📚 Mastering Core Programming for Edexcel A-Level Computer Science | 精通 Edexcel A-Level 计算机科学核心编程
Programming is the heart of Edexcel A-Level Computer Science. This revision guide consolidates the essential constructs, data structures, algorithms, and object-oriented principles you need for the exam. Work through each section, practise tracing pseudocode, and aim for precision in every answer.
编程是 Edexcel A-Level 计算机科学的核心。本复习指南整合了你考试所需的基本结构、数据结构、算法和面向对象原则。逐节学习，练习追踪伪代码，并在每道答案中力求准确。
1. Programming Constructs: Sequence, Selection, Iteration | 编程结构：顺序、选择、迭代
At the heart of every procedural program are three control constructs: sequence, selection, and iteration. Sequence executes statements in order; selection uses if-else or switch-case to branch; iteration repeats blocks with for, while, or repeat loops. Edexcel exam questions frequently ask you to convert between pseudocode and flowcharts for these constructs.
每个过程式程序的核心都有三种控制结构：顺序、选择、迭代。顺序按先后次序执行语句；选择使用 if-else 或 switch-case 分支；迭代通过 for、while 或 repeat 循环重复执行代码块。Edexcel 考试题经常要求你在这三种结构的伪代码与流程图之间转换。
When writing pseudocode, make the opening and closing of each construct explicit, for example using ENDIF, ENDFOR, and ENDWHILE. This mirrors Edexcel’s preferred style and helps you avoid missing logic when tracing.
在编写伪代码时，要明确写出每种结构的开始与结束，例如使用 ENDIF、ENDFOR 和 ENDWHILE。这符合 Edexcel 偏好的风格，并帮助你在追踪时不遗漏逻辑。
2. Data Types and Variables | 数据类型与变量
Variables store values that can change during execution. Common primitive data types include integer, real/float, Boolean, character, and string. Edexcel pseudocode often uses type declarations or comments to indicate data types, and questions test type compatibility, casting, and variable scope.
变量存储在执行过程中可以改变的值。常见的原始数据类型包括整数、实数/浮点数、布尔值、字符和字符串。Edexcel 伪代码常使用类型声明或注释标明数据类型，考题会测试类型兼容性、类型转换以及变量作用域。
Local variables exist only inside a subroutine, while global variables are accessible throughout the program. Using global variables can lead to side effects, so Edexcel marks responses higher when you limit their use and pass parameters explicitly.
局部变量仅存在于子程序内部，而全局变量可在整个程序中访问。使用全局变量可能导致副作用，因此当你限制使用全局变量并显式传递参数时，Edexcel 会给分更高。
| Data type | Example |
|---|---|
| Integer | 42, -7 |
| Real/Float | 3.14, -0.5 |
| Boolean | TRUE, FALSE |
| Character | ‘A’, ‘9’ |
| String | “Hello” |
3. Operators and Expressions | 运算符与表达式
Expressions combine operators and operands. Arithmetic operators (+, -, *, /, MOD, DIV), comparison operators (=, <>, <, >, <=, >=), and logical operators (AND, OR, NOT) form the basis of conditions and calculations. Precedence rules determine evaluation order, and parentheses override default precedence.
表达式由运算符和操作数组成。算术运算符（+、-、*、/、MOD、DIV）、比较运算符（=、<>、<、>、<=、>=）和逻辑运算符（AND、OR、NOT）构成条件和计算的基础。优先级规则决定求值顺序，圆括号可以改变默认优先级。
parentheses → arithmetic (MOD, DIV, *, /, +, -) → comparison → logical (NOT, AND, OR)
圆括号 → 算术（MOD、DIV、*、/、+、-）→ 比较 → 逻辑（NOT、AND、OR）
Be careful with integer division: DIV returns the whole-number quotient, while MOD returns the remainder. For example, 17 DIV 5 equals 3, and 17 MOD 5 equals 2. Edexcel often includes these in trace table questions.
