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(emacs-lisp-intro.info)nth

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`nth'
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   The `nthcdr' function takes the CDR of a list repeatedly.  The `nth'
function takes the CAR of the result returned by `nthcdr'.  It returns
the Nth element of the list.

   Thus, if it were not defined in C for speed, the definition of `nth'
would be:

     (defun nth (n list)
       "Returns the Nth element of LIST.
     N counts from zero.  If LIST is not that long, nil is returned."
       (car (nthcdr n list)))

(Originally, `nth' was defined in Emacs Lisp in `subr.el', but its
definition was redone in C in the 1980s.)

   The `nth' function returns a single element of a list.  This can be
very convenient.

   Note that the elements are numbered from zero, not one.  That is to
say, the first element of a list, its CAR is the zeroth element.  This
is called `zero-based' counting and often bothers people who are
accustomed to the first element in a list being number one, which is
`one-based'.

   For example:

     (nth 0 '("one" "two" "three"))
         => "one"
     
     (nth 1 '("one" "two" "three"))
         => "two"

   It is worth mentioning that `nth', like `nthcdr' and `cdr', does not
change the original list--the function is non-destructive.  This is in
sharp contrast to the `setcar' and `setcdr' functions.