You’ve already seen the basic functions for getting at the elements of a list: 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 **FIRST**s and **REST**s—there’s no way to get to the nth element of a list without following n **CDR** references.11

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:

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

Table 12-1. Other List Functions