Vectors As Sequences（作为序列的向量）

As mentioned earlier, vectors and lists are the two concrete subtypes of the abstract type sequence. All the functions I'll discuss in the next few sections are sequence functions; in addition to being applicable to vectors--both general and specialized--they can also be used with lists.

正如早先所提到的，向量和列表是抽象类型序列的两种具体子类型。接下来几节里讨论的所有函数都是序列函数：除了可以应用于向量（无论是通用还是特化的）之外，它们还可应用于列表。

The two most basic sequence functions are LENGTH, which returns the length of a sequence, and ELT, which allows you to access individual elements via an integer index. LENGTH takes a sequence as its only argument and returns the number of elements it contains. For vectors with a fill pointer, this will be the value of the fill pointer. ELT, short for element, takes a sequence and an integer index between zero (inclusive) and the length of the sequence (exclusive) and returns the corresponding element. ELT will signal an error if the index is out of bounds. Like LENGTH, ELT treats a vector with a fill pointer as having the length specified by the fill pointer.

两个最基本的序列函数是 LENGTH，其返回一个序列的长度；ELT，其允许通过一个整数索引来访问个别元素。LENGTH 接受序列作为其唯一的参数并返回它含有的元素数量。对于带有填充指针的向量，这些是填充指针的值。ELT 是元素（element）的简称，它接受序列和从 0 到序列长度（左闭右开区间）的整数索引并返回对应的元素。ELT 将在索引超出边界时报错。和 LENGTH 一样，ELT 也将把一个带有填充指针的向量视为其具有该填充指针所指定的长度。

(defparameter *x* (vector 1 2 3))

(length *x*) ==> 3
(elt *x* 0)  ==> 1
(elt *x* 1)  ==> 2
(elt *x* 2)  ==> 3
(elt *x* 3)  ==> error

ELT is also a SETFable place, so you can set the value of a particular element like this:

ELT 也是一个支持 SETF 的位置，因此可以像这样来设置一个特定元素的值：

(setf (elt *x* 0) 10)

*x* ==> #(10 2 3)