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  ANSI Common Lisp   14 Conses   14.2 Dictionary of Conses

14.2.30 ldiff, tailp Function

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.

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.

 (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)

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:

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:

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