August 2021
Intermediate to advanced
450 pages
9h 36m
Chinese
Content preview from C++程序设计：原理与实践（基础篇）（原书第2版）
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6.4 文法
对于如何理解表达式的含义，已经有标准的解决方法了：首先读入符号，将它们组合为单词。因此，如果键入
程序应该产生如下单词列表：
一个单词就是一个字符序列，用来表示一个基本单元，例如数字或者运算符。
在产生单词之后，我们的程序必须保证对整个表达式正确解析。例如，我们知道表达式45+11.5/7的计算顺序是45+（11.5/7）而不是（45+11.5）/7，但如何告诉程序这些有用的规则呢（例如除法比加法优先级高）？标准的方法就是设计一个文法（grammar）来定义表达式的语法，然后在程序中实现这些文法规则。例如：
这是一个简单的规则集合，最后一条规则读作“一个Number是一个浮点常量”，倒数第二条规则表明“一个Primary是一个Number，或者是'（'后接一个Expression再接一个'）'”。针对Expression与Term的规则类似，都是依赖其后的规则来定义。
如6.3.2节，我们从C++定义中借用了如下几类单词：
·浮点常量：与C++定义相同，如3.14、0.274e2和42等。 ...
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