📚 A-Level Edexcel Programming: Core Concepts and Exam Skills | A-Level Edexcel 编程：核心概念与考试技巧
Edexcel A-Level programming exams test both coding fluency and theoretical understanding. You need to read pseudocode, trace algorithms, compare data structures, and justify efficiency. This revision guide covers the core programming topics most often assessed, with bilingual explanations to support both English and Chinese learners.
Edexcel A-Level 编程考试既考查编码熟练度，也考查理论理解。你需要阅读伪代码、追踪算法、比较数据结构并解释效率。本复习指南涵盖最常考查的核心编程主题，用双语解释帮助中英文学习者。
1. Programming Basics and Data Types | 编程基础与数据类型
In Edexcel programming questions, you must select appropriate data types for variables. The main primitive types are integer, real or float, Boolean, character and string. Composite types include arrays, records and sets. Strong typing, used in many exam pseudocode languages, requires each variable to have a declared type, which helps catch errors.
在 Edexcel 编程题中，必须为变量选择合适的数据类型。主要的原始类型有整数、实数或浮点数、布尔型、字符和字符串。复合类型包括数组、记录和集合。许多考试伪代码语言采用强类型，要求每个变量声明类型，这有助于发现错误。
| Data Type | 数据类型 | Example | 示例 |
|---|---|
| Integer | 整数 | 42, -7, 0 |
| Real/Float | 实数 | 3.14, -0.5 |
| Boolean | 布尔 | True, False |
| Character | 字符 | ‘A’, ‘7’, ‘$’ |
| String | 字符串 | “hello”, “TutorHao” |
2. Sequence, Selection and Iteration | 顺序、选择与迭代
Programs are built from three control structures: sequence, selection and iteration. Sequence executes statements in order. Selection uses IF, ELSE IF, ELSE and CASE to branch. Iteration uses FOR, WHILE and REPEAT UNTIL to repeat blocks. Trace tables are a common exam tool; they record variable values after each line of pseudocode.
程序由三种控制结构构成：顺序、选择和迭代。顺序按顺序执行语句。选择使用 IF、ELSE IF、ELSE 和 CASE 进行分支。迭代使用 FOR、WHILE 和 REPEAT UNTIL 重复代码块。追踪表是常见的考试工具，记录每行伪代码执行后的变量值。
IF score >= 60 THEN grade ← ‘Pass’ ELSE grade ← ‘Fail’
You should be able to convert between flowchart diagrams and pseudocode, and to determine how many times a loop runs for a given input.
你应当能够在流程图和伪代码之间转换，并能确定给定输入下循环执行的次数。
3. Subprograms and Parameter Passing | 子程序与参数传递
A subprogram can be a function or a procedure. A function returns a single value and can appear in expressions. A procedure performs a task but does not return a value. Parameters may be passed by value or by reference. By value copies the argument, so changes inside the subprogram do not affect the caller. By reference passes the memory address, so changes do affect the original variable.
子程序可以是函数或过程。函数返回一个值并可出现在表达式中。过程执行任务但不返回值。参数可以按值或按引用传递。按值复制实参，所以子程序内部的更改不影响调用者。按引用传递内存地址，所以更改会影响原始变量。
- Pass by value | 按值传递: safe, no side effects | 安全，无副作用
- Pass by reference | 按引用传递: efficient for large data, allows modification | 适用于大数据，允许修改
4. Recursion and Stack Frames | 递归与栈帧
Recursion occurs when a subprogram calls itself. Every recursive solution needs a base case to terminate and a recursive case that moves towards the base case. For example, factorial n can be defined as n times factorial of n minus 1, with factorial of 0 equal to 1. Each call uses a stack frame, so deep recursion can cause stack overflow.
递归发生在子程序调用自身时。每个递归方案都需要终止的基准情形和向基准情形推进的递归情形。例如，n 的阶乘可定义为 n 乘以 n-1 的阶乘，0 的阶乘为 1。每次调用使用一个栈帧，因此递归过深会导致栈溢出。
factorial(n) = n × factorial(n – 1), factorial(0) = 1
Exam questions may ask you to trace a recursive function or to convert a recursive algorithm into an iterative one.
考试题可能要求你追踪递归函数，或将递归算法转换为迭代算法。
5. Object-Oriented Programming in Edexcel | Edexcel 中的面向对象编程
Object-oriented programming models real-world entities as classes and objects. Encapsulation keeps an object’s data private and exposes only necessary methods. Inheritance lets a subclass reuse and extend a parent class. Polymorphism allows the same method name to behave differently in different subclasses. Edexcel exams may ask you to identify these features in class diagrams or code.
面向对象编程将现实世界实体建模为类和对象。封装保持对象的数据私有，仅公开必要的方法。继承允许子类复用并扩展父类。多态使同一方法名在不同子类中有不同行为。Edexcel 考试可能要求你在类图或代码中识别这些特征。
- Encapsulation | 封装: data hiding and access methods | 数据隐藏与访问方法
- Inheritance | 继承: subclass extends superclass | 子类扩展父类
- Polymorphism | 多态: same interface, different implementation | 同一接口，不同实现
- Abstraction | 抽象: hide complex details | 隐藏复杂细节
6. Searching Algorithms | 搜索算法
Linear search inspects each element from the start until it finds the target or reaches the end. Its worst-case time complexity is O(n). Binary search works on a sorted list by comparing the middle element and discarding half the list each time. Its time complexity is O(log₂ n). For large sorted data, binary search is much faster.
线性搜索从开头检查每个元素，直到找到目标或到达末尾。其最坏时间复杂度为 O(n)。二分搜索在有序列表上工作，比较中间元素并每次舍弃一半列表。时间复杂度为 O(log₂ n)。对大型有序数据，二分搜索快得多。
A binary search algorithm sets low to 0, high to length – 1, then repeats while low <= high: mid = (low + high) DIV 2, compare, and adjust low or high.
二分搜索算法将 low 设为 0，high 设为长度减 1，然后当 low <= high 时重复：mid = (low + high) DIV 2，比较并调整 low 或 high。
7. Sorting Algorithms | 排序算法
Bubble sort repeatedly compares adjacent pairs and swaps them if they are in the wrong order. Insertion sort takes one unsorted item at a time and inserts
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