ldiff list object ⇒ result-list
tailp object list ⇒ generalized-boolean
| list | a list,
which might be a dotted list. |
| object | an object. |
| result-list | a list. |
| generalized-boolean | a generalized boolean. |
If object is the same as some tail of list, tailp returns true; otherwise, it returns false.
If object is the same as some tail of list, ldiff returns a fresh list of the elements of list that precede object in the list structure of list; otherwise, it returns a copy_2 of list.
(let ((lists '#((a b c) (a b c . d))))
(dotimes (i (length lists)) ()
(let ((list (aref lists i)))
(format t "~2&list=~S ~21T(tailp object list)~
~44T(ldiff list object)~
(let ((objects (vector list (cddr list) (copy-list (cddr list))
'(f g h) '() 'd 'x)))
(dotimes (j (length objects)) ()
(let ((object (aref objects j)))
(format t "~& object=~S ~21T~S ~44T~S"
object (tailp object list) (ldiff list object))))))))
|>
|> list=(A B C) (tailp object list) (ldiff list object)
|> object=(A B C) T NIL
|> object=(C) T (A B)
|> object=(C) NIL (A B C)
|> object=(F G H) NIL (A B C)
|> object=NIL T (A B C)
|> object=D NIL (A B C)
|> object=X NIL (A B C)
|>
|> list=(A B C . D) (tailp object list) (ldiff list object)
|> object=(A B C . D) T NIL
|> object=(C . D) T (A B)
|> object=(C . D) NIL (A B C . D)
|> object=(F G H) NIL (A B C . D)
|> object=NIL NIL (A B C . D)
|> object=D T (A B C)
|> object=X NIL (A B C . D)
⇒ NIL
Should be prepared to signal an error of type type-error if list is not a proper list or a dotted list.
If the list is a circular list, tailp will reliably yield a value only if the given object is in fact a tail of list. Otherwise, the consequences are unspecified: a given implementation which detects the circularity must return false, but since an implementation is not obliged to detect such a situation, tailp might just loop indefinitely without returning in that case.
tailp could be defined as follows:
(defun tailp (object list)
(do ((list list (cdr list)))
((atom list) (eql list object))
(if (eql object list)
(return t))))
and ldiff could be defined by:
(defun ldiff (list object)
(do ((list list (cdr list))
(r '() (cons (car list) r)))
((atom list)
(if (eql list object) (nreverse r) (nreconc r list)))
(when (eql object list)
(return (nreverse r)))))