10.2 Arrays 数组概念简答

Scope

本页整理 array terminology、1D/2D structure selection、使用数组的好处及算法描述用词。Array pseudocode、linear search 与 bubble sort 代码留到专项。

Core terms

• Array：a fixed collection of elements of the same data type under one identifier。
• Element：one stored value in the array。
• Index/subscript：the value used to identify/access an element。
• Lower bound：smallest valid index。
• Upper bound：largest valid index。
• Dimension：number of indexes required to identify one element。

ThisArray[n] has one dimension；Grid[row, column] has two dimensions。

Choosing 1D or 2D

Use a 1D array for a single list/sequence, such as 30 marks or 12 month names。Use a 2D array for tabular/grid data with two coordinates, such as students by tests, seats by row/column or game-board positions。

每个 array 的 elements 必须是同一 type。若每个 entity 同时有 STRING、DATE、REAL 等不同 fields，应先定义 record，再使用 array of records。

Benefits of arrays

• Multiple values use one identifier, reducing the number of separate variable names。
• Elements can be accessed using an index。
• A loop can iterate through elements, simplifying repeated processing。
• Searching, sorting and organising data are easier to implement。
• Program design, testing and maintenance become easier when the number of items changes。

Three-mark pattern

An array stores many values under one identifier. An index allows individual values to be accessed, and a loop can iterate through all elements, simplifying processing.

Describing array processing

即使题目说 “Do not include pseudocode”，算法描述仍应按步骤写：

• Initialise an index/counter and any accumulator or current best value。
• Iterate from lower bound to upper bound。
• Use the loop counter as the array index。
• Compare/process the current element。
• Update the result when the condition is met。
• Output/return after the loop。

例如 largest search 应把 Max 初始化为 an existing array element，而不是任意 0，否则全负数数据可能产生错误结果。

Exam checklist

• Index、lower bound、upper bound 是否用词准确？
• 1D/2D 的选择是否联系数据形状？
• 是否误用 array 存储 mixed types？
• Benefit 是否具体写出 index 和 loop？
• 算法描述是否有 initialisation、iteration、comparison、update、output？

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完整 array declarations、search、sort 和 processing pseudocode 留到代码专项。