注意整数除法：DIV 返回整数商，而 MOD 返回余数。例如，17 DIV 5 等于 3，17 MOD 5 等于 2。Edexcel 经常在追踪表题中包含这些运算。
4. Arrays and Lists | 数组与列表
Arrays store multiple elements of the same type in contiguous memory locations. A one-dimensional array is indexed from 0 or 1 depending on the language; two-dimensional arrays model tables and matrices. Edexcel questions often require tracing algorithms that iterate through arrays to find maximum, minimum, or sum.
数组将多个相同类型的元素存储在连续内存位置。一维数组根据语言从 0 或 1 开始索引；二维数组可用于建模表格和矩阵。Edexcel 题目常要求追踪遍历数组的算法，以查找最大值、最小值或求和。
Lists are dynamic structures that can grow and shrink. Common operations include append, insert, remove, and sort. In Edexcel pseudocode, list indexing is often shown as myList[2], but you must check whether the question uses 0-based or 1-based indexing.
列表是动态结构，可以增删元素。常见操作包括 append、insert、remove 和 sort。在 Edexcel 伪代码中，列表索引通常写作 myList[2]，但你必须检查题目使用的是从 0 开始还是从 1 开始的索引。
- 1D array: a fixed-size container for elements of one data type
- 2D array: a table with rows and columns, accessed as matrix[row][column]
- List: a dynamic collection with add, remove, and sort methods
- 一维数组：一种固定大小的容器，存放单一数据类型的元素
- 二维数组：具有行和列的表格，通过 matrix[row][column] 访问
- 列表：一种动态集合，具有添加、删除和排序方法
5. Functions and Procedures | 函数与过程
Subroutines break programs into manageable blocks. A function returns a value, while a procedure performs an action without returning a value. Parameters can be passed by value or by reference; by-reference allows modifications to affect the original variable. Edexcel expects you to define, call, and trace subroutines accurately in pseudocode.
子程序将程序分解为可管理的模块。函数返回值，而过程执行动作但不返回值。参数可以按值传递或按引用传递；按引用传递允许修改影响原变量。Edexcel 要求你能够准确地定义、调用和追踪伪代码中的子程序。
By-value parameters create a copy, so changes inside the subroutine do not affect the caller’s variable. By-reference parameters share the same memory location, so changes persist after the subroutine ends. Use these distinctions to explain side effects and return values.
按值传递参数会创建副本，因此子程序内部的修改不会影响调用者的变量。按引用传递参数共享同一内存位置，因此修改会在子程序结束后保留。利用这些区别来解释副作用和返回值。
A well-designed function should be self-contained and have a single clear purpose. Edexcel marking rewards modular design, meaningful identifiers, and the correct use of return statements.
设计良好的函数应当自包含并且具有单一明确目的。Edexcel 评分会奖励模块化设计、有意义的标识符和 return 语句的正确使用。
6. Recursion | 递归
Recursion occurs when a subroutine calls itself. Every correct recursive algorithm must have a base case to stop the recursion and a recursive case that reduces the problem. Classic examples include factorial, Fibonacci, and binary search. Recursive solutions are elegant but may use more memory due to call stack growth.
递归发生在子程序调用自身时。每个正确的递归算法必须有一个基准情形来停止递归，以及一个递归情形来缩小问题规模。经典例子包括阶乘、斐波那契数列和二分查找。递归解法简洁优雅，但由于调用栈增长可能占用更多内存。
factorial(n) = n × factorial(n-1), with factorial(0) = 1
factorial(n) = n × factorial(n-1)，且 factorial(0) = 1
When tracing recursion, write down each call and its return value. The call stack unwinds in reverse order, so factorial(3) becomes 3 × factorial(2), then 3 × 2 × factorial(1), and finally 3 × 2 × 1 = 6. Edexcel often asks you to complete a trace table for such recursive calls.
