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cl:functions:ldiff [2019/11/11 05:00]
cl:functions:ldiff [2020/05/31 16:00] (current)
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 +====== Function LDIFF, TAILP ======
 +  * **ldiff** //list object// → //​result-list//​
 +  * **tailp** //object list// → //​generalized-boolean//​
 +====Arguments and Values====
 +  * //list// - a //​[[CL:​Glossary:​list]]//,​ which might be a //​[[CL:​Glossary:​dotted list]]//.
 +  * //object// - an //​[[CL:​Glossary:​object]]//​.
 +  * //​result-list//​ - a //​[[CL:​Glossary:​list]]//​.
 +  * //​generalized-boolean//​ - a //​[[CL:​Glossary:​generalized boolean]]//​.
 +If //object// is the //​[[CL:​Glossary:​same]]//​ as some //​[[CL:​Glossary:​tail]]//​ of //list//, **tailp** returns //​[[CL:​Glossary:​true]]//;​ otherwise, it returns //​[[CL:​Glossary:​false]]//​.
 +If //object// is the //​[[CL:​Glossary:​same]]//​ as some //​[[CL:​Glossary:​tail]]//​ of //list//, **ldiff** returns a //​[[CL:​Glossary:​fresh]]//​ //​[[CL:​Glossary:​list]]// ​ of the //​[[CL:​Glossary:​elements]]//​ of //​[[CL:​Glossary:​list]]// ​ that precede **[[CL:​Functions:​object]]** in the //​[[CL:​Glossary:​list structure]]//​ of //list//; otherwise, it returns a //​[[CL:​Glossary:​copy]]//​ of //list//.
 +([[CL:​Special Operators:​let]] ((lists '#((a b c) (a b c . d))))
 +  ([[CL:​Macros:​dotimes]] (i ([[CL:​Functions:​length]] lists)) ()
 +    ([[CL:​Special Operators:​let]] ((list ([[CL:​Functions:​aref]] lists i)))
 +      ([[CL:​Functions:​format]] t "​~2&​list=~S ~21T(tailp object list)~44T(ldiff list object)~%"​ list)
 +      ([[CL:​Special Operators:​let]] ((objects ([[CL:​Functions:​vector]] list ([[CL:​Functions:​cddr]] list) ([[CL:​Functions:​copy-list]] ([[CL:​Functions:​cddr]] list)) '(f g h) '() 'd 'x)))
 + ([[CL:​Macros:​dotimes]] (j ([[CL:​Functions:​length]] objects)) ()
 +   ([[CL:​Special Operators:​let]] ((object ([[CL:​Functions:​aref]] objects j)))
 +     ([[CL:​Functions:​format]] t "​~&​ object=~S ~21T~S ~44T~S"​ object ([[CL:​Functions:​tailp]] object list) ([[CL:​Functions:​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)</​o>​
 +<​r>​[[CL:​Constant Variable:​nil|NIL]]</​r>​
 +====Side Effects====
 +====Affected By====
 +====Exceptional Situations====
 +Should be prepared to signal an error of type type-error if //list// is not a //​[[CL:​Glossary:​proper list]]// or a //​[[CL:​Glossary:​dotted list]]//.
 +====See Also====
 +**[[CL:​Functions:​set-difference|Function SET-DIFFERENCE]]**
 +====Example Implementation====
 +**tailp** could be defined as follows:
 +([[CL:​Macros:​defun]] tailp (object list)
 +  ([[CL:​Macros:​do]] ((list list ([[CL:​Functions:​cdr]] list)))
 +      (([[CL:​Functions:​atom]] list) ([[CL:​Functions:​eql]] list object))
 +    ([[CL:​Special Operators:​if]] ([[CL:​Functions:​eql]] object list)
 + ([[CL:​Macros:​return]] t))))
 +and **ldiff** could be defined by:
 +([[CL:​Macros:​defun]] ldiff (list object)
 +  ([[CL:​Macros:​do]] ((list list (cdr list))
 +       (r '() ([[CL:​Functions:​cons]] ([[CL:​Functions:​car]] list) r)))
 +      (([[CL:​Functions:​atom]] list)
 +       ​([[CL:​Special Operators:​if]] ([[CL:​Functions:​eql]] list object) ([[CL:​Functions:​nreverse]] r) ([[CL:​Functions:​nreconc]] r list)))
 +    ([[CL:​Macros:​when]] ([[CL:​Functions:​eql]] object list)
 +      ([[CL:​Macros:​return]] ([[CL:​Functions:​nreverse]] r)))))
 +If the //list// is a //​[[CL:​Glossary:​circular list]]//, **tailp** will reliably //​[[CL:​Glossary:​yield]]//​ a //​[[CL:​Glossary:​value]]// ​ only if the given //object// is in fact a //​[[CL:​Glossary:​tail]]//​ of //list//. Otherwise, the consequences are unspecified:​ a given //​[[CL:​Glossary:​implementation]]//​ which detects the circularity must return //​[[CL:​Glossary:​false]]//,​ but since an //​[[CL:​Glossary:​implementation]]//​ is not obliged to detect such a //​[[CL:​Glossary:​situation]]//,​ **tailp** might just loop indefinitely without returning in that case.