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ldiff, tailp

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.

ldiff, tailp

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)))))

Expanded Reference: ldiff, tailp​

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