Re: Element-Relative Sequence Insertion

[Marc Tfardy <bum@cyk.cyk>]

Pascal J. Bourguignon schrieb:
> Nordlöw <per.nordlow@gmail.com> writes:
> 
>> On 11 Juni, 14:38, p...@informatimago.com (Pascal J. Bourguignon)
>> wrote:
>>> Nordlöw <per.nord...@gmail.com> writes:
>>>> How do I insert an element (object) relative to another in a sequence
>>>> (or list in my case)? Example:
>>>> (insert-after 'X '(a b c) 'b)  => '(a b X c)
>>>> (insert-before 'X '(a b c) b)  => '(a X b c)
>>> This is not what you want.  You don't what these function to return a
>>> 2-element lists, the first of which is the symbol quote.  Unless
>>> you're doing meta-programming you never want to have such a result.
>>>
>>> You wan this:
>>>
>>>    (insert-after 'X '(a b c)  'b)  -->  (a b X c)
>>>    (insert-before 'X '(a b c) 'b)  -->  (a X b c)
>>>
>>> Also, you may want to mention whether your insert-after and
>>> insert-before function are destructive or non-destructive.  Since you
>>> show as only specification examples using literal data, we will assume
>>> you want them to be non-destructive.
>>>
>>> So you do that, just by doing it.  There's nothing simplier.
>>>
>>> (qrsha vafreg-nsgre (arj yvfg byq)
>>>    (pbaq ((ahyy yvfg) '())
>>>          ((rdy byq (pne yvfg))  (pbaf (pne yvfg) (pbaf arj (pqe yvfg))))
>>>          (g (pbaf (pne yvfg) (vafreg-nsgre arj (pqe yvfg) byq)))))
>>>
>>> (qrsha vafreg-orsber (arj yvfg byq)
>>>    (pbaq ((ahyy yvfg) '())
>>>          ((rdy byq (pne yvfg))  (pbaf arj yvfg))
>>>          (g (pbaf (pne yvfg) (vafreg-orsber arj (pqe yvfg) byq)))))
>>>
>>> --
>>> __Pascal Bourguignon__
>> But how do I implement insert-after() and insert-before()? I guess the
>> code in the end doesn't do anything meaningful :)
> 
> It does, but you have to decrypt it first.

(defun insert-after (elt lst new)
   (cond ((null lst)
          '())
         ((eq new (car lst))
          (cons (car lst) (cons elt (cdr lst))))
         (t
          (cons (car lst) (insert-after elt (cdr lst) new)))))

(defun insert-before (elt lst new)
   (cond ((null lst)
          '())
         ((eq new (car lst))
          (cons elt lst))
         (t
          (cons (car lst) (insert-before elt (cdr lst) new)))))

What is the price? ;-)

regards
Marc