《代码大全》学习笔记——第十八章，表驱动法

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mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-border-insideh:.5pt solid windowtext; mso-border-insidev:.5pt solid windowtext; mso-para-margin:0cm; mso-para-margin-bottom:.0001pt; text-align:justify; text-justify:inter-ideograph; mso-pagination:none; font-size:10.0pt; font-family:"Times New Roman"; mso-ansi-language:#0400; mso-fareast-language:#0400; mso-bidi-language:#0400;} --> <!-- [endif]--> 第十八章，表驱动法

前言

表驱动法是一种编程模型（ scheme ）——从表中查找信息而不是使用逻辑语句（ if 或 else ）。

对于表驱动法一定要慎用。如果逻辑很复杂，导致逻辑判断链很长，使用表驱动法有助于降低复杂度。否则，表驱动法只会增加复杂度。

18.1 表驱动法使用总则

使用表驱动法的两个问题：

<!-- [if !supportLists]-->1、 <!-- [endif]-->如何从表中查询条目。三种方法：直接访问（ Directory access ）表，索引访问（ Index access ）表，阶梯访问（ Stair-step access ）表。

<!-- [if !supportLists]-->2、 <!-- [endif]-->表中要保存什么。可能是数据，函数指针，对象实例等。

18.2 直接访问表

灵活的消息格式：

这里介绍了一个通用的表驱动法。核心的思想仍然是通过抽象，隔离变化，使变化的影响降低到最低，使代码对变化是友好的。

构造查询键值：

<!-- [if !supportLists]-->1、 <!-- [endif]-->复制信息从而能够直接使用键值。

<!-- [if !supportLists]-->2、 <!-- [endif]-->转换键值以使其能够直接使用。对键值进行转换，得到直接的索引值。这里可以在增加一个新的查询表。

<!-- [if !supportLists]-->3、 <!-- [endif]-->把键值转换提取层单独的子程序。

18.3 索引访问表

18.4 阶梯访问表

基本思想：表中的记录对于不同的数据范围有效，而不是对不同的数据点有效。

注意细节：

<!-- [if !supportLists]-->1、 <!-- [endif]-->留心端点。

<!-- [if !supportLists]-->2、 <!-- [endif]-->考虑用二分查找代替顺序查找。

<!-- [if !supportLists]-->3、 <!-- [endif]-->考虑用索引访问代替阶梯访问。索引访问速度快，占内存。阶梯访问速度慢，省内存。

<!-- [if !supportLists]-->4、 <!-- [endif]-->把阶梯中的查询操作提取成单独的子程序。

18.5 表查询的其他示例

CHECKLIST: Table-Driven Methods 核对表：表驱动法 <!-- [if !supportLists]-->1、 <!-- [endif]-->你考虑过把表驱动法作为复杂逻辑的替代方案吗？ <!-- [if !supportLists]-->2、 <!-- [endif]-->你考虑过把表驱动法作为复杂继承结构的替代方案吗？ <!-- [if !supportLists]-->3、 <!-- [endif]-->你考虑过把数据存储在程序外部，以便在不用修改代码的情况下，更改这些数据吗？ <!-- [if !supportLists]-->4、 <!-- [endif]-->如果无法用一种简单的数组索引去访问表，那么你把计算访问键值的功能提取成单独的子程序，而不是在代码中重复的计算键值了吗？

本章要点

<!-- [if !supportLists]-->1、 <!-- [endif]-->表提供了一种复杂的逻辑和继承结构的替代方案。如果你发现自己对某个程序的逻辑和继承关系感到困惑时，那么问问自己它是否可以通过一个查询表来简化。

<!-- [if !supportLists]-->2、 <!-- [endif]-->使用表的一项关键决策时如何去访问表。你可以采取直接访问，索引访问，或者阶梯访问。

<!-- [if !supportLists]-->3、 <!-- [endif]-->使用表的另一项关键决策时把什么内容放在表中。

把表驱动法写成一个基础库。