Re: Element-Relative Sequence Insertion

[Marc Tfardy]

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