nth
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 zero-based counting 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.