追踪递归时，写出每次调用及其返回值。调用栈按相反顺序展开，因此 factorial(3) 变为 3 × factorial(2)，然后是 3 × 2 × factorial(1)，最后得到 3 × 2 × 1 = 6。Edexcel 常要求你为这种递归调用填写追踪表。
7. Object-Oriented Programming: Classes and Objects | 面向对象编程：类与对象
Object-oriented programming (OOP) models real-world entities using classes and objects. A class is a blueprint that defines attributes and methods; an object is an instance of a class. For example, a Car class may have attributes like colour and speed, and methods like accelerate and brake. Edexcel questions may ask you to interpret class diagrams and write object instantiation code.
面向对象编程使用类和对象对现实世界实体建模。类是定义属性和方法的蓝图；对象是类的实例。例如，Car 类可以具有 colour 和 speed 等属性，以及 accelerate 和 brake 等方法。Edexcel 题目可能要求你解读类图并编写对象实例化代码。
Attributes are usually private to protect data integrity, so they are accessed through public getter and setter methods. This concept is called encapsulation. In pseudocode, you may see CLASS, PRIVATE, PUBLIC, and NEW keywords used to define and create objects.
属性通常是私有的，以保护数据完整性，因此通过公共的 getter 和 setter 方法访问。这个概念称为封装。在伪代码中，你可能会看到 CLASS、PRIVATE、PUBLIC 和 NEW 关键词用于定义和创建对象。
8. Inheritance and Polymorphism | 继承与多态
Inheritance allows a subclass to inherit attributes and methods from a superclass, promoting code reuse. Overriding lets a subclass provide a specific implementation of a superclass method. Polymorphism means ‘many forms’: the same method name can behave differently depending on the object type. Edexcel questions typically use class hierarchies to test these concepts.
继承允许子类从超类继承属性和方法，促进代码复用。重写允许子类提供超类方法的特定实现。多态意为 ‘多种形态’：同一方法名根据对象类型可以表现出不同行为。Edexcel 题目通常使用类层次结构来考查这些概念。
For instance, an Animal superclass may define a makeSound method, and Dog and Cat subclasses override it to produce ‘Woof’ and ‘Meow’. A loop calling makeSound on a list of Animal objects will produce the correct sound for each subtype without knowing its exact class.
例如，Animal 超类可以定义 makeSound 方法，而 Dog 和 Cat 子类重写该方法以产生 ‘Woof’ 和 ‘Meow’。对一个 Animal 对象列表循环调用 makeSound 时，无需知道每个对象的确切类，就能为每个子类型产生正确的声音。
Inheritance creates an ‘is-a’ relationship: a Dog is an Animal. Composition creates a ‘has-a’ relationship: a Car has an Engine. Edexcel may ask you to justify the choice between inheritance and composition in a given design scenario.
继承创建 ‘is-a’ 关系：Dog 是一个 Animal。组合创建 ‘has-a’ 关系：Car 有一个 Engine。Edexcel 可能要求你在给定设计场景中说明选择继承还是组合的理由。
9. Searching Algorithms | 查找算法
Searching algorithms locate a target value within a collection. Linear search checks every element sequentially until a match is found; it works on unsorted data with average time O(n). Binary search repeatedly divides a sorted list in half, achieving O(log n) time but requiring sorted data. Edexcel exams often ask for trace tables and comparisons of these algorithms.
查找算法用于在集合中定位目标值。线性查找逐个检查元素直到找到匹配项；它适用于未排序数据，平均时间复杂度为 O(n)。二分查找不断将有序列表分成两半，时间复杂度为 O(log n)，但要求数据已排序。Edexcel 考试常要求使用追踪表和比较这两种算法。
Linear search: O(n) | Binary search: O(log n)
线性查找：O(n) | 二分查找：O(log n)
For binary search, compare the target with the middle element. If the target is smaller, search the left half; if larger, search the right half. Repeat until found or the search
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