Info Catalog eintr: nthcdr eintr: car cdr & cons eintr: setcar

eintr: nth

 
 7.4 ‘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.
 

Info Catalog eintr: nthcdr eintr: car cdr & cons eintr: setcar