List-Manipulation Functions（列表处理函数）

With that background out of the way, you’re ready to look at the library of functions Common Lisp provides for manipulating lists.

有了前面这些背景知识，现在就可以开始学习 Common Lisp 为处理列表而提供的函数库了。

You’ve already seen the basic functions for getting at the elements of a list: FIRST and REST. Although you can get at any element of a list by combining enough calls to REST (to move down the list) with a FIRST (to extract the element), that can be a bit tedious. So Common Lisp provides functions named for the other ordinals from SECOND to TENTH that return the appropriate element. More generally, the function NTH takes two arguments, an index and a list, and returns the nth (zero-based) element of the list. Similarly, NTHCDR takes an index and a list and returns the result of calling CDR n times. (Thus, (nthcdr 0 ...) simply returns the original list, and (nthcdr 1 ...) is equivalent to REST.) Note, however, that none of these functions is any more efficient, in terms of work done by the computer, than the equivalent combinations of FIRSTs and RESTs—there’s no way to get to the nth element of a list without following n CDR references.

前面已经介绍了获取列表中元素的基本函数：FIRST 和 REST。尽管可以通过将足够多的 REST 调用（用于深入列表）和一个 FIRST 调用（用于抽取元素）组合起来以获得一个列表中的任意元素，但这样可能有点冗长。因此，Common Lisp 提供了以从 SECOND 到 TENTH 的由其他序数命名的函数来返回相应的元素。而函数 NTH 则更为普遍，它接受两个参数，一个索引和一个列表，并返回列表中第 n 个（从 0 开始）元素。类似地，NTHCDR 接受一个索引和一个列表并返回 n 次调用 CDR 的结果。 （这样，(nthcdr 0 ...) 简单地返回最初的列表，而 (nthcdr 1 ...) 等价于 REST。）但要注意的是，就计算机完成的工作而言，这些函数都不会比等价的 FIRST 和 REST 组合更高效，因位无法在没有跟随 n 个 CDR 引用的情况下得到一个列表的第 n 个元素。

The 28 composite CAR/CDR functions are another family of functions you may see used from time to time. Each function is named by placing a sequence of up to four As and Ds between a C and R, with each A representing a call to CAR and each D a call to CDR. Thus:

另一个不时会用到的函数家族是 28 个复合 CAR/CDR 函数。每个函数都是通过将由最多四个 A 和 D 组成的序列放在 C 和 R 之间来命名的，其中每个 A 代表对 CAR 的调用而每个 D 代表对 CDR 的调用。因此我们可以得到：

(caar list) === (car (car list))
(cadr list) === (car (cdr list))
(cadadr list) === (car (cdr (car (cdr list))))

Note, however, that many of these functions make sense only when applied to lists that contain other lists. For instance, CAAR extracts the CAR of the CAR of the list it’s given; thus, the list it’s passed must contain another list as its first element. In other words, these are really functions on trees rather than lists:

但要注意，这其中许多函数仅当应用于含有其他列表的列表时才有意义。例如，CAAR 抽取出给定列表的 CAR 的 CAR，因此传递给它的列表必须含有另一个列表，并将该列表用作其第一个元素。换句话说，这些函数其实是用于树而不是列表的：

(caar (list 1 2 3))                  ==> error
(caar (list (list 1 2) 3))           ==> 1
(cadr (list (list 1 2) (list 3 4)))  ==> (3 4)
(caadr (list (list 1 2) (list 3 4))) ==> 3

These functions aren’t used as often now as in the old days. And even the most die-hard old-school Lisp hackers tend to avoid the longer combinations. However, they’re used quite a bit in older Lisp code, so it’s worth at least understanding how they work.

现在这些函数不像以前那样常用了。并且即便是最顽固的守旧 Lisp 黑客也倾向于避免使用过长的组合。尽管如此，它们还是被用在很多古老的 Lisp 代码上，因此至少应当去理解它们的工作方式。

The FIRST-TENTH and CAR, CADR, and so on, functions can also be used as SETFable places if you’re using lists nonfunctionally.

如果你正在非函数式地使用列表，这些 FIRST-TENTH 和 CAR、CADR 等函数也可被用作 SETF 的位置。

Table 12-1 summarizes some other list functions that I won’t cover in detail.

表 12-1 总结了其他一些我不会详细介绍的列表函数。

Table 12-1. Other List Functions