📚 Mastering Edexcel A-Level Programming: From Core Constructs to Algorithms | 精通Edexcel A-Level编程：从核心结构到算法
This article consolidates the core programming ideas assessed in Edexcel A-Level Computer Science. It is suitable for revision of Paper 2 topics and for strengthening your programming project skills.
本文整合了Edexcel A-Level计算机科学考试中的核心编程思想。它适用于Paper 2主题复习，也有助于加强你的编程项目能力。
1. Variables, Constants and Data Types | 变量、常量与数据类型
A variable is a named storage location whose value can change during execution. A constant is fixed once assigned, and using constants improves maintainability by preventing accidental modification.
变量是一个命名的存储位置，其值在执行过程中可以改变。常量在赋值后固定不变，使用常量可防止意外修改并提高可维护性。
Edexcel pseudocode commonly uses the five standard data types: Integer, Real/Float, Boolean, Character, and String. Choosing the correct type affects memory usage and the operations that can be performed.
Edexcel伪代码通常使用五种标准数据类型：整数、实数/浮点、布尔、字符和字符串。选择正确的类型会影响内存使用以及可以执行的操作。
Type conversions may be implicit or explicit. In many languages, dividing two integers may produce a real result, while explicit casting such as INT(3.7) truncates the decimal part.
类型转换可以是隐式的或显式的。在许多语言中，两个整数相除可能产生实数结果，而显式转换如INT(3.7)会截断小数部分。
2. Selection and Iteration | 选择与迭代
Selection uses IF…THEN…ELSE constructs to choose between alternative paths. Nested IF statements can express multi-branch logic, but CASE/SWITCH structures are often clearer.
选择结构使用IF…THEN…ELSE来在多个路径之间进行选择。嵌套IF可以表达多分支逻辑，但CASE/SWITCH结构通常更清晰。
Iteration repeats a block of code. Definite iteration (FOR loops) runs a known number of times, while indefinite iteration (WHILE and REPEAT…UNTIL) continues until a condition changes.
迭代重复执行一段代码。确定迭代（FOR循环）运行已知次数，不确定迭代（WHILE和REPEAT…UNTIL）一直持续到条件改变。
When comparing loop types, remember that a WHILE loop checks the condition at the start, so it may execute zero times. A REPEAT…UNTIL loop checks at the end, so the body always runs at least once.
比较循环类型时，记住WHILE循环在开始时检查条件，因此可能执行零次。REPEAT…UNTIL循环在结束时检查条件，所以循环体至少执行一次。
3. Arrays, Lists and Records | 数组、列表与记录
An array is a finite, ordered collection of elements of the same data type. Arrays allow direct access by index, usually starting at 0, which gives O(1) read/write time.
数组是一个有限的、有序的、同类型元素集合。数组允许按索引直接访问，通常从0开始，因此读写时间为O(1)。
Lists are dynamic structures that can grow and shrink. They support insertion and deletion more flexibly than static arrays, though access to an element by position may be O(n) in a linked implementation.
列表是可以动态增长和缩小的结构。它们支持比静态数组更灵活的插入和删除，但在链式实现中按位置访问元素可能需要O(n)时间。
A record combines fields of different types under one name, such as a Student record containing name, age and grade. It is a simple way to model real-world entities.
记录将不同类型的字段组合在一个名称下，例如包含姓名、年龄和成绩的Student记录。它是建模现实世界实体的一种简单方式。
4. Functions, Procedures and Parameter Passing | 函数、过程与参数传递
A function returns a value, while a procedure performs a task without returning a value. Edexcel pseudocode often uses SUBROUTINE for reusable blocks.
函数返回一个值，而过程执行任务但不返回值。Edexcel伪代码通常使用SUBROUTINE表示可重用代码块。
Parameters can be passed by value or by reference. By value copies the argument, so changes inside the routine do not affect the original. By reference passes the address, so modifications persist.
参数可以按值传递或按引用传递。按值传递会复制实参，因此例程内部的更改不会影响原始变量。按引用传递会传递地址，因此修改会保留。
Using meaningful identifiers, local variables and clear pre/post-conditions makes subroutines easier to test and reuse. Modular programming supports divide-and-conquer problem solving.
使用有意义的标识符、局部变量和明确的前置/后置条件使得子程序更易于测试和复用。模块化编程支持分而治之的问题求解。
5. Recursion and the Call Stack | 递归与调用栈
A recursive subroutine calls itself with a smaller input. Every valid recursive solution needs a base case to stop the recursion and a recursive case that moves toward the base case.
递归子程序用更小的输入调用自身。每个有效的递归方案都需要一个停止递归的基本情况，以及一个向基本情况推进的递归情况。
Each recursive call is placed on the call stack. The stack stores return addresses, parameters and local variables. If the base case is missing or unreachable, stack overflow can occur.
每个递归调用都会放入调用栈。栈保存返回地址、参数和局部变量。如果缺少基本情况或基本情况不可达，就可能发生栈溢出。
Recursion can be elegant for problems such as factorial, Fibonacci and tree traversal, but it may use more memory than an equivalent iterative solution.
递归对于阶乘、斐波那契和树遍历等问题可能十分简洁，但它可能比等价的迭代方案使用更多内存。
6. Searching Algorithms: Linear and Binary Search | 搜索算法：线性搜索与二分搜索
Linear search checks each element in order. It works on unsorted data and has O(n) worst-case time. Binary search requires sorted data and repeatedly halves the search interval.
线性搜索按顺序检查每个元素。它适用于未排序数据，最坏情况时间复杂度为O(n)。二分搜索要求数据已排序，并反复将搜索区间减半。
For binary search, compare the target with the middle element. If it is smaller, search the left half; if larger, search the right half. The maximum number of comparisons is about log₂ n + 1.
对于二分搜索，将目标值与中间元素比较。如果目标较小则搜索左半部分；如果较大则搜索右半部分。最大比较次数约为log₂ n + 1。
Binary search time = O(log n)
二分搜索时间 = O(log n)
7. Sorting Algorithms: Bubble, Insertion and Merge Sort | 排序
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