nthcdr function takes the cdr of a list repeatedly.
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)))
nth was defined in Emacs Lisp in subr.el,
but its definition was redone in C in the 1980s.)
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.
(nth 0 '("one" "two" "three")) ⇒ "one" (nth 1 '("one" "two" "three")) ⇒ "two"
It is worth mentioning that
cdr, does not change the original list—the function is
non-destructive. This is in sharp contrast to the