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ANSI Common Lisp  14 Conses 14.2 Dictionary of Conses
| 14.2.30 ldiff, tailp |
Function |
- Syntax:
-
ldiff
list object
 result-list
tailp
object list
generalized-boolean
- Arguments and Values:
-
list - a list,
which might be a dotted list.
object - an object.
result-list - a list.
generalized-boolean - a generalized boolean.
- Description:
-
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 copy2 of list.
- Examples:
-
(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)~%" list)
(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
- Side Effects:
-
Neither ldiff nor tailp modifies either of its arguments.
- Exceptional Situations:
-
Should be prepared to signal an error of type type-error if list is not a proper list or a dotted list.
- See Also:
-
set-difference
- Notes:
-
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)))))
- Allegro CL Implementation Details:
-
None.
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