在开始介绍表达式的细节之前，有必要先简单了解表达式能够计算的数据类型。
Before turning to the details of expression formation, it is useful to take a quick survey of the types of data that expressions can manipulate.
在第三章和第四章继续详细介绍表达式和语句的细节之前，非常有必要介绍一个本章例子中已经展示了的高层次的概念。
Before continuing on to consider the details of expressions and statements in Chapters 3 and 4, it is important to introduce one more high-level concept illustrated by the examples in this chapter.
他们想要介绍世界各地人们交流和表达自己的方式。
They want to introduce the way that people communicate and express themselves around the world.
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