📚 Programming Fundamentals for Edexcel A-Level Computer Science | Edexcel A-Level 计算机科学编程基础
Programming questions in Edexcel A-Level Computer Science test your ability to read, trace, write and evaluate code. You are expected to use precise pseudocode and to understand how constructs such as selection, iteration, functions and data structures behave in a high-level language like Python.
Edexcel A-Level 计算机科学中的编程题考查你阅读、跟踪、编写和评估代码的能力。你需要掌握精确的伪代码，并理解选择、迭代、函数和数据结构等构造在 Python 等高级语言中的行为。
1. Programming Constructs: Sequence, Selection, Iteration | 编程结构：顺序、选择、迭代
All programs can be expressed using three building blocks: sequence, selection and iteration. Sequence means statements run in order; selection chooses between paths using conditions; iteration repeats code until a condition is met.
所有程序都可以用三种基本结构表示：顺序、选择和迭代。顺序表示语句按顺序执行；选择根据条件在路径之间做出决定；迭代重复代码直到满足条件。
In pseudocode, selection is written as IF…THEN…ELSE…ENDIF or CASE…OF, while iteration is written as FOR…ENDFOR, WHILE…ENDWHILE or REPEAT…UNTIL. Trace tables are used in the exam to record variable values as each instruction is executed.
在伪代码中，选择写作 IF…THEN…ELSE…ENDIF 或 CASE…OF，迭代写作 FOR…ENDFOR、WHILE…ENDWHILE 或 REPEAT…UNTIL。考试中使用跟踪表记录每条指令执行后的变量值。
2. Variables, Constants and Data Types | 变量、常量与数据类型
A variable is a named memory location whose value can change during execution; a constant has a value that cannot be changed. Edexcel pseudocode requires clear choice of data types: integer, real, Boolean, character and string.
变量是有名称的内存位置，其值在执行过程中可以改变；常量的值不能改变。Edexcel 伪代码要求明确选择数据类型：整数、实数、布尔值、字符和字符串。
Type mismatches and integer division are common exam pitfalls. In Python, 7 / 2 gives 3.5 while 7 // 2 gives 3, and the modulo operator % gives the remainder 1.
类型不匹配和整数除法是常见的考试陷阱。在 Python 中，7 / 2 得到 3.5，而 7 // 2 得到 3，取模运算符 % 得到余数 1。
3. Operators and Boolean Logic | 运算符与布尔逻辑
Programs use arithmetic operators (+, −, ×, ÷, DIV, MOD), relational operators (=, ≠, <, >, ≤, ≥) and logical operators AND, OR and NOT. Boolean expressions evaluate to TRUE or FALSE and drive selection and iteration conditions.
程序使用算术运算符（+、−、×、÷、DIV、MOD）、关系运算符（=、≠、<、>、≤、≥）以及逻辑运算符 AND、OR 和 NOT。布尔表达式的结果为 TRUE 或 FALSE，并驱动选择和迭代条件。
De Morgan’s laws help simplify complex conditions: NOT (A AND B) becomes (NOT A) OR (NOT B); NOT (A OR B) becomes (NOT A) AND (NOT B). Trace tables often include Boolean columns for these evaluations.
德摩根定律有助于简化复杂条件：NOT (A AND B) 等价于 (NOT A) OR (NOT B)；NOT (A OR B) 等价于 (NOT A) AND (NOT B)。跟踪表通常包含这些逻辑求值的布尔列。
4. String Manipulation and Formatting | 字符串处理与格式化
String handling appears frequently in file processing and data validation. Common operations include length, substring, concatenation, character indexing, character conversion and case changes.
字符串处理经常出现在文件处理和数据验证中。常见操作包括求长度、取子串、连接、字符索引、字符转换和大小写转换。
In Python, strings are immutable and zero-indexed: text[0] returns the first character, text[-1] the last, and slicing text[0:3] returns the first three characters. Concatenation uses the + operator.
在 Python 中，字符串是不可变的且从零开始索引：text[0] 返回第一个字符，text[-1] 返回最后一个字符，切片 text[0:3] 返回前三个字符。连接使用 + 运算符。
5. Arrays, Lists and Records | 数组、列表与记录
Arrays store multiple values of the same type in contiguous memory, accessed by an index. Edexcel also uses records, which group related fields of different types under one name.
数组在连续内存中存储多个相同类型的值，通过索引访问。Edexcel 还使用记录，将不同类型但相关的字段放在一个名称下。
Lists in Python are dynamic and allow mixed types, making them useful for building stacks, queues and hash tables. A 2D array is often modelled as a list of lists.
Python 中的列表是动态的并允许混合类型，因此常用于构建栈、队列和哈希表。二维数组通常建模为列表的列表。
6. Functions, Procedures and Parameter Passing | 函数、过程与参数传递
A function returns a value, while a procedure does not. Both can accept parameters, which are passed either by value or by reference. By value passes a copy, so the original variable is unchanged.
函数返回值，过程不返回值。两者都可以接受参数，参数可以按值传递或按引用传递。按值传递传递的是副本，因此原始变量不会改变。
In Python, integers, strings and floats are passed by value effectively; mutable objects such as lists behave more like pass-by-reference because changes inside the function affect the original object.
在 Python 中，整数、字符串和浮点数实际上按值传递；列表等可变对象的行为更接近按引用传递，因为函数内部的修改会影响原对象。
7. Recursion and Stack Frames | 递归与栈帧
Recursion occurs when a function calls itself to solve smaller instances of a problem. Every recursive solution needs a base case to stop, otherwise a stack overflow occurs.
递归是指函数调用自身来求解规模更小的问题。每个递归解都需要一个基准条件来停止，否则会发生栈溢出。
A recursive factorial is written as factorial(n) = n × factorial(n − 1) with base case factorial(0) = 1. Each call creates a stack frame storing local variables and the return address.
递归阶乘写作 factorial(n) = n × factorial(n − 1)，基准条件为 factorial(0) = 1。每次调用都会创建一个栈帧，保存局部变量和返回地址。
8. Sorting and Searching Algorithms | 排序与搜索算法
Linear search checks every element until a match is found; binary search repeatedly halves a sorted list. Bubble sort passes through the list swapping adjacent elements, while merge sort divides and conquers.
线性搜索逐个检查每个元素直到找到匹配项；二分搜索反复将有序列表减半。冒泡排序遍历列表并交换相邻元素，而归并排序采用分治策略。
You must be able to trace these algorithms with pencil and paper, writing the state of the list after each pass or split. Exam questions often ask for the number of comparisons in the worst case.
你必须能够用纸笔跟踪这些算法，写出每一轮或每次分割后的列表状态。考试题经常要求计算最坏情况下的比较次数。
9. Big O Notation and Efficiency | 大 O 表示法与效率
Big O notation describes how time or space grows with input size n. It ignores constant factors and lower-order terms, for example 3n² + 2n + 1 is O(n²).
大 O 表示法描述时间或空间如何随输入规模 n 增长。它忽略常数因子和低阶项，例如 3n² + 2n + 1 为 O(n²)。
Linear search is O(n), binary search is O(log n), and bubble sort is O(n²) in the worst case. Merge sort is O(n log n), which is much faster for large lists.
线性搜索是 O(n)，二分搜索是 O(log n)，冒泡排序最坏情况下是 O(n²)。归并排序是 O(n log n)，对于大型列表要快得多。
10. File Handling and Exceptions | 文件处理与异常
Files are processed by opening a handle, reading or writing data, and closing the file. Common modes are read, write, append and update. Failure to close can corrupt data.
文件处理需要打开句柄、读取或写入数据，然后关闭文件。常见模式有读、写、追加和更新。不关闭文件可能导致数据损坏。
Exception handling uses TRY…EXCEPT…FINALLY structures to catch errors such as missing files, invalid input or division by zero without crashing the program.
异常处理使用 TRY…EXCEPT…FINALLY 结构来捕获错误，例如文件缺失、无效输入或除零错误，而不会导致程序崩溃。
11. Object-Oriented Programming Concepts | 面向对象编程概念
Object-oriented programming organises code into classes that define attributes and methods. Objects are instances of classes. Encapsulation hides internal state behind public methods.
面向对象编程将代码组织为定义属性和方法的类。对象是类的实例。封装将内部状态隐藏在公有方法之后。
Inheritance allows a derived class to reuse and extend a base class; polymorphism lets the same method name behave differently in different classes, often through overriding.
继承允许派生类重用并扩展基类；多态使得同一方法名在不同类中表现出不同行为，通常通过方法重写实现。
12. Programming Paradigms and Pseudocode | 编程范式与伪代码
Edexcel expects you to read and write pseudocode that is independent of any one language. The main paradigms are imperative, procedural, object-oriented and event-driven.
Edexcel 要求你能够阅读和编写独立于具体语言的伪代码。主要范式有命令式、过程式、面向对象和事件驱动。
In the exam, keep pseudocode simple: meaningful identifiers, one statement per line, indentation for blocks, and clear conditions. Do not mix Python syntax with pseudocode unless the question asks for it.
在考试中，保持伪代码简洁：使用有意义的标识符、每行一条语句、块结构缩进、条件清晰。除非题目要求，不要把 Python 语法与伪代码混用。
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