思 A-Level 编程：核心编码技巧与考试技能
In Edexcel A-Level Computer Science, programming is not just about typing code; it is about designing precise, efficient solutions to computational problems. This revision guide covers the core programming techniques you will need for Paper 1 and Paper 2, including data structures, algorithms, recursion, object-oriented principles, and exam strategies.
在爱德思 A-Level 计算机科学中，编程不仅仅是输入代码，更是为计算问题设计精确、高效的解决方案。本复习指南涵盖 Paper 1 和 Paper 2 所需的核心编程技术，包括数据结构、算法、递归、面向对象原则以及考试策略。
1. Computational Thinking and Problem Decomposition | 计算思维与问题分解
Programming in Edexcel A-Level Computer Science begins with computational thinking: decompose a problem into smaller parts, recognise patterns, abstract away irrelevant detail, and design an algorithm before writing code.
爱德思 A-Level 计算机科学中的编程始于计算思维：先将问题分解成更小的部分，识别模式，抽象掉无关细节，再在写代码之前设计算法。
A good algorithm must be clear, finite, and precise. It should also be represented using pseudocode, flowcharts, or structured English so that examiners can follow your logic even if syntax is imperfect.
好的算法必须清晰、有限且精确。它还应使用伪代码、流程图或结构化英语表示，这样即使语法不完美，考官也能理解你的逻辑。
2. Variables, Constants and Data Types | 变量、常量与数据类型
Variables store values that can change while a program runs, whereas constants hold fixed values. Every variable has a data type such as integer, real, Boolean, character, or string, and the choice of type affects memory use and the operations available.
变量存储程序运行时可能变化的值，常量则保存固定值。每个变量都有数据类型，如整数、实数、布尔、字符或字符串，选择类型会影响内存使用和可执行的操作。
Scope is also important: local variables exist only inside a function or procedure, while global variables can be accessed anywhere. Overusing global variables can make a program harder to debug and maintain.
作用域也很重要：局部变量只存在于函数或过程内部，全局变量则可以在任何地方访问。过度使用全局变量会使程序更难调试和维护。
3. Control Structures: Sequence, Selection, Iteration | 控制结构：顺序、选择与迭代
Sequence means instructions run one after another. Selection uses if, else if, and switch/case to make decisions. Iteration repeats instructions using count-controlled loops such as FOR or condition-controlled loops such as WHILE.
顺序意味着指令一条接一条执行。选择使用 if、else if 和 switch/case 来进行判断。迭代使用计数控制循环（如 FOR）或条件控制循环（如 WHILE）重复执行指令。
Choosing the right loop matters: use a FOR loop when the number of repetitions is known in advance, and a WHILE loop when repetition depends on a condition that may change during execution.
选择合适的循环很重要：当重复次数事先已知时使用 FOR 循环；当重复取决于执行过程中可能变化的条件时使用 WHILE 循环。
4. Functions, Procedures and Parameters | 函数、过程与参数传递
Functions and procedures are named blocks of code that promote modularity and reusability. A function returns a value, while a procedure does not, though both can accept parameters.
函数和过程是命名代码块，能提高模块化和可重用性。函数返回一个值，过程不返回值，但两者都可以接受参数。
Parameters can be passed by value or by reference. Pass by value copies the data, so changes inside the subroutine do not affect the original variable. Pass by reference shares the memory location, allowing the subroutine to modify the caller’s variable.
参数可以按值传递或按引用传递。按值传递会复制数据，因此子程序内部的更改不会影响原始变量；按引用传递共享内存位置，子程序可以修改调用者的变量。
5. Recursion and the Call Stack | 递归与调用栈
Recursion is a technique where a function calls itself to solve a smaller version of the same problem. A correct recursive algorithm must have a base case to stop the recursion and a recursive case that moves towards the base case.
递归是一种函数调用自身来解决同一问题更小版本的技术。正确的递归算法必须有停止递归的基准情形，以及向基准情形靠近的递归情形。
For example, the factorial of n can be defined recursively as:
例如，n 的阶乘可以递归定义为：
n! = n × (n − 1)! for n > 1, and 1! = 1
Each recursive call is placed on the call stack. If the base case is missing or unreachable, the stack can overflow, causing a runtime error.
每次递归调用都会放入调用栈。如果缺少基准情形或基准情形不可达，栈可能会溢出，导致运行时错误。
Recursion produces elegant solutions for tree traversal, backtracking, and divide-and-conquer algorithms, but it can be less memory-efficient than iteration.
递归可以为树遍历、回溯和分治算法生成优雅的解决方案，但它可能比迭代占用更多内存。
6. Arrays, Lists and Records | 数组、列表与记录
Arrays store multiple values of the same data type under one name and use an index to access each element. Lists are similar but can often grow and shrink dynamically. Records store fields of different data types about one entity.
数组在一个名称下存储多个相同数据类型的值，并使用索引访问每个元素。列表类似，但通常可以动态增减。记录存储一个实体的不同数据类型字段。
A 2D array can model a grid or matrix, such as a game board or a spreadsheet. Accessing an element requires two indices: array[row][column].
二维数组可以模拟网格或矩阵，例如游戏棋盘或电子表格。访问元素需要两个索引：array[行][列]。
Knowing how to traverse arrays with loops, insert and delete elements, and search for a value is essential for Paper 2 algorithmic questions.
了解如何用循环遍历数组、插入和删除元素以及搜索某个值，是 Paper 2 算法题的关键。
7. Sorting and Searching Algorithms | 排序与查找算法
Sorting algorithms arrange data in ascending or descending order. Bubble sort repeatedly compares adjacent items and swaps them if they are in the wrong order. Insertion sort builds a sorted portion by inserting each new item into its correct place.
排序算法将数据按升序或降序排列。冒泡排序反复比较相邻项并在顺序错误时交换。插入排序通过将每个新项插入正确位置来构建已排序部分。
Merge sort uses divide and conquer: it splits the list in half, sorts each half recursively, and then merges the two sorted halves. This gives a worst-case time complexity of O(n log₂ n), whereas bubble and insertion sorts are O(n²) in the worst case.
归并排序使用分治法：将列表一分为二，递归地对每一半排序，然后合并两个有序半部分。它的最坏时间复杂度为 O(n log₂ n)，而冒泡和插入排序的最坏情况是 O(n²)。
| Algorithm | Best case | Worst case | Typical use |
|---|---|---|---|
| Bubble sort | O(n) | O(n²) | Simple, small data sets |
| Insertion sort | O(n) | O(n²) | Nearly sorted data |
| Merge sort | O(n log₂ n) | O(n log₂ n) | Large, stable sorting |
Searching can be linear, checking every item one by one, or binary, which works on sorted arrays by repeatedly halving the search interval. Binary search has O(log₂ n) time complexity.
查找可以是线性，逐个检查每个项；也可以是二分，在有序数组上重复将搜索区间减半。二分查找的时间复杂度为 O(log₂ n)。
8. Object-Oriented Programming Basics | 面向对象编程基础
Object-oriented programming (OOP) organises code around classes and objects. A class is a blueprint that defines attributes and methods; an object is an instance of a class.
面向对象编程围绕类和对象组织代码。类是定义属性和方法的蓝图；对象是类的实例。
Encapsulation hides internal state and requires access through methods. Inheritance allows a child class to reuse and extend a parent class. Polymorphism lets different classes respond to the same method name in their own way.
封装隐藏内部状态并要求通过方法访问。继承允许子类重用和扩展父类。多态让不同类以自己的方式响应同一个方法名。
Constructors initialise new objects, and methods such as getters and setters provide controlled access to private attributes.
构造函数初始化新对象，getter 和 setter